What is a quick catch?
Charles Carroll
In another thread Magnus Burbanks writes that he pairs partner's catch is
measurably quicker than his.
Reading this I was once again reminded that I am somewhat unclear on what
people mean by "a quick catch." Indeed I am not sure I even know what people
mean by the catch.
Consider the definition proposed by Drs. Volker Nolte and Valery Kleshnev:
"The Catch is defined as the furthest point of the handle towards the
stern." (Rowing Biomechanics Newsletter, March 2008)
Let's assume Nolte and Kleshnev are right in defining the catch as a point
in the stroke. Doesn't this present a problem? Am I correct in thinking that
quickness is an attribute than can only apply to the speed with which
something moves through an interval between two points? And if so, then how
can a point, which in and of itself has no extension, be quick? It doesn't
make sense to me.
I would think that a catch can only be called quick if it were a process,
that is, an interval between two points in the rowing stroke. If so, then
what are these points and what is this process?
It is easy to understand blade entry as a process. For example, couldn't we
define blade entry as that interval between when the tip of the blade first
touches the water to when the blade is immersed at its correct depth?
Indeed Carl is forever reminding us that if we want to go fast we must enter
the blades quickly, among other things.
So do people really mean quick blade entries when they talk about quick
catches?
Or do they mean something else?
For example, maybe by quick catches people really mean loading the blades
quickly. And again I refer to Carl, who also reminds us that blades don't
load themselves - it is we who load the blades. So does a quick catch mean
we load the blades quickly?
And if so, then how do we load the blades? Do they load during blade entry?
Or do they load after the blade reaches its correct depth? Or is loading the
blade a process that begins with blade entry and ends when the load on the
blade will support full pressure applied against the pins?
What is the best way to load a blade? And how do we load a blade quickly?
Cordially,
Charles
Walter Martindale
wrote:
I am pretty sure that if you use what others call a quick catch, the
quick entry of Nolte/Kleshnev will fit.
I don't have anything empirical about this, but if the blades go in
quickly at that "turn around" point or at that "catch" as defined by N/
K (not NK) the blades will almost load themselves as lift applies
force normal to the direction of travel through the water. The
sculler has to hang onto them to keep them from being pulled from his/
her hands by the lift, and the initial foot stretcher reaction comes
from this loading applied by the blade's lift.
Pull the oar into the water (i.e., have it travel aft before it hits
the water) and you mess this up.
No time. Off to lake for daysheet, if tomorrow's races aren't blown
off.
Whee.
W
carolinetu
1. Hesitating before putting the blades in the water
2. Dropping the hands on the way forward, "skying" the blade so it
has a long way to travel before it hits the water
3. Lifting the shoulders rather than raising the arms, which results
in opening up the body too early rather than driving with the legs
Fix these, and you should end up with a "quick" catch. At least,
that's my theory.
Caroline
MagnusBurbanks
compression (i.e. seat momentarily stationary on its rails) to the
moment when the acceleration of the shell turns positive, both of
which can be observed with the accelerometer data. This can be seen on
the (for now) only shell/rower acceleration plot I have up at the
moment at http://www.slidingseat.net/accel/accel.html#accelintro where
the point of full compression and the point where the shell
acceleration "goes vertical".
I used to count video frames from the point of full compression to the
point at which the blades were fully buried, but at 25 blurry fps that
was less than ideal.
One of the studies I intend to do is to collect and publish a gallery
of different subjects from this catch-speed viewpoint, ranging from
very good to less good or inexperienced scullers and crews.
Cheers, Magnus
Alexander Lindsay
"Walter Martindale" <wmar...@gmail.com> wrote in message
news:1d7a4b43-8d95-46ef...@e4g2000prn.googlegroups.com...
Sorry, but this must be wrong.
When a blade enter the water, it is forced by the movement of the boat to
rotate about the pin. This pushes the handle towards the bows, and towards
the sculler/oarsman. Yes, initially the blade moves forward through the
water, but it does not move forward relative to the boat. The idea that
"sculler has to hang onto them to keep them from being pulled from his/
her hands by the lift", cannot be true. It may perhaps feel like that, but
it cannot be true.
Also "blades will almost load themselves as lift applies force normal to the
direction of travel through the water" is not true (pace "almost"). The
lift force on the blade is not created by the movement of the blade through
the water; it is a reaction to the force applied to the blade by the man
pulling on the handle. Intil the load is applied to the handle there can be
no significant lift force. Of course, if no force is applied to the handle,
there must be a tiny force on the blade to cause the angular acceleration of
the oar, but it is in the wrong direction.
I think we see here examples of the confusion that can arise when comments,
which may be useful as coaching aids, are taken to be descriptions of
physical reality. For example compare what is written above with the views
of a local coach (England), who says he was taught it at an ARA coaching
course, that you should drop the blade into the water and then not pull
until you feel the handle pushing against your hands. And he seems to
believe it!
Alexander Lindsay
Tinus
wrote:
> For my purposes the catch is the interval between the moment of full
> compression (i.e. seat momentarily stationary on its rails) to the
> moment when the acceleration of the shell turns positive
I am sure you meant the moment when the acceleration of the centre of
mass turns positive. The shell can easily stay at a negative
acceleration for quite a while even though work is generated by the
action of moving the blade against the water.
On Oct 2, 2:43 am, "Charles Carroll" <charles_carr...@comcast.net>
wrote:
> Dear all,
>
> Reading this I was once again reminded that I am somewhat unclear on what
> people mean by "a quick catch." Indeed I am not sure I even know what people
> mean by the catch.
I personally prefer to view/define the catch (and release) from a
kinematic viewpoint in the same way as Magnus.
This can be especially interesting for the release. If the recovery
starts before the drive has ended (the mass of the body starts to move
towards stern before the drive, defined by force being applied by the
blade in stern wards direction, has ended) then one might speak of
something strange as a release taking a negative amount of time. For
the release it is very well possible to develop a technique/style in
which this is possible. I believe this style is very successful for
the rowing machine (contrary to the very popular long strokes).
For the catch it becomes more difficult but one can imagine a rowing
style in which the seat still moves towards stern while the upper body
is already moving towards bow applying force to the water. In this way
a rower has started the drive before even changing the direction of
the momentum of the body relative to the boat.
Of course one can also use other viewpoints like using the movement of
the body as starting point and define the catch as the period during
which the rower moves the handle/hands in upwards direction in order
to get the blades into the water.
Walter Martindale
wrote:
> "Walter Martindale" <wmart...@gmail.com> wrote in message
I'm all for getting force applied to the handles from the moment of
contact with the bottom edge of the blade. The description I've used
above is more of a thought experiment - perhaps it's not the best, and
it's certainly not valid at shallow catch angles of normal human
scullers and any sweep rowers.
If the ARA coaching courses are teaching that you should wait until
the handle is pushing against your hands at the catch, then please
(from the perspective of "I cheer for two teams, mine, and anyone
competing against England") keep doing it.
A quick entry, if I recall correctly, with the handles moving "up and
away", seems to get load on the blades during the entry if it's timed
correctly with the foot-stretcher changing (relative) direction.
Things that contribute to slow entries include things mentioned above,
too tight grips, hesitation after reaching full length before putting
the blades in the water, thinking about the hands going up instead of
thinking about the blades going in the water, trying to be a dressage
sculler, and physical unfitness - if that's a word.
Walter
- A Canuck, resident in New Zealand...
Donal
the blades allows them to bury quite nicely by themselves....let the
hands follow the blades to the water...
then the legs can drive through the lower body horizontally at any
time from when the blade is 1/4 1/2 or fully covered
donal
Pete
wrote:
> Let's assume Nolte and Kleshnev are right in defining the catch as a point
> in the stroke. Doesn't this present a problem?
Yes.
> Am I correct in thinking that
> quickness is an attribute than can only apply to the speed with which
> something moves through an interval between two points?
Yes.
> And if so, then how
> can a point, which in and of itself has no extension, be quick? It doesn't
> make sense to me.
The catch isn't a point, or at least the only way you can define it to
be so is to ignore the way everyone else in the rowing world talks
about it.
> I would think that a catch can only be called quick if it were a process,
> that is, an interval between two points in the rowing stroke. If so, then
> what are these points and what is this process?
In a single, it makes some sense to say that it's between the furthest
forward the sculls reach, and the point where they are fully covered;
but you could equally well talk about loading the blade, and probably
several other ways to define it would make sense. In a crew boat, it's
even more problematic. I'll stick to the first...
> It is easy to understand blade entry as a process. For example, couldn't we
> define blade entry as that interval between when the tip of the blade first
> touches the water to when the blade is immersed at its correct depth?
Yes, but that's not the same as a 'quick catch' as per normal speech.
Often a very slow catch goes with quick blade entry: someone who
reaches frontstops skying and then slams the blade in can have a very
quick blade entry. Of course, that blade entry probably occurs when
the rower has moved back a foot down the slide, well after the rest of
any hypothetical crew have put their blades in the water...
> Indeed Carl is forever reminding us that if we want to go fast we must enter
> the blades quickly, among other things.
>
> So do people really mean quick blade entries when they talk about quick
> catches?
No.
> Or do they mean something else?
Yes, see above.
> For example, maybe by quick catches people really mean loading the blades
> quickly.
Again that's different, if you want the blades to load fast then just
kick the catch. Your stroke won't be very long, but it'll load fast,
somewhere around half slide...
> And again I refer to Carl, who also reminds us that blades don't
> load themselves - it is we who load the blades. So does a quick catch mean
> we load the blades quickly?
IMO, no. I think those are separate processes. But that's really just
about definition.
> And if so, then how do we load the blades? Do they load during blade entry?
Yes.
> Or do they load after the blade reaches its correct depth?
Yes.
Or is loading the
> blade a process that begins with blade entry and ends when the load on the
> blade will support full pressure applied against the pins?
Yes.
> What is the best way to load a blade?
As fast as possible subject to minimising slip.
> And how do we load a blade quickly?
Now, there you have a good question... :-)
Blade entry should take place rapidly, as soon as possible after full
extension (what K+N call 'catch'), with no requirement to push the
blades into the water (which can only happen if you are braking the
boat with the sculls), and loading the oars should be as fast as
possible without causing slip (you can start to load before the oars
are covered, but I don't know of any serious attempt to make
computations on this: my opinion, as someone who occasionally has to
actually deal with this kind of thing, is that any CFD simulation of
the catch may be of interest to oar designers (who can plug in a
typical force profile and maybe get something not totally useless) but
is a waste of time for those who want to know what the 'best way to
load' is.
If you want to shell out for all the kit, then you will certainly
learn something from observing your force profile in the boat; timed
pieces (or speedcoach / HR data, if you're consistent with HR /
exertion) will tell you a bit. You'll also learn something from the
old-school method of keeping an eye on what your puddles look like;
and that's free...
Pete
Teaplant
oars go from touching the water to fully buried is equivalent to the
quickness of a catch. Indeed also that coming down from a large
altitude towards the water means a slow catch.
The slowness of a catch (IMHO) is dictated by the time lag between the
start of the rise in the stern-ward force on the stretcher and the
rise in useful bow-ward force on the gates. A top quality oarsmen
rowing at training cadence probably getting these two events to happen
within milliseconds of one another - simultaneously to the naked eye.
Smacking the blades into the water doesn't make the catch quicker
since the effort of doing this vertical movement seems to make the leg
drive start too early (+loses body posture) and the bow-ward force on
the pins does not reach maximum until later when the blade has
stabilised in the water at whatever depth.
Carrying the blades too near the water doesn't help either since there
is inevitably some lowering of the hands to get the oar square and
even if the blade is uniformly 1mm from the surface, the time taken to
bury the spoon properly is actually longer if it is barely moving
vertically at the moment of contact with the surface.
Most catastrophically I think that if the body posture is wrong there
is a serious time lag between pushing on the stretcher and getting a
good grip on the water - without which the useful forward force on the
pin is very weak.
Fast catches to my eye are the result of a strong and unchanging body
posture during the the approach to the catch and early drive phase.
Carrying the oar well clear of the surface and allowing it to descend
smoothly during the final part of the slide, and a relatively deep
bury of the blade. The crucial factor is phasing the drive to begin
at the right moment during this otherwise smooth transition around the
front end. It is so much easier to see when you stop imagining the
oar as moving forward and backwards, rather than moving forward then
stopping, forward then stopping etc...
Humble opinion of course
teaplant.
MagnusBurbanks
> On Oct 2, 12:03 pm, MagnusBurbanks <magnus.burba...@googlemail.com>
> wrote:
>
> > For my purposes the catch is the interval between the moment of full
> > compression (i.e. seat momentarily stationary on its rails) to the
> > moment when the acceleration of the shell turns positive
>
> I am sure you meant the moment when the acceleration of the centre of
> mass turns positive. The shell can easily stay at a negative
> acceleration for quite a while even though work is generated by the
> action of moving the blade against the water.
Actually when I wrote it I did indeed mean the full-compression to
_shell_ positive acceleration, but that was before I had sufficiently
developed my processing to look at the data from a centre of mass
viewpoint. But reading your comments I agree the full-compression to
CoM positive acceleration interval is better.
Cheers, Magnus
Carl Douglas
There's no immediate connection between an initial rise of force on the
stretcher & a rise in bow-ward force on the gates/pins. That this is
not understood even here shows the dumbing effect of the serial
regurgitation of certain coaching mantras, such as "The blades go in,
the legs go down!". Today's rowers can,t be blamed for accepting these
mantras. After all, why test the truth of what everyone else takes as
gospel? But, hey, let's go break aother icon!
The first moments of the catch require not the slightest force on the
stretcher. The catch can even be taken (if you wish) while still "going
forward" (i.e. the stretcher still being pulled towards you). While it
may make life simpler to ignore this rather simple fact, to do that is
to ignore an important part of the rowing action - that it entails
inertial as well as directly mechanical interactions.
Why do we ignore important the inertial interactions which are
inescapable, essential parts of the rowing action. Rowing is about
intermittently adding energy to an elongated lump of bone, muscle &
gristle (with various moderately weighty extensions), all loosely placed
upon a lightweight boat, by use of a light & flexible sticks. The boat
is completely free to move under any small or large load & responds
immediately. The body is heavy & dense & responds only slowly to those
same loads, due to its inertia. And we try to move ourselves by what
can only be termed a bootstrapping process. It needs a most carefuly
judged sequence of muscle movements to accomplish this well, not a
damned great stamp on the unsupported stretcher of the unsupported boat.
You have no load on the pins until you have already loaded the handles &
blades &, thereby, bent the shafts. And the load is only proportional
to the bend in the shafts. But by good technique you can get lots of
load onto the handles & blades a relatively long time before you put the
slightest load on the stretcher. Why is this so little understood or
worked towards?
If you want to increase the check on your boat, then take the catch by
interacting your entire body mass first against the lightweight boat.
But do first ponder how long it takes that impulse, with which you hit
the stretcher, to pass right up through said mass of bone & gristle &
down along those hitherto unloaded arms. Now add in the time it then
takes to build anything like enough flex in the oar to support even part
of the load that you've already banged against the stretcher.
You cn only hit the stretcher because the legs can react forcefully
against the large inertia of your body mass. It's a great way to kick
the boat, but the least direct way there is to get any load up to your
hands. If you hit the stretcher, the body hardly feels it but the boat
is kicked back sharply - a big check. That's inertia in action - the
iescapable phenomenon you are encouraged to ignore. If you kick
_really_ hard, you may even send the boat back enough to drive the pin
into the hitherto unloaded oar. Wow!
Kicking the boat is a truly inept way to try to get an early load onto
the handles. It involves first moving everything you have available to
move before any load can reach the oars. It simply doesn't do the job.
There's a better, more logical way to load up the handles, & thus the
blades & the pins. But it takes just a little thought.
React against the same high inertia of the same lump of body, only this
time apply the initial load not through the legs, which couldn't be
further from the handles, but with the fingers (right there on the
handles), the arms (next in line) & the shoulders? That way you get
almost instant connection & move so much less of your body mass in the
process.
Immediately I hear all sorts of shouts about the imagined evils of
bent-arm catches & using the weakest muscles. Tosh! The arms need
hardly bend, especially since the action at the catch is as much (or
more, in a sculling boat)) a sideways one as one along the boat & the
movement required, if quickly taken (which the arms can do but an an
impulse begun at the other end of the body cannot), is rather small.
And you needn't worry about using the relatively weaker arm muscles
early: catches are _never_ particularly hard, because the initial load
starts from zero &, due to oar flexing, takes significant time to build
up. And as that build-up is occurring you can then make an
appropriately programmed application of load to the stretcher as a
balancing load is building everywhere else.
As in so much of rowing, we're asked to think in static postures &
steady forces. We are, quite wrongly, expected to believe that a load
applied at the feet gets immediately to the hands. No one explains (or
understands) the consequent delays - how having to first energise a
large mass is both inefficient & imposes time lags on transmission of
the forces you need to apply at the blades. Yet all this is
understandable with no more than school maths!
In short, making a catch by first moving your bum by stamping on the
stretcher is a pretty cack-handed way to load up the blades.
Cheers -
Carl
PS Magnificant work, Magnus!
C
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
Carl Douglas
Thanks Alexander for underlining the essential point that lift exists
only when _you_ apply a load, it exists in exact equlity to that load &
is absent in the absence of load.
There is no earthly point in that popular fiction that you need to wait
before you load the blades. In the few instants covering the process of
a clean entry the water is (feels) as hard as rock & the interaction
between it & the blade can take all the load you care to put on it. The
oarshaft, in comparison, is highly flexible & you only get load (& lift)
if you geet it bent, so hanging about around the catch for some mystical
process to occur is doubly pointless.
Cheers -
Carl
Teaplant
>
> There's no immediate connection between an initial rise of force on the
> stretcher & a rise in bow-ward force on the gates/pins. That this is
> not understood even here shows the dumbing effect of the serial
> regurgitation of certain coaching mantras, such as "The blades go in,
> the legs go down!". Today's rowers can,t be blamed for accepting these
> mantras. After all, why test the truth of what everyone else takes as
> gospel? But, hey, let's go break aother icon!
>
You know, Carl, given that we are discussing the definition of the
catch and speed thereof, it is simply not helpful to define it your
own way and then (as if by magic) declare someone else's previous
suggestion as the result of their misunderstanding of your new
definition. I clearly misunderstand your definition of the catch
since you hadn't suggested it when I wrote my reply. This is not
news, nor does it make me think you know more about it than I do,
however often you type it.
I think you're right that there is no immediate connection between
these two forces. I also suggested that it is the phasing of numerous
other factors that lead to the onset of these two forces - one can in
principal be moved in time independently of the other. I also never
ever suggested anyone 'stamp' on anything and especially not 'after'
the blades go in. In fact I believe I suggested that these things
should be part of a smooth process. I even included the word 'useful'
when describing the rise in the bow-ward force on the gate (sadly you
trimmed that word out when making your criticism). I quite agree that
nothing useful is done until the blade is loaded and this happens
after the entry to the water. A 'catch' is a process, not a moment in
time. A moment in time cannot be fast or slow. This is why I made
some attempt at defining a pair of moments in time, the lag between
which gives some measure of the efficiency of the' front end' of the
stroke.
I maintain that if a useful forward force appears on the pins a very
short time or even coincidentally with the rise in sternward force on
the stretcher, this is what could be termed a fast catch. I don't
think a useful forward force is likely to appear before the stretcher
force starts to build. A large time lag the other way could often be
the 'stamp' that Carl quite rightly hates. The changes one makes to an
oarsman's rowing mechanics to shorten this time interval can lead to a
faster boat (if the changes don't negatively affect other aspects of
the stroke cycle. The key culprits causing a large lag between these
moments would be upper torso/shoulder/head movement during or shortly
after the point of maximum compression, arm bending at the catch,
carrying the blade too near the water on the recovery. Combinations
of these faults seem to be the cause of what blade-watching coaches
call skying, rowing in, digging etc... but as I have argued before,
these are the result of inboard faults of body position, not bona fide
faults of their own. Do explain how my physics contradicts my
definition of the terms used, but please stop explaining how my
physics is wrong if you change the definition of the words I have
used.
teaplant.
Walter Martindale
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Well, Carl, I'm not really sure what you're on about. Unless a boat
is a piece of mush, the pins and the foot stretcher are physically
connected whether the rigger slides on the boat or the seat slides on
the boat, and any push on the footstretcher is (should be? has to be?)
transmitted through the pins to the oar sleeve. If the blade is in
the air when this happens, water gets missed and the boat gets pushed
aft of the movement of the CoM. If the blade is in contact with the
water, a propulsive force has to be transmitted. The bit of mush
between the foot stretcher and the oar handle needs to provide a good
link between the foot stretcher and the handle so that there's no
waste more of energy than already happens because of drag, slippage,
and other stuff. Are you going on about a matter of degree? That big
hunk of mass on the recovery doesn't move towards the stern - it moves
the stern towards the finish line under it through the connection with
the feet - you know that. A really hard crush of a foot stretcher
before the blades are buried will check the boat, but a really hard
crush on the foot stretcher (which probably takes about 0.12 or .13
second to go from 0 to something) when the blades are on the way into
the water with an entry that takes about 0.10s from contact to full
immersion. If the human in the middle isn't linked between the feet
and handle, you get a shot seat. If other things go off out of
sequence, you get lost effort and reduced peak forces.
What we need to do is get hold of the information, most likely
proprietary, that is being collected from the best rowers and scullers
in the world to see what they're doing. Are they perfect? probably
not. Are they "optimised?" most likely not. They are, however, the
best of today, and the people in the respective national rowing
federations are working hard towards improvement. I know of work
being done at Karapiro using force gates, accelerometers, and video,
but I don't know much detail: what detail I do know was gained while
under a confidentiality agreement, which I continue to respect.
without a fully documented kinetic analysis, using force measurements
at the pin, handle, foot stretcher, and inverse dynamics to all the
body segments, much of what we do here is more or less a (well thought
out) thought experiment.
Not sure what all the ranting is about...
Don't really care that much at the moment - it's late, and we're
having a training camp.
W
Carl Douglas
You say, "Sadly you trimmed that word out when making your criticism".
No I didn't! I trimmed not a word.
I found your first paragraph, above a bit hard to fathom, but nothing
that I'd said criticised you. Had I disagreed with you, I'd have said
so, & why. But I did not. I treated the wider generality & what I
wrote supported your the point there that:
"Smacking the blades into the water doesn't make the catch quicker since
the effort of doing this vertical movement seems to make the leg drive
start too early".
I explored the simple futility of trying to load up the hands by first
loading the feet. "Stamp" is my shorthand for what I so often hear
floating across the water: "Drive the legs _down_ at the catch!", with
added imprecations to sharpen the impact, is a familiar noise from
passing megaphones. "Stamp" is a fair interpretation of that
instruction, while many in the UK have heard commentaries with exactly
the comment I quoted in my previous posting.
Caroline does say:
"Lifting the shoulders rather than raising the arms, which results
in opening up the body too early rather than driving with the legs".
Donal does say:
"then the legs can drive through the lower body horizontally at any
time from when the blade is 1/4 1/2 or fully covered".
Both I respect as good friends, With both I may here disagree. They're
welcome to tell me I'm wrong & I doubt we'll fall out over it either
way. Usenet is such a fine place to discuss that which we feel strongly
about precisely because its conventions encourage directness over
politesse, content over style.
So what's this about catch definitions? That I'm interested in how you
& others see things does not mean I can't add my own views. It's a
2-way street, so let's compare views.
However, I do very much disagree with your new statement, above:
"I don't think a useful forward force is likely to appear before the
stretcher force starts to build".
What, I wonder, constitutes a "useful force", & by extension what would
be a useless force? Words are airy-fairy but numbers might prove
meaningful. Why must the stretcher be loading up before that "useful
force" can appear? Experiments in boats & on a Rowperfect seem to
contradict your statement, so I'd like to hear on what you base it. In
my book any load on the blades is "useful" & the catch which doesn't
start at the feet gives an early & ample load?
You may have missed my point - that at the catch to be driving the legs
is not in principle so different to pulling on the handles. Both rely
in that first instance on the high inertia of the rest of the body.
What divdes then is their effect on the load on the blades & the load on
the boat as a function of time. One method gets the load early on the
blades & a later onto the stretcher. The other puts an early load on
the stretcher but a later load onto the blades. Since it's the blades
that move the boat, I think the case perfectly clear.
Incidentally, I believe that the catch is about getting the blades
loaded well forward, & that a quick catch is one that achieves this in
the shortest possible time. Anything else, I would suggest, is either
decorative, inconsequential, irrelevant or may even be counterproductive.
Cheers-
Carl
Teaplant
> stretcher & a rise in bow-ward force on the gates/pins. That this is
> not understood even here shows the dumbing effect of the serial
> regurgitation of certain coaching mantras, such as "The blades go in,
> the legs go down!". Today's rowers can,t be blamed for accepting these
> mantras. After all, why test the truth of what everyone else takes as
> gospel? But, hey, let's go break aother icon!
I would say that by paraphrasing my response in these terms you were
directly criticising my understanding of the situation... perhaps you
still disagree. the key here remains a difference in vocabulary I
suspect (the catch is not a point in time or space in my definition
remember) so I feel this still represents an undue criticism of my
understanding of your definition.
>However, I do very much disagree with your new statement, above:
>"I don't think a useful forward force is likely to appear before the
>stretcher force starts to build".
>What, I wonder, constitutes a "useful force", & by extension what would
>be a useless force? Words are airy-fairy but numbers might prove
>meaningful. Why must the stretcher be loading up before that "useful
>force" can appear? Experiments in boats & on a Rowperfect seem to
>contradict your statement, so I'd like to hear on what you base it. In
>my book any load on the blades is "useful" & the catch which doesn't
>start at the feet gives an early & ample load?
'useful' forward force (on the *pin*, not the blades btw) is a force
which accelerates the centre of mass of the system. pulling on the
handles with ones feet hovering in mid-air not connected to the
stretcher will accelerate the boat forwards, accelerate the oarsman
sternwards and will not do much that is useful to the centre of mass
of the system - agreed. You most definitely need a sternwards force
on the stretcher to generate a large sternwards force on the water and
therefore large bow-wards force on the pin (albeit not always straight
away, hence this topic).
is this not sensible?
teaplant.
Tinus
> "I don't think a useful forward force is likely to appear before the
> stretcher force starts to build".
> What, I wonder, constitutes a "useful force", & by extension what would
> be a useless force? Words are airy-fairy but numbers might prove
> meaningful. Why must the stretcher be loading up before that "useful
> force" can appear? Experiments in boats & on a Rowperfect seem to
> contradict your statement, so I'd like to hear on what you base it. In
> my book any load on the blades is "useful" & the catch which doesn't
> start at the feet gives an early & ample load?
Using Newton's third law: If the body is to apply a (boward) force on
the handles than the handles must apply a force on body in the
opposite (sternward) direction. Is the body accelerating towards the
stern during this phase of the catch (which would happen without a
bowards force on the feet from the stretcher)? I've always had the
idea that the body decelerates relative to the shell during this phase
of the stroke. I can only imagine that it must be extremely hard to
get the blades loaded in the right direction while still moving
towards stern or even accelerating towards stern.
Is it also really bad to check the boat? I don't believe that it is
directly bad by itself but instead because it is indirectly bad by
being a marker of other bad technique.
I would say it is not directly bad using the same arguments which can
be used for the idea 'the way in which one moves during the recovery
has little effect on the average boat speed'. In the same way as the
profile for the speed of the rower relative to the shell during the
recovery has little effect on the average boat speed velocity, this is
also true for the drive.
On the other hand if one checks the boat it is a feedback for the
rower wasting time and space to catch. If one would catch such that
the boat is not checked (earlier) than one has to move a shorter
distance back and forward and one needs less time for the same stroke/
drive under water.
Carl Douglas
>> There's no immediate connection between an initial rise of force on the
>> stretcher & a rise in bow-ward force on the gates/pins. That this is
>> not understood even here shows the dumbing effect of the serial
>> regurgitation of certain coaching mantras, such as "The blades go in,
>> the legs go down!". Today's rowers can,t be blamed for accepting these
>> mantras. After all, why test the truth of what everyone else takes as
>> gospel? But, hey, let's go break aother icon!
>
> I would say that by paraphrasing my response in these terms you were
> directly criticising my understanding of the situation... perhaps you
> still disagree. the key here remains a difference in vocabulary I
> suspect (the catch is not a point in time or space in my definition
> remember) so I feel this still represents an undue criticism of my
> understanding of your definition.
Errr? Must we work by your rules, then?
I might have to criticise your understanding of how a technical
discussion has to be conducted. You're allowed to challenge what I say,
if you can fault it by rational means. I'm allowed to challenge what
you say, provided I advance rational arguments. That's how differences
of opinion over matters of science are resolved. I had, actually, not
criticised you. This denate is about playing the ball, not the man, &
it would do well for us to return to just that.
>
>> However, I do very much disagree with your new statement, above:
>> "I don't think a useful forward force is likely to appear before the
>> stretcher force starts to build".
>> What, I wonder, constitutes a "useful force", & by extension what would
>> be a useless force? Words are airy-fairy but numbers might prove
>> meaningful. Why must the stretcher be loading up before that "useful
>> force" can appear? Experiments in boats & on a Rowperfect seem to
>> contradict your statement, so I'd like to hear on what you base it. In
>> my book any load on the blades is "useful" & the catch which doesn't
>> start at the feet gives an early & ample load?
>
> 'useful' forward force (on the *pin*, not the blades btw) is a force
> which accelerates the centre of mass of the system.
Actually wrong. The boat/crew system's only propulsive interaction with
its wider surroundings is in the regular interaction between blades &
water. The pins are merely a part of the linkage used in our peculiar
sport to achieve that interaction.
By your analysis, the kayaker would be propelling himself with his hand,
not by contact with the water, the aircraft by some inner part of its
engine spool & the car by some widget in its back axle. Please let us
at least get the simple science right, & in the process show a bit of
respect for Newton in the process. You will go nowhere in a shell
unless you interact with the outside world. All the force which propels
your boat, other than on a windy day, is that exerted between the blades
& the water.
pulling on the
> handles with ones feet hovering in mid-air not connected to the
> stretcher will accelerate the boat forwards, accelerate the oarsman
> sternwards and will not do much that is useful to the centre of mass
> of the system - agreed.
No. It has much to do with the relative disparity of the masses
involved - as when the #37 bus at 30mph hits you on your bike. The bus
& its passengers hardly notice - just a bit of noise & almost no
deceleration - but you are hurled across the road by the impact. Newton
again.
And that is the point. Because you are so much the heavier part of the
system, what the other bits of that system do is heavily dependent on
your actions. Your mass, in its inertial reactions, is a very important
player in the actual dynamics of your rowing.
Work has to be done to change the velocity of a mass. If you drive the
stretcher before your blade is to any extent loaded, that results in the
mass of your heaviest limbs going (relatively) one way, together with
the boat - a big check - & your own body mas moving but little in the
other direction (again relative to the system as a whole). That's work
which accomplishes nothing except delay in the onset of load on the blade.
It takes a finite amount of rapid movement, for a finite time, to make a
catch - as we both agree, & that is the subject of this thread - because
the blade is loaded only in proportion to the bend you put into its
shaft. Slow won't do, as you know, since the oar moves & accelerates
even if you don't load it. So you must move your hands faster to get
enough bend into the shafts to be fully loaded in a respectable time.
You also have to do work, both to bend the shaft, & further in
proportion to the current load & speed of the handles. If you want the
hand acceleration done smartly & over a rather short distance, which it
can & should be (please try it in a boat, rather than seeking reasons
why it can't be done), then you will do best if you achieve this with
the closest, lightest & fastest linkages rather than investing energy in
an early acceleration of 80% of the system mass - that bit comes later.
Rowing is certainly about storing kinetic energy in your body mass. But
the time to try to accelerate that mass comes only as soon as you can
load the blades to react against that acceleration. With no load, it
can't be done. Get the load first onto the blades (& of course that
heavily involves the pins, as key parts of the linkage, just as you get
huge forces in any gearbox) and as that builds you quickly build teh
load onto the stretcher.
This is about matching your reactions in such a way that you can
minimise the lags in the system. Some bits act slowly but are very
strong while others can act fast but will do only the first bit.
You most definitely need a sternwards force
> on the stretcher to generate a large sternwards force on the water and
> therefore large bow-wards force on the pin (albeit not always straight
> away, hence this topic).
>
> is this not sensible?
>
> teaplant.
That's perfectly sensible, but it's not the whole picture. You cannot
generate those big forces until you have first done the initial loading
of the oars. You have the tools to do both jobs & should use them in
the optimal order. The big hammer is vital, but you have first to have
something you can usefully hit. Like fluid mechanics, the responses of
dynamic systems are rarely as intuitive as we'd expect, so no wonder
it's a struggle to learn to use them to best advantage. But saying "It
can't be so" does nothing to increase that understanding. You have to
grapple with it.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Carl Douglas
If, for this analysis, we ignore other quite important factors to
concentrate attention on boat check, then what matters most is the
duration x intensity of the check. A really bad check, if it is very
short, is not necessarily worse than a broader but lesser check. Just
ask how much faster (which certainly is uneconomical) you need to go to
plug the hole that check put into your velocity/time curve.
That said, check, with its inevitable sudden bouncing & or pitching
movements, does of itself waste energy - in the disruption of the
smoother flow past the boat, in perhaps adversely affecting its drag
coefficient by changing its trim for a moment, & by generating
additional surface waves. So we have many good reasons to row smoothly.
Cheers -
carl
freeheelfunhog
two of your premises
1) that distance from the hands to the feet means a delay in load
transmission
It seems to me that distance is really irrelevant (unless you somehow
believe that signal from brain to legs is that much different than
brain to hands/arms). You seem to assume that movement in your legs/
bum isn't virtually instantaneous with movement in your hands/arms,
but you don't share why this might be.
I suppose if you don't maintain some rigidity in your back and
shoulders, your bum might shoot out (toward bow) and your shoulders
might cave in, but this is a transmission problem that will manifest
whether you catch with your hands or with your legs. I'll concede that
my assumption in this analysis is that you must maintain sufficient
rigidity in your back and shoulders, i.e., that you are not Gumby. I
think if you agree with that assumption, it's hard to argue that the
transmission from stretcher to handle isn't virtually instantaneous.
Of course, this is a testable hypothesis: assuming straight arms and
blades in the water, does force applied to stretchers occur at the
same time as force on the handles? Anyone with that data?
2) that being "an elongated lump of bone, muscle & gristle" somehow
leads to inefficient application of forces
Are you suggesting that applying a load at the stretcher somehow gets
absorbed or delayed by stretching of tendons and ligaments and muscles
by the time it reaches the hands? I'd like to see proof of that. That
does sound like the "Gumby hypothesis".
You seem to suggest that "with no more than school maths" that I might
be able to understand. Show me. Please.
Finally, I think you've created a false dilemma between "stomp" on the
foot stretchers and catching with the hands. Is there something other
than stomping?
Torsten
Teaplant
> > 'useful' forward force (on the *pin*, not the blades btw) is a force
> > which accelerates the centre of mass of the system.
>
> Actually wrong. The boat/crew system's only propulsive interaction with
> its wider surroundings is in the regular interaction between blades &
> water. The pins are merely a part of the linkage used in our peculiar
> sport to achieve that interaction.
>
> By your analysis, the kayaker would be propelling himself with his hand,
> not by contact with the water, the aircraft by some inner part of its
> engine spool & the car by some widget in its back axle. Please let us
> at least get the simple science right, & in the process show a bit of
> respect for Newton in the process. You will go nowhere in a shell
> unless you interact with the outside world. All the force which propels
> your boat, other than on a windy day, is that exerted between the blades
> & the water.
vocab still, Carl. I am describing the forces on the BOAT, not on the
blade/water system (other than to connect my reasoning to yours). The
propulsive force on the boat appears on the pins and is provided by
the sleeve of the oar. The oarsman ONLY interacts with the boat and
the oars - things he is in contact with. The water/oar interface is
not part of this 'free body diagram'.
The forward force on the kayaker system DOES act on his hand and is
exerted by the reaction force from paddle.
The forward force on the car DOES act on the tyres, and is the
reaction force from the frictional force the tyres exert backwards on
the road.
If you read what I have written before with this in mind, are you
still sure I don't understand simple science? My definitions apply to
forces on the boat (majorly the force on the stretcher and the force
on the pins). How the blade loads up contributes to the magnitude and
time scale of these forces but I don't need to be explicit in my
definition - although I agree that its important in making the catches
quicker. As I wrote right at the start, the reason that catches are
'slow' can be due to "bladework" as well as "bodywork". I just don't
use these in my definition of catch speed. The skill of a coach would
be in knowing how an oarsman needs to change either or both of these
factors to be more effective (I would certainly like to be better at
that!). It may well be that you can define a catch solely by
considering the oar - this seems to be where you prefer to operate. I
suspect that it will even lead to the same conclusions. However, in my
definition I consider only forces on the boat... you can disagree with
the relevance of my definition of 'catch interval' but please stop
criticising my 'simple science' unless you restrict your explanations
to forces on the boat as I have done.
As for your discussion of the relative masses of boat and rower, I can
confirm that I can spend several hours a week coaching a rowing crew
in which the moving mass of the rowers is less than the inanimate mass
of the boat (+"static fittings") and it most definitely moves along
just fine. A 4+ with two rowing and two stabilising seems to row
along just fine. Great test bed for working on fast catches actually.
teaplant.
Tinus
discussion which is different from a 'quick catch'. Namely a 'late
catch'. This introduces a dualism which allows the catch to be seen as
both an interval and a point. Could this solve some confusion between
the people here?
Carl Douglas
Hi Torsten -
If you yank hard on the end of a short rope, your hand feels a hard &
immediate impact. That's because, although all ropes are elastic, even
Kevlar ropes, the shortness of that rope allowed for very little change
of length under load. So no significant amount of energy went into
stretching the rope & everything you put into yanking it was felt at
both ends of it.
If you yank the end of 50m of rope, it's a quite different story.
Nearly all your effort is spent in stretching the rope, even into
reacting against the rope's own mass to move it by whatever amount the
local degrees of stretch permit. And the impulse you tried to deliver
from your end is largely dissipated.
Those are the dynamic, the inertial, consequences of interposing a large
& lengthy mass between the point of initial impulse & that to which the
impulse is to be delivered.
If you don't mind about immediate delivery & care only about the
eventual peak load, then it won't matter how long, or how elastic, or
heavy, that rope is. But the longer, more stretchy & heavier the rope,
the longer it will take for the full load to build up at the other end.
You might think you are holding everything in your arms & body rigid,
but that rigidity is a relative concept. And you have to expend real
energy to achieve & maintain it. But if I were to hit the backs of your
knees while you were standing up, holding yourself rigid, it would still
take an appreciable time delay before that impact spread out (& all the
time softened) to reach & cause you to spill your full glass of beer.
But if i delivered that blow directly to the hand holding the glass, or
better still to the glass itself, then the result for the beer would be
both immediate & more dramatic. Maybe in either case the result for me
would be "unfortunate"!
Real systems have real dynamic lags & losses, even when everything is
very stiff & light. Human systems are never stiff & they are not light.
That's why it makes poor sense, when you want an immediate or swift
result, to try to move the hands by pushing the feet. It takes a lot
more energy anyway, & it really slows & diminishes the build up of load
at the hands. It also has a heap of other, undesirable, dynamic
consequences.
However, once you do have the load on & building, through the early but
transient use of the lighter & more local-to-the-handle parts of your
body, which can only work that way by inertial interactions with the
much larger mass of your body, you are able to make (& you must make) a
smooth & relatively lossless transition to using all the other, more
remote but stronger parts of the body to provide the main thrust.
Dynamics is not a concept which gets a lot of consideration, even when
coaches are (incorrectly) urging rowers to "accelerate the boat every
stroke". Yet the better understanding of these inertial interactions
can be key to the effective execution of so many athletic movements.
In rowing we welcome quite small improvements, so to blind ourselves to
the dynamically adverse consequences of deliberately assuming that
inertia & hysteresis are absent from rowing - when so very evidently
they are omnipresent - is very short-sighted. The potential gains of
fully understanding this are not necessarily small, but so often with us
humans it is a case that, if we don't understand something, we prefer to
pretend it doesn't exist rather than roll up our sleeves & get to grips
with it.
I hope that goes some way to answering your question? If my use of the
term "Stamp" seems imperfect, nevertheless I hope you may now see the
logic of the argument?
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Alexander Lindsay
"Carl Douglas" <ca...@carldouglas.co.uk> wrote in message
news:7j3et0F...@mid.individual.net...
May I be brave enough to add my pennyworth to Carl's comments? He has
covered you doubts about the "stretching of tendons and ligaments" bit.
There is also the matter of "inertial interactions"
You say "You seem to suggest that "with no more than school maths" that I
might
be able to understand. Show me. Please."
OK. I have some experience in teaching maths to those who find it
difficult. Let's do maths with no numbers;
Force = mass x acceleration. (Newton)
Relevant forces on the body are the stretcher force (forwards) and the
handle force (backwards)
When you accelerate the body at the catch (or indeed at any other time) then
Net forward force on body = stretcher force - handle force = body mass
x body acceleration.
So the bigger the body mass is, and the more you accelerate it, the larger
is the term "stretcher force - handle force".
While the body is accelerating, the handle force is unavoidably less than
the stretcher force, because part of the latter is needed to cause the body
acceleration.
Of course this sort of non-numerical argument is not much use until you put
in the numbers, which I will resist. But these effects are not trivial
because in a racing rowing boat the body mass is so relatively huge.
I think Carl is suggesting that by reducing the body acceleration at the
catch, you can briefly make the handle force large without making the
stretcher force large, which is what causes check on the boat. This is what
he is referring to when he mentions "inertial interactions".
Does that help?
Alexander Lindsay
freeheelfunhog
Climbers everywhere give thanks to the rope engineers for some rope
stretch. But the body is not a rope and it is not 50m long. We're not
made of polypropylene. I can't imagine leg bones and arm bones stretch
enough to be measurable for the purposes of this discussion. Does the
connective tissue stretch enough to cause a significant delay in
transmission you suggest? I can't see it.
Would a rowing start be an acceptable test of your hysteresis
hypothesis? Put gauges on foot stretchers and oar handle. Measure a
straight arm catch. Measure a hand/arm initiated catch. See what the
difference is with respect to time.
I agree with you that it takes real energy to hold your arms and body
rigid, but that is something you are going to have to do anyway.
Delaying this by initiating the catch with your hands/arms is just a
delay.
Furthermore, if we were to accept your Gumby argument, then the delay
in transmission you suggest would, itself, just be delayed by
initiating the catch with the hands.
And, if you initiate the catch with your hands/arms, you are going to
have to hold your arms rigid in a non-straight position (or do you
imagine that after the slight bend, they would return to straight
again). That, too, must require energy.
And, please don't be whacking my knees while I'm drinking a beer. I'm
usually relaxed when drinking a beer, not rigid in preparation for a
whack. However, if I had my arms straight and I were rigid, yes, the
beer might spill virtually instantaneously if you whacked my knees.
With regard to the logic of your argument, yes I see it. But, as with
syllogisms, if you don't accept the premises, you probably won't
accept the conclusion. I don't accept your hysteresis premise, and
therefore I don't accept your conclusion.
Torsten
Walter Martindale
As well, the mushy bone-muscle-ligament-tendon system is under some
degree of control of a nervous system which can prepare the bone-
muscle-ligament-tendons (and skin, and guts, and circulatory system,
and kidneys, and, and, and) for the forces involved, so that less
energy is wasted being Gumby. The human body, at movement speeds and
forces involved in rowing, is not really a passive mass. The control
of the human part of the movement system can be learned. That's what
coaches are (meant to be) for - helping people discover how to use
their bodies to move the bowball towards the finish line (and, of
course, the rest of the boat). I do recognize that a big part of this
discussion is our (coaches) imperfect understanding of what's really
going on within the human body and its interaction with the water and
boat.
I've challenged biomechanics researchers to find out what's really
going on, and then help us coaches figure out how to make it better
based on the results of the studies. Those who are studying the
information, and who are funded all or in part by national rowing
federations, tend to keep the information to themselves (well, within
the federation) and only release selected information to refereed
journals.
W
freeheelfunhog
Another thought.
Let's assume that you do, in fact, initiate force with the arms. Don't
you still have to brace with your shoulders and back to resist. These
body parts are all connected; can you really pull the handle expecting
to pry the boat without resisting with the shoulders and back? I can't
see how.
If you accept that leg and arm bones don't really stretch (or at least
not enough to be of relevance for this discussion), and you're already
engaging your arms, shoulders and back with an arm-initiated force,
what is left to stretch and how much delay in transmission is it
likely to cause?
Torsten
Carl Douglas
> Carl,
>
> Climbers everywhere give thanks to the rope engineers for some rope
> stretch. But the body is not a rope and it is not 50m long. We're not
> made of polypropylene. I can't imagine leg bones and arm bones stretch
> enough to be measurable for the purposes of this discussion. Does the
> connective tissue stretch enough to cause a significant delay in
> transmission you suggest? I can't see it.
Perhaps you are not looking? Only of your body consisted only of bone,
bolted end-to-end in a straight line, would it start to approach the
sort of stiffness that your way of thinking (without analysis or proof,
it seems) presupposes.
In reality your body is designed to cushion impacts & sharp loads so
that bits don't break or get overloaded. How many intervertebral joints
do you have, made of sacs of fluid & held in place by elastic tendons?
What about the cartilage in knee, hip & other joints?
But you deliberately (I suppose) ignore the most obvious fact of all -
your body at the catch is about as coiled up as it can be. And it is
braced by muscles which are long & elastic (or they'd snap under impact
loads). So it is in exactly the form of a spring, with the difference
that it is a heavy spring whose responses are not inert but are
determined by messages flashing back & forth along nerves & controlled
by the brain, helped by your long practice & conditioning. But it
remains a very elastic spring. So, however good it is at generating
loads in response to neural instructions, it remains a poor transmitter
of sudden changes in load - like any spring. And the softer (& heavier)
you make a spring, the more it will damp & slow down any rapid change in
applied load. You see this in all manner of mechanical systems. Its
why you want to minimise the ratio of sprung to unsprung mass in road
vehicles, so the combination of dampers & springs can isolate the car
from the road bumps & the wheels will stay in contact with the bumpy
road. It is behind the simple designs of oscillator circuits we used to
play with in school physics lessons. it is relevant to sound generation
& to acoustic suppression. You see it in the generation of waves on
water. Springs & damping are all around us, & we are each of us -
however much we prefer to disbelieve it, heavy, somewhat damped springs.
Meanwhile, you have completely ignored Alexander's very well explained
remarks about the consequences of imposing a large body mass between the
impulse generated & that which the handles feel. He really does know
his subject, & if he thought I had the wrong end of the stick I feel
sure he'd correct me.
>
> Would a rowing start be an acceptable test of your hysteresis
> hypothesis? Put gauges on foot stretchers and oar handle. Measure a
> straight arm catch. Measure a hand/arm initiated catch. See what the
> difference is with respect to time.
Well, I just gave you one - with a bit of a kick to it. You want me to
spit out numbers when in any case you're disinclined to get properly
involved with it - I guess that's your problem 'cos, like Alexander, I'm
disinclined to spend too much time on it. It is all too easy to keep
saying "It's not so" when what you have been taught by rowers who are
non-scientists, & what you may yourself have bought into to the extent
that you now teach it, is challenged. But that doesn't make it right
to do so.
So why don't you set up such an experiment, since you don't believe the
hoary old mathematician & the not much less hoary engineer?
>
> I agree with you that it takes real energy to hold your arms and body
> rigid, but that is something you are going to have to do anyway.
> Delaying this by initiating the catch with your hands/arms is just a
> delay.
A delay on what? In fact, you seem there to have accepted that one can
effectively initiate a catch with fingers, hands & arms, which implies
that you also accept that the body mass provides, inertially, an
effective platform for such a process.
>
> Furthermore, if we were to accept your Gumby argument, then the delay
> in transmission you suggest would, itself, just be delayed by
> initiating the catch with the hands.
No, it would not. Please explain why you think it would? Would an
explosion under your feet have the same effect on the oar handle as one
right next to it - if so, then an awful lot of explosives experts have
got a lot of re-learning to do.
>
> And, if you initiate the catch with your hands/arms, you are going to
> have to hold your arms rigid in a non-straight position (or do you
> imagine that after the slight bend, they would return to straight
> again). That, too, must require energy.
Of course your arms, as with everything else in your body, but less so
than the oar itself, are elastic. And nothing is achieved without
expending energy. Besides, your arms are never held dead straight in
real life, so why would they be dead straight in rowing - except that
folk like to take every action to some ill-considered ultimate. Small
arm flexes impose very low arm muscle loadings (simple trigonometry will
tell you that). And, no, you won't be holding your arms rigid, or bent.
as with every other part of your body (think about walking & running,
if you will) there is a constant variation in muscular lengths & loads,
not a straight-line extension & contraction process.
>
> And, please don't be whacking my knees while I'm drinking a beer. I'm
> usually relaxed when drinking a beer, not rigid in preparation for a
> whack. However, if I had my arms straight and I were rigid, yes, the
> beer might spill virtually instantaneously if you whacked my knees.
Sorry to spill your beer, my friend. As for your second statement -
you'd be quite wrong. There would be a real delay.
>
> With regard to the logic of your argument, yes I see it. But, as with
> syllogisms, if you don't accept the premises, you probably won't
> accept the conclusion. I don't accept your hysteresis premise, and
> therefore I don't accept your conclusion.
I would venture to suggest that the problem is yours. You don't want to
consider what you immediately wish to discount. You are welcome to take
that view - it's only a sport, not a matter of life & death. but it
would be more fun were you to be adventurous & see where that line of
enquiry might take you.
Cheers -
Carl
>
> Torsten
Carl Douglas
It might do for some of us.
For myself, I do not recognise "points" in a stroke since the action is
continuous, involving body mass movements which carry their own momentum
& should not stop, just vary with greater or lesser rapidity.
However, we each need mental models which ought themselves to be open to
continuous review. So if this kind of a Copenhagen interpretation will
do it for more of us, that's fine.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
freeheelfunhog
> freeheelfunhog wrote:
> > Carl,
>
> > Climbers everywhere give thanks to the rope engineers for some rope
> > stretch. But the body is not a rope and it is not 50m long. We're not
> > made of polypropylene. I can't imagine leg bones and arm bones stretch
> > enough to be measurable for the purposes of this discussion. Does the
> > connective tissue stretch enough to cause a significant delay in
> > transmission you suggest? I can't see it.
>
> Perhaps you are not looking? Only of your body consisted only of bone,
> bolted end-to-end in a straight line, would it start to approach the
> sort of stiffness that your way of thinking
Let me be clear: I don't think the body is made of steel, or just
bone. I'll accept that there is some stretchiness. I just don't have
reason to believe it is enough to make the claim that one should start
each stroke with an effort initiated by the arms.
> (without analysis or proof,
> it seems) presupposes.
Analysis, yes. Proof, no. Proof for your position and mine would come
from a test, which I have proposed to someone capable of actually
undertaking it.
> In reality your body is designed to cushion impacts & sharp loads so
> that bits don't break or get overloaded. How many intervertebral joints
> do you have, made of sacs of fluid & held in place by elastic tendons?
> What about the cartilage in knee, hip & other joints?
I think we can agree on that. However, I don't see how you can take an
arm-initiated stroke without somehow bracing with shoulders, back, and
to some extent the legs--your spring, if you will. In other words, all
those tendons, ligaments, joints, sacs, etc are likely in play already
to some degree.
Furthermore, given that we are talking a small fraction of a second
between an an arm-initiated force and a leg-driven force, I suspect
the body (muscles, joints, tendons) would be braced before you
actually initiated either.
>
> But you deliberately (I suppose) ignore the most obvious fact of all -
> your body at the catch is about as coiled up as it can be. And it is
> braced by muscles which are long & elastic (or they'd snap under impact
> loads). So it is in exactly the form of a spring, with the difference
> that it is a heavy spring whose responses are not inert but are
> determined by messages flashing back & forth along nerves & controlled
> by the brain, helped by your long practice & conditioning. But it
> remains a very elastic spring. So, however good it is at generating
> loads in response to neural instructions, it remains a poor transmitter
> of sudden changes in load - like any spring. And the softer (& heavier)
> you make a spring, the more it will damp & slow down any rapid change in
> applied load. You see this in all manner of mechanical systems. Its
> why you want to minimise the ratio of sprung to unsprung mass in road
> vehicles, so the combination of dampers & springs can isolate the car
> from the road bumps & the wheels will stay in contact with the bumpy
> road. It is behind the simple designs of oscillator circuits we used to
> play with in school physics lessons. it is relevant to sound generation
> & to acoustic suppression. You see it in the generation of waves on
> water. Springs & damping are all around us, & we are each of us -
> however much we prefer to disbelieve it, heavy, somewhat damped springs.
The thing about the arms is that they are connected to the shoulders,
which are connected to the back, which are connected to the legs. Hard
to separate them, especially when you're coiled, as you describe it.
Show me how the arms can initiate a stroke without involving
shoulders, back, etc.
>
> Meanwhile, you have completely ignored Alexander's very well explained
> remarks about the consequences of imposing a large body mass between the
> impulse generated & that which the handles feel. He really does know
> his subject, & if he thought I had the wrong end of the stick I feel
> sure he'd correct me.
Forgive me if I haven't had time to respond to each message.
>
> > Would a rowing start be an acceptable test of your hysteresis
> > hypothesis? Put gauges on foot stretchers and oar handle. Measure a
> > straight arm catch. Measure a hand/arm initiated catch. See what the
> > difference is with respect to time.
>
> Well, I just gave you one - with a bit of a kick to it.
Knocking my beer...
>You want me to
> spit out numbers
You wrote that it was all explainable with simple maths.
> when in any case you're disinclined to get properly
> involved with it
Hmm. I wonder what "properly" is. For several months, I tried to row
this way. I found it didn't contribute to my speed/efficiency and
actually made the stroke more complicated. A friend and reader of this
forum had suggested this and we both tried it out. Both of us found it
wanting.
> - I guess that's your problem 'cos,
Sure, I'll accept that.
> like Alexander, I'm
> disinclined to spend too much time on it.
No, you don't seem like you're spending much time on it. :-)
> It is all too easy to keep
> saying "It's not so"
Reasoned discourse is hardly that.
>when what you have been taught by rowers who are
> non-scientists,
How do you know who I have been taught by?
> & what you may yourself have bought into to the extent
> that you now teach it, is challenged.
How do you know that I teach rowing? Maybe you're writing
metaphorically, addressing the wider world?
> But that doesn't make it right
> to do so.
>
> So why don't you set up such an experiment,
Trying to see if someone can run the experiment. A bit difficult when
I don't possess strain gauges.
>since you don't believe the
> hoary old mathematician & the not much less hoary engineer?
> > I agree with you that it takes real energy to hold your arms and body
> > rigid, but that is something you are going to have to do anyway.
> > Delaying this by initiating the catch with your hands/arms is just a
> > delay.
>
> A delay on what? In fact, you seem there to have accepted that one can
> effectively initiate a catch with fingers, hands & arms, which implies
> that you also accept that the body mass provides, inertially, an
> effective platform for such a process.
I'm not sure how to parse this, but see below...
>
> > Furthermore, if we were to accept your Gumby argument, then the delay
> > in transmission you suggest would, itself, just be delayed by
> > initiating the catch with the hands.
>
> No, it would not. Please explain why you think it would?
At some point, you will still use the legs, back and arms, so if we
believe what you're saying, you will be engaging your large body of
mass with its spring and its stretchiness and its resulting
hysteresis. In other words, with an arm-initiated force you have just
delayed the hysteresis. You haven't eliminated it.
Yes, accepted.
> You don't want to
> consider what you immediately wish to discount.
Consider, yes. I'll accept when either compelling argument or proof is
available.
> You are welcome to take
> that view - it's only a sport, not a matter of life & death. but it
> would be more fun were you to be adventurous & see where that line of
> enquiry might take you.
I have fun experimenting with my rowing technique, equipment and
training and it's quite "an adventure" discussing this with you.
Torsten
>
> Cheers -
> Carl
>
>
>
> > Torsten
>
> --
> Carl Douglas Racing Shells -
> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
> Find: http://tinyurl.com/2tqujf
> Email: c...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Tinus
> > it seems) presupposes.
>
> Analysis, yes. Proof, no. Proof for your position and mine would come
> from a test, which I have proposed to someone capable of actually
> undertaking it.
>
> > So why don't you set up such an experiment,
>
> Trying to see if someone can run the experiment. A bit difficult when
> I don't possess strain gauges.
Aren't there enough experiments already which have measured angle
velocity of joints? If I look at one example which is easily
accessible on the internet, the rowing biomechanics newsletter 2005/03
(http://www.biorow.com/RBN_en_2005.htm), I can see no sign of an early
initiation of the catch by the arms. I also can't remember other of
such diagrams showing signs of early initiation by the arms. I've
measured this using video of myself and some other people as well. The
best thing of motion in the arms (during the blade entry and early
phase of the stroke) that I could find was an extension instead of an
extraction. The only thing which I can imagine that fixes this would
be that the initiation of the drive by changing/decreasing the elbow
angle would have to be done in such a short period that it's not
measurable by simple video analysis. Also, as the angle around the
shoulders increases during the entry of the blade and the early phase
of the drive, it could be possible for the angle of the elbow to
decrease without changing the horizontal distance between hands and
shoulders.
I also wonder what Carl claims: 'A way how people should row' or 'a
way how people could row'. I can't see the benefit of an initiation of
load by using the arms as being trivial. It is blurred by a large
complexity. If one uses the arms to move the handle without a
simultaneous increase of force on the stretcher the result will be
that the body is displaced towards the stern (or alternatively the
stern towards the rower). Does this mean that in order for the rower
to get the load on the blades he has to move towards stern? And why is
it beneficial not to push the stretcher early. One way or another the
rower has to push the stretcher slowing down the boat. I guess it
could be even better to do this as early as possible in order to
decrease drag caused by a high boat peak velocity. Also if the rower
is accelerating towards stern (relative to the boat) because of an
initiation of the load on the blade by a force on the handles (from
the arms) which is not supported by a force on the stretcher than the
needed acceleration for the rower to move back towards bow again needs
to be higher. I've learned a couple of days ago that high acceleration
might possibly not be a good thing because it has effect on drag.
Tinus
> The best thing of motion in the arms (during the blade entry and early
> phase of the stroke) that I could find was an extension instead of an
> extraction.
extension instead of contraction
Carl Douglas
> On Oct 7, 1:36 pm, Carl Douglas <c...@carldouglas.co.uk> wrote:
>> freeheelfunhog wrote:
>>> Carl,
>>> Climbers everywhere give thanks to the rope engineers for some rope
>>> stretch. But the body is not a rope and it is not 50m long. We're not
>>> made of polypropylene. I can't imagine leg bones and arm bones stretch
>>> enough to be measurable for the purposes of this discussion. Does the
>>> connective tissue stretch enough to cause a significant delay in
>>> transmission you suggest? I can't see it.
>> Perhaps you are not looking? Only of your body consisted only of bone,
>> bolted end-to-end in a straight line, would it start to approach the
>> sort of stiffness that your way of thinking
>
> Let me be clear: I don't think the body is made of steel, or just
> bone. I'll accept that there is some stretchiness. I just don't have
> reason to believe it is enough to make the claim that one should start
> each stroke with an effort initiated by the arms.
Belief has little relevance in science or engineering - it is an
abdication of the spirit of enquiry. The converse of your statement is
that you have no valid grounds beyond incredulity to oppose the proposition.
>
>> (without analysis or proof,
>> it seems) presupposes.
>
> Analysis, yes. Proof, no. Proof for your position and mine would come
> from a test, which I have proposed to someone capable of actually
> undertaking it.
Some things hardly need proof, the principle being so firmly established
- unless you can refute Alexander's points which really do demand your
measured consideration before you take this argument further. Maybe
Newton was wrong after all?
>
>> In reality your body is designed to cushion impacts & sharp loads so
>> that bits don't break or get overloaded. How many intervertebral joints
>> do you have, made of sacs of fluid & held in place by elastic tendons?
>> What about the cartilage in knee, hip & other joints?
>
> I think we can agree on that. However, I don't see how you can take an
> arm-initiated stroke without somehow bracing with shoulders, back, and
> to some extent the legs--your spring, if you will. In other words, all
> those tendons, ligaments, joints, sacs, etc are likely in play already
> to some degree.
Whatever load they may at that moment be bearing is exactly balanced by
other loads - or you would be unable to sustain your springlike posture.
A system balanced only by springs (stressed tendons), as you have in
effect just proposed, will move easily under an applied load unless
something simultaneously adjusts the balance of those springs. Of
course that can happen, but what a lot of effort you wish to go to to
make a poor argument stick.
And still you have chosen to duck entirely the inertial effects - the
energy absorbed in moving the larger mass in preference to the smaller,
which has perforce to spread out & thus delay the eventual application
of a full load by your preferred method.
>
> Furthermore, given that we are talking a small fraction of a second
> between an an arm-initiated force and a leg-driven force, I suspect
> the body (muscles, joints, tendons) would be braced before you
> actually initiated either.
In a stroke taking ~800 milliseconds, do you not think that fractions of
a second which, despite disputing their relevance, you decline quantify
beyond calling them small, are of some significance?
As you say, everything is interconnected. And you seem again to
recognise that masses do matter
>
>> Meanwhile, you have completely ignored Alexander's very well explained
>> remarks about the consequences of imposing a large body mass between the
>> impulse generated & that which the handles feel. He really does know
>> his subject, & if he thought I had the wrong end of the stick I feel
>> sure he'd correct me.
>
> Forgive me if I haven't had time to respond to each message.
That is one message that deserves a reply.
>
>>> Would a rowing start be an acceptable test of your hysteresis
>>> hypothesis? Put gauges on foot stretchers and oar handle. Measure a
>>> straight arm catch. Measure a hand/arm initiated catch. See what the
>>> difference is with respect to time.
>> Well, I just gave you one - with a bit of a kick to it.
>
> Knocking my beer...
You'll have to get over that spillage in time. It was only a virtual
beer, after all ;) No point in crying over spilt virtual beer, although
spilling the real stuff might be another matter....
>
>> You want me to
>> spit out numbers
>
> You wrote that it was all explainable with simple maths.
And so it is. Alexander gave you some of it but you declined to give it
your attention because he invited you to insert the numbers. I have to
suppose your response would similarly not justify the effort on my part.
>
>> when in any case you're disinclined to get properly
>> involved with it
>
> Hmm. I wonder what "properly" is. For several months, I tried to row
> this way. I found it didn't contribute to my speed/efficiency and
> actually made the stroke more complicated. A friend and reader of this
> forum had suggested this and we both tried it out. Both of us found it
> wanting.
Perhaps we need to know what you tried. Some detail might clarify. A
bland statement does not.
>
>> - I guess that's your problem 'cos,
>
> Sure, I'll accept that.
>
>> like Alexander, I'm
>> disinclined to spend too much time on it.
>
> No, you don't seem like you're spending much time on it. :-)
Are you suggesting, just because I have the good manners to consider
your comments with care & to respond to them in detail, & in the spirit
with which I trust they were intended, that I have time hanging on my
hands & nothing better to do?? Does your Nordic name in fact signify a
troll? I think we should be told, so no more time need be wasted on
feeding you.
Oddly enough, my thinking is done & organised in a spare part of my
brain while I am working. Putting it into text is a matter of a few
minutes - short relaxation over a coffee or, as now, while scratching my
head over jobs for the morrow before heading to bed.
>
>> It is all too easy to keep
>> saying "It's not so"
>
> Reasoned discourse is hardly that.
>
>> when what you have been taught by rowers who are
>> non-scientists,
>
> How do you know who I have been taught by?
Not many coaches are scientists or engineers. Scientists & engineers
understand Newton's laws of motion. But I was not pinning an accusation
on you - this thread has (well, may have) more than 2 readers.
>
>> & what you may yourself have bought into to the extent
>> that you now teach it, is challenged.
>
> How do you know that I teach rowing? Maybe you're writing
> metaphorically, addressing the wider world?
Very possibly.
>
>> But that doesn't make it right
>> to do so.
>
>> So why don't you set up such an experiment,
>
> Trying to see if someone can run the experiment. A bit difficult when
> I don't possess strain gauges.
Good. Actually you can do a lot without strain gauges. Consider the
possibility of flexible materials of high stiffness (so very small
deflections under load) with those small deflections being measured by
dial gauges the reading on which are recorded on video - wholly
mechanical, cheap, reliable, instant responses (within this context) &
low hysteresis. I've done work like this & it works wonderfully well.
You don't have to use electronics for a job like this if you think
carefully & laterally - but that's not to knock the sort of high quality
work that can be done if you know your stuff & will spend the funds,
time & trouble to set up good circuitry
>
>> since you don't believe the
>> hoary old mathematician & the not much less hoary engineer?
>
>
>>> I agree with you that it takes real energy to hold your arms and body
>>> rigid, but that is something you are going to have to do anyway.
>>> Delaying this by initiating the catch with your hands/arms is just a
>>> delay.
>> A delay on what? In fact, you seem there to have accepted that one can
>> effectively initiate a catch with fingers, hands & arms, which implies
>> that you also accept that the body mass provides, inertially, an
>> effective platform for such a process.
>
> I'm not sure how to parse this, but see below...
>
>>> Furthermore, if we were to accept your Gumby argument, then the delay
>>> in transmission you suggest would, itself, just be delayed by
>>> initiating the catch with the hands.
>> No, it would not. Please explain why you think it would?
>
> At some point, you will still use the legs, back and arms, so if we
> believe what you're saying, you will be engaging your large body of
> mass with its spring and its stretchiness and its resulting
> hysteresis. In other words, with an arm-initiated force you have just
> delayed the hysteresis. You haven't eliminated it.
You should not believe, but you should understand what I am saying since
it tells no lies. Anyway, if the arms are able to act before the
impulse can arrive from the feet after having to accelerate & load the
entire body mass (which I think is what you mean by "delayed the
hysteresis") then you are some way to accepting my thesis.
Your hands can only move, under leg drive, at whatever current speed &
acceleration that your legs are able to generate in the mass of your
body. Your boat will initially move much more but in the other
direction, which is hardly what you want. So what do you still have
against using small arm & finger movements to initiate blade loading?
followed through the body from shoulders down to the feet or, better
still, meeting at some point down the body with the slightly later
impulse initiated by the leg drive? Argue as you might, there is no
question that the foot impulse must react first on the stretcher & only
later & much slower on the hands. Which is why you strive so hard to
defend your case by introducing yet more beliefs that this or that will
be insignificant while that & the other will kindly intervene to defeat
Newton.
We don't spoon-feed science! Besides, you now have enough to put real
answers together, provided you pay due attention to Alexander's quite
straightforward explanations. Technical discussions require each party
to engage fully with the other, to wrestle with the difficult bits, to
accept the intellectual challenge. Otherwise it is all too easy to just
say "No".
>
>> You are welcome to take
>> that view - it's only a sport, not a matter of life & death. but it
>> would be more fun were you to be adventurous & see where that line of
>> enquiry might take you.
>
> I have fun experimenting with my rowing technique, equipment and
> training and it's quite "an adventure" discussing this with you.
>
> Torsten
Which is a fair closing remark. It wasn't meant to be easy, but it
should be interesting.
Cheers -
Carl
>> --
>> Carl Douglas Racing Shells -
>> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
>> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
>> Find: http://tinyurl.com/2tqujf
>> Email: c...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
>> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
>
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
freeheelfunhog
wrote:
> OK. I have some experience in teaching maths to those who find it
> difficult. Let's do maths with no numbers;
>
> Force = mass x acceleration. (Newton)
> Relevant forces on the body are the stretcher force (forwards) and the
> handle force (backwards)
So far I'm with you...
> When you accelerate the body at the catch (or indeed at any other time) then
> Net forward force on body = stretcher force - handle force = body mass
> x body acceleration.
> So the bigger the body mass is, and the more you accelerate it, the larger
> is the term "stretcher force - handle force".
> While the body is accelerating, the handle force is unavoidably less than
> the stretcher force, because part of the latter is needed to cause the body
> acceleration.
Interestingly, Kleshnev (vol 9, #99) found that "in a boat, stretcher
force is about 40% higher than the handle force but that on an ergo
the forces are nearly equal." I'm not sure what to make of this
difference (between boat and ergo), but he ascribes it to things like
seat height.
Judging by the actual graphs, you would see that his comment appears
to refer to peak forces. In any event, his empirical findings tend to
corroborate your statements. (Curiously, as depicted there is a
negative force on the stretcher: how does that happen?)
(Reference: www.biorow.com/RBN_en_2009.../2009RowBiomNews06.pdf)
A more elaborate discussion appears here: w4.ub.uni-konstanz.de/cpa/
article/view/486/426
>
> Of course this sort of non-numerical argument is not much use until you put
> in the numbers, which I will resist. But these effects are not trivial
> because in a racing rowing boat the body mass is so relatively huge.
Agreed.
>
> I think Carl is suggesting that by reducing the body acceleration at the
> catch, you can briefly make the handle force large without making the
> stretcher force large, which is what causes check on the boat.
This is one of several places where I am stumped. Take a race start:
by this last reasoning, if I initiate force with the legs, I should
experience check: I should travel sternwards. Empirically, I do not. I
tested this a few times with my stern a couple inches off the dock.
Photos and video seem to corroborate that I am not rowing with an arm-
initiated force. Maybe an arm-initiated force is too subtle to see and
I've been doing it all along.
> This is what
> he is referring to when he mentions "inertial interactions".
Got it.
>
> Does that help?
Thank you for that.
Torsten
>
> Alexander Lindsay
Steve S
settled in the paper that will download if one types this Web address
into a browser (Acrobat browser plug-in needed):
http://espace.library.uq.edu.au/eserv/UQ:8277/bsplash_macrossan.pdf
The title is "Back-splash in rowing-shell propulsion". While the
primary objective seems to be to bury once-and-for-all the notion that
back splash is desirable, the authors characterize "quick catch" with
precision, and provide some guidance about the physiology and rigging
involved.
There is a fair amount of math, but it is not essential to
understanding the key points, which are made clear in the prose.
Carl Douglas
Thanks for that latter reference, Torsten. (Is something missing from
the preceding one?)
The negative foot force shown is inescapable if the rower is to bring
his/her CoM & front-stops closer together, or even to hold in the
front-stops position (if so inclined), due to the net deceleration of
the system under fluid drag. As we have often discussed, the rower has
to pull on the legs to reach frontstops & teh suggestions often heard
that pressure on the stretcher helps compression are nonsense. The foot
force should, in a simplistic, ideal world, remain negative until the
the blades are generating an axial force component exactly matching the
fluid drag. Leg drive & other actions introduce additional forces
tending to alter the actual balance of forces.
freeheelfunhog
>
> Thanks for that latter reference, Torsten. (Is something missing from
> the preceding one?)
http://www.biorow.com/RBN_en_2009_files/2009RowBiomNews06.pdf
t.
Charles Carroll
> continuous
Carl,
Have you found a way, then, to conceive of motion without matter?
Let us return to those thrilling days of yesteryear and renew our
acquaintance with one of the great works of Philosophiae Naturalis, the
renowned "Principia Mathematica."
The first definition: "The quantity of matter is the measure of the same,
arising from its density and bulk conjointly."
The second definition: "The quantity of motion is the measure of the same,
arising from the velocity and quantity of mater conjointly."
And the point?
It seems to me that matter is defined before motion because it is impossible
to think of motion without matter.
And when matter is put into motion can we not always select units of motion
to study?
And isn't a unit, however great or small, always bounded by a beginning and
end, that is, bounded two points?
So, assuming the action of the rowing stroke is continuous, as you say, how
can you understand it without recognizing points?
Do you remember the beginning of Tolstoy's famous Chapter 1 in Book 11 of
"War and Peace?"
"FOR THE HUMAN MIND the absolute continuity of motion is inconceivable. The
laws of motion of any kind only become comprehensible to man when he
examines units of this motion, arbitrarily selected [...]"
Cordially,
Charles
Carl Douglas
You are a cunning fellow, Charles, but I still don't buy your
provocative case ;)
Tolstoy was a great writer but not, I think, a great scientist. When an
artist writes on science in that way, why I am slightly reminded of
Bishop "Soapy" Wilberforce pronouncing on heredity? What he writes may
sound good, but how does it add up? He feels it to be true, so it must
be true? That just ain't science.
As for the order in which units are defined, may I remind you that light
has no mass, but it has velocity, energy & a characteristic frequency,
all of which we can measure after the fact.
Now to "points" in a stroke. A musical note from an instrument
certainly has a starting point, just as a race has a start, & both must
have an end. But during the note & the race there is, unless we slip
up, no interruption or cessation, only smooth variations in
amplitude/power, frequency/stroke rate, timbre/stroke characteristics
(as in speed of catch or force profile). The catch, after the very
first stroke (& maybe not even then), comes with no instantaneous change
in any force or load - everything must perforce be progressive. So
where are these points of which you speak, except as frozen frames from
a truly continuous sequence.
That doesn't mean the sequence does not involve quite rapid changes.
And I do think we can very usefully analyse whereabouts & how those
changes occur. But does that mean the catch has a beginning or end? I
think not.
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: http://tinyurl.com/2tqujf
Tinus
> But during the note & the race there is, unless we slip
> up, no interruption or cessation, only smooth variations in
> amplitude/power, frequency/stroke rate, timbre/stroke characteristics
> (as in speed of catch or force profile).
Sounds a bit like Zeno's paradox upside down. I see a point in
ignoring points. Points decrease the appreciation of the fluid
movement of the rowing stroke and make coaches focus too much on
synchronizing position and less on speed and acceleration. But, do
you, besides ignoring them, also really believe points do not exist in
the rowing stroke?
E.g. besides points at which abrupt changes occur we could also have
points like maxima or points at which direction of velocity or
acceleration change. Or maybe something like the point at which the
brains start to send signals to the muscles to start a certain
movement instead of a kinematic description.
As the catch is a process of limited time isn't it a mathematical
necessity for boundaries (either open or closed) to exist? I would
agree with an argument like those boundaries being arguable but an
argument like the catch encompassing the entire stroke (and thus
having no beginning or end) seems absurd to me.
Carl Douglas
Nice points, Tinus, & well put.
Yes, there's too much emphasis by some on supposedly static points or,
I'd prefer to say, postures. You're never static at those parts of the
cycle. Everything in the stroke is the result of dynamic processes
which we can no more hold than we can remain suspended in the middle of
a jump. We go through those apparent postures only because we're
moving; we should never try to hold them & they only make sense when
they happen through motion.
No, I don't really think there are "points" in the stroke. I do think
there are more-evident transitions, which I think you're describing,
when one phase of motion blends more swiftly into another. The catch is
such a transition, but is no more a point than is a tighter bend in one
part of an endless belt. In the case of the belt the radius may quite
quickly change but it does not change abruptly because the structure of
the belt ensures a smooth transition. Similarly, the forces that you &
nature are applying during the stroke, & the constantly varying momentum
of all the different body parts & bits of equipment that result from the
dynamic nature of the rowing action, ensure that every phase of the
stroke cycle must blend into the next. A blend cannot be a point, nor
even a start.
The nearest I can see to the start of a catch is the signal from the
brain that starts the changes by which the recovery will fuse into the
catch. I still don't see that as a point in the cycle, but others may.
If they do, it still is not a visible point & will have been passed by
some milliseconds or more before its consequences are detectable.
That last may be getting to be a bit too philosophical? However, I'd
suggest that the last thing we need to do in rowing is to try to go from
one static posture to another. How can a catch be taken well if first
we stop & then we "take a catch" from that supposedly strong position?
That's not what you're saying but it is how some do read the catch.
Tinus
> That last may be getting to be a bit too philosophical? However, I'd
> suggest that the last thing we need to do in rowing is to try to go from
> one static posture to another. How can a catch be taken well if first
> we stop & then we "take a catch" from that supposedly strong position?
> That's not what you're saying but it is how some do read the catch.
I agree that we should not focus on the stroke being a sequence of
postures. But, I personally dislike the statement of the body being
continuously in motion (which I associate with speed). Strictly it can
be true if you look at the motion of the body segments and directions
separately. Some part of the body is always in motion. However, for
every stroke there are two "points" at which the whole body mass has
zero horizontal speed relative to the shell.
I like this notion as it introduces some "paradox" in the rowing
motion which can be solved if we just accept it and try to work with
it as good as possible. Doing this allows to increase efficiency. The
"paradox" is the idea 'the drive ends if the handle stops moving but
in order to end the drive the handle needs to move'. Should we really
teach rowers to have some speed relative to the shell (continuous
motion) at these points during which the body has actually virtually
no speed?
Differently stated: In order to make the drive we need to move the
handle towards the bow but we also need to stop doing this at some
"point" and move the handle back again in order to make the next
stroke. It is stated that phases of the stroke smoothly blend but the
processes of moving backwards and forwards are by definition not
blended. I believe that the idea of the body in continuous motion
might help in order to get a smooth motion without pauses. But, if we
want to balance the contrary processes of both a backwards movement
and a forward movement than the notion that there is a point, at which
the body has no speed relative to the shell, might help better. This
does not mean that there is a pause. It just changes the focus of
continuous speed to continuous acceleration. The latter being a better
description of the phases around catch and the release. Or... am I
wrong in putting to much stress on speed when one speaks of continuous
motion. Could I also see continuous motion as either one of non zero
speed and non zero acceleration?
Maybe it is just semantics but I feel that changing the focus from
continuous speed/motion to continuous acceleration is helpful. It's a
bit the same like getting rid of the focus on a succession of
positions is being helpful. Some people who have been told to be in
continuous motion don't stop (which is different from pausing) their
speed with all kinds of consequences.
Another thing which rowers might try is to move their hands in a
circle in order to stay in 'continuous motion'. The catch might better
be described as a back and forward motion instead of a smooth circle
motion. The former involves an irregularity in the motion.
> Similarly, the forces that you &
> nature are applying during the stroke, & the constantly varying momentum
> of all the different body parts & bits of equipment that result from the
> dynamic nature of the rowing action, ensure that every phase of the
> stroke cycle must blend into the next.
Similarly the forces describing any motion can be regarded as dynamic
in nature and constantly varying. One could describe the ticking
motion of the hands of a clock as ultimately being a continuous
(varying!) motion but would it make sense?
Carl Douglas
So the rebound of a ball from an elastic surface (e.g. a trampoline, but
also the ground) is not a continuous process, nor its passage through
its apogee?
I think that at no point in the stroke cycle is there both zero velocity
& zero acceleration (either WRT boat or to a constant velocity reference
frame) affecting all or even most parts of the body. The changes in
direction are rarely exact reversals, & any exact reversal is normally a
continuous process with changes of acceleration smoothly (however
swiftly) preceding smooth changes in velocity - even when there is
nearly stasis.
I believe that the idea of the body in continuous motion
> might help in order to get a smooth motion without pauses. But, if we
> want to balance the contrary processes of both a backwards movement
> and a forward movement than the notion that there is a point, at which
> the body has no speed relative to the shell, might help better. This
> does not mean that there is a pause. It just changes the focus of
> continuous speed to continuous acceleration. The latter being a better
> description of the phases around catch and the release. Or... am I
> wrong in putting to much stress on speed when one speaks of continuous
> motion. Could I also see continuous motion as either one of non zero
> speed and non zero acceleration?
I think I see exactly your dilemma, & to some extent I share it. See later.
>
> Maybe it is just semantics but I feel that changing the focus from
> continuous speed/motion to continuous acceleration is helpful. It's a
> bit the same like getting rid of the focus on a succession of
> positions is being helpful. Some people who have been told to be in
> continuous motion don't stop (which is different from pausing) their
> speed with all kinds of consequences.
>
> Another thing which rowers might try is to move their hands in a
> circle in order to stay in 'continuous motion'. The catch might better
> be described as a back and forward motion instead of a smooth circle
> motion. The former involves an irregularity in the motion.
I'm sure that makes a useful model.
In coaching it is of utmost importance to provide a model comprehensible
to the student. Rowers are not examined on their understanding of the
physics of the stroke but on how well they move a boat & fit into their
crews, so they neither need nor want the full theoretical treatment.
But here we have been discussing not what we might need to tell a rower,
but what we think is actually happening. If we get our minds right
inside the machine we have the chance to learn how it really does work.
If thus enlightened then, if a coach is also good at translating our
requirements into language that makes sense to this or that rower (&
each needs it spelt out slightly differently), they can tell their crew
an appropriate set or subsets of plausible half-truths which tick all
the necessary mental boxes for each rower rower.
That is where the mantras start. What each rower is told in order to
get them doing roughly what their coach desired when they were training
for competition is, years later, spouted back, parrot-fashion & without
the same insight as when first delivered, to another bunch of younger
rowers. And that time it makes less sense all round. Thoughtful rowers
can't go to that coach for a deeper exploration of what the real
objective of the instruction was, because the coach never knew it in teh
first place.
In short, you can be a better teacher if you really understand your
subject rather than being able only to repeat what it says in the book
>
>> Similarly, the forces that you &
>> nature are applying during the stroke, & the constantly varying momentum
>> of all the different body parts & bits of equipment that result from the
>> dynamic nature of the rowing action, ensure that every phase of the
>> stroke cycle must blend into the next.
>
> Similarly the forces describing any motion can be regarded as dynamic
> in nature and constantly varying. One could describe the ticking
> motion of the hands of a clock as ultimately being a continuous
> (varying!) motion but would it make sense?
The ticking hands do, I think, represent a truly interrupted motion
since, until the next hand movement, there's a period of zero motion _&_
zero acceleration.
Tinus
> So the rebound of a ball from an elastic surface (e.g. a trampoline, but
> also the ground) is not a continuous process, nor its passage through
> its apogee?
You could look at different parameters (speed, position, kinetic
energy, acceleration) and they are not all continuous or smooth
functions. A pendulum gives a good example as it is perfectly
described by a sinusoidal function. The position, a sinusoid, is a
nice smooth function. The directional speed, the derivative and also
sinusoid, is smooth as well. However, the speed defined without
direction (absolute value of speed), which looks much like a cycloid,
is not a smooth function. http://www.wolframalpha.com/input/?i=abs(sin(x))
.
The position of a rebound ball is also practically described by the
absolute value of a sinusoid and not smooth. The speed of a rebounding
ball is, when looking without much detail, much like the derivative of
the position described like the derivative of the absolute value of a
sinusoid. Thus, it is not only not smooth but also discontinuous. When
looking with more detail one could describe the motion of the ball as
an harmonic motion and the change of speed is not really
instantaneous. Both views can be practical depending on the situation
they do not really oppose each other but just describe the same
situation differently.
I believe that in rowing there must be occasions at which one could
view the motion as non smooth or discontinuous. At sometimes it could
be useful to describe the catch as a bouncing movement at other times
the catch is best describe as a motion which is continuously changing.
The term 'changing motion' instead 'motion' can be important (for the
understanding of the coach). The speed of a bouncing ball, when it's
position is viewed as the absolute value of a spheroid may not be
continuous but it's acceleration is.
It is for instance in this earlier mentioned description of the motion
of the handle (Kleshnev and Nolte) where the position appears to be
non smooth and the speed seems practically discontinuous at the points
of catch and finish (sharp turns). http://www.biorow.com/RBN_en_2008_files/2008RowBiomNews03.pdf
> > Some people who have been told to be in
> > continuous motion don't stop (which is different from pausing) their
> > speed with all kinds of consequences.
>
> That is where the mantras start. What each rower is told in order to
> get them doing roughly what their coach desired when they were training
> for competition is, years later, spouted back, parrot-fashion & without
> the same insight as when first delivered,
> In short, you can be a better teacher if you really understand your
> subject rather than being able only to repeat what it says in the book
I agree that it is this where the notion of continuous motion/speed
could possibly go wrong. One can tell the rower to keep on moving.
This often works well in having the rowers keep a contentiously
(changing) motion. But if the rower exaggerates and is still having
speed while there needs to be a turning "point" than there needs to be
a correction given. I personally feel that a lot "errors" are made by
trying to move (too fast) at the end of the stroke or the end of the
recovery while the focus should be trying change the movement (which
involves stopping the movement) and turn around. Around the endpoints
of the stroke the impulse of the rower's motion relative to the boat
is low.
> The ticking hands do, I think, represent a truly interrupted motion
> since, until the next hand movement, there's a period of zero motion _&_
> zero acceleration.
Looking closer one could say that the ticking hands are not completely
in zero motion and acceleration as they experience dynamic forces like
vibration. Or one could say that there is no interruption as there is
some small force left such that the motion is still smooth and the
phases of standing still and moving to the next minute or second are
blended. Also, one could say that while the handles are standing still
there is still some movement in the clockwork mechanism. Or one could
say that while the hands are standing still and experience no
acceleration the derivative of the acceleration (which just happens to
have no important physical relevance or fame) is still non zero
(ultimately some derivative needs to be non zero other wise the hands
would be standing still for ever).