Kernschlag or Schubschlag? Slam or Squeeze?
Charles Carroll
Just some idle thoughts on a lazy Sunday afternoon:
Consider a 150 lbs burlap sack of coffee beans.
Wouldn't dropping the whole, unopened sack on to the end of the lever
be analogous to "a solid stroke with a hard beginning," i.e.
Kernschlag? It would be equivalent to slamming the sack of coffee
beans down hard on to the lever.
On the other hand pouring the beans from the sack on to the end of the
lever would be analogous to a "thrust-stroke," i.e. Schubschlag? The
150 lbs of beans, instead of slamming, would squeeze the lever
downwards. You could use a quick squeeze, which would be a fast pour
accelerated rationally. Or you could use a slow squeeze, which would
be equivalent to rowing at low pressure.
But either way we are left with two methods for beginning the initial
phases of drive. And which is the better choice? A solid stroke with a
hard beginning or a thrust-stroke? Kernschlag or Schubschlag? Slam or
squeeze?
According Valery Kleshnev's research, "It is very important to
increase force up to 70% of its maximal value as quickly as possible."
(RBN December 2004) The principle benefit of a solid stroke with a
hard be-ginning is that it increases force very quickly against the
oar handles.
The problem, however, with Kernschlag is that your timing has to be
perfect. If you delay until you are backwatering, you will lose any
benefit that might have come from early force application. But if you
slam against the stretcher too early, that is, before you have fully
established the essential tensile connection between the mass of water
behind the backs of the blades and the backs of the blades themselves,
then much of your effort will go into making the blade tear and slip
through the water as opposed to moving the boat.
On the other hand, a thrust-stroke, or squeezing against the
stretcher, gives you more control as you apply force. Schubschlag
allows you to preserve the tension, or negative pressure, acting
across the convex back of the blade. This ensures that most your
effort will be used for moving the boat as opposed to stirring up
water. As Tom Neumeir says, "Feel as if you are caressing the water,
instead of chopping it."
Can science enlighten us on this?
Cordially,
Charles
Charles Carroll
I am not into words. Every rowing book around the world is the same
but we all row differently. Me explaining it to me you will be your
own interpretation of my words but that may not be my interpretation.
There are many ways to do it - whether you are a pusher or a puller do
it to high level and you have a chance of being competitive
From an Interview with Tim McLaren, US Rowing Head Coach Part:1. See
http://www.rowperfect.co.uk/2011/05/23/interview-tim-mclaren-head-coach-rowing-part1/
Zbigniew A.
> are many ways to do it - whether you are a pusher or a puller do it to
> high level and you have a chance of being competitive
Here i think "pusher or puller" is not about "slam or squeeze". I
believe it is about whether we focus on pulling the oars, or else, on
pushing the footstretcher in a first place.
Slam or squeeze?
certainly not slam, me thinking. Water is soft, if you just slam it with
blades it will give way. Squeeze... I'm not an expert of English but i
don't think it's a good word either...
Have any of you try to push a car that get stuck in a snow? (Charles you
are excused, you live in California.) When you find your feet standing
on ice covered with a slippery snow, you need to apply the force in a
special way... I think that's the way!
--
Yours Virtually, Zibi
Charles Carroll
Zibi,
Have I misunderstood Carl?
If you have an "essential tensile connection between the mass of water
behind the backs of the blades and the backs of the blades
themselves," won't it support any load you can put on the blade?
Carl writes: "Water is very hard (high modulus of elasticity) so it
doesn't give; when the blade tries to move away from the water behind
it, at first that water behind it develops internal tension, exactly
enough across the area of the blade to balance the applied load, & its
pressure falls."
Cordially,
Charles
Carl Douglas
Actually, Zibi, water is extremely hard & does not give way. The faster
you engage with it the less it moves, because it is dense & to move it
has to flow, but to flow it has to accelerated from nothing, and the
faster you engage with it the more it has to accelerate.
If, however, you engage with water slowly (= more gently) there is more
time for it to flow out of the way & much less force is required to
achieve this slower movement.
A blade already immersed is thus extremely resistant to sudden loads.
This is well understood by users of under water explosives, where
water's density & unyielding nature enhances the percussive effect of a
blast against a ship's hull of other structure.
What you may be thinking is that to drive a blade by loading it when
still above the water is rather unproductive, & I'd agree. But that's
because the blade scrapes into the water in your effort to load it
before it is adequately immersed. Timing is everything.
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: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
Walter Martindale
wrote:
I haven't unpacked it but somewhere there's an old FISA coach article
from around 1990, I think, that discusses three force-time profile
'families'. The early hammer the foot stretcher type was supposed to
generate higher lactates, a late peak force profile didn't get enough
time under a higher peak force to provide enough impulse, and
something in between, with the peak force in the first half of the
stroke but not slamming the stretcher was best - I can't remember if
it was shown to be best or hypothesized to be best...
Cheers,
W
Charles Carroll
So the faster you engage with water, the less it moves.
Am I right then to infer from this that the faster you engage with
water, the less opportunity you will give the blade to tear and slip
through the water?
In other words, is increasing force as quickly as possible a way to
curtail slip?
And if the answer is yes, then haven't you put forth a powerful
argument for "a solid stroke with a hard beginning?"
Warmest regards,
Charles
Carl Douglas
The position of the blade in the stroke arc is time-determined. If the
boat keeps moving (as with a crew shell) then the work you do hardly
affects what the blade does in the first 1/4 of the stroke. And that's
because "lift" largely prevents a loaded blade from moving face-first.
That sets the scene for what I have to say next:
If you build a load slowly in your stroke, 2 things will happen:
1. It will take longer for you to build up to full load, so the blade
will have moved under less load than might have been the case, & as a
result your opportunity to maximise the work done within the stroke arc
is reduced.
2. More of the work you do during the stroke will be unproductive -
because you have then to do it in the stalled mid-stroke phase, where
the propulsive efficiency is least.
To not load the blade early & fast is thus the waste of an opportunity
to do more of your work where the return is highest.
Even so, an oarblade is a less than ideal hydrofoil, because it has a
low aspect ratio - which means it is relatively short vertically
(perpendicular to the flow) but rather long from tip to root (in line
with the flow. An efficient foil will be longer in span or major chord
(i.e. across the flow) & relatively shorter in minor chord (from leading
to trailing edge) - which is what you see in commercial aircraft wings
&, even more, so in glider wings. It is good to have a high aspect
ratio, for that same total area, as the tip losses are proportionately
lower. These tip losses are energy losses in the tip vortex flow which
is generated by fluid flowing off around the tip from the higher
pressure surface towards the lower pressure surface.
For this reason it is even more worth getting your load on early. In
normal flow situations we are dealing with a steady state situation - a
fully developed flow which has had time to establish itself & settle
down in a steady pressure field. In this case, the way the fluid flows
is determined by the wider distribution of that pressure field, & flows
have time & distance within which to shape themselves.
Near the catch you have highly non-steady-state flows. The water didn't
know what was about to hit it & the pressure & consequent flow fields
have had no time to establish themselves. So the blade is cutting into
& working within something much more like concrete & much less compliant
that water feels when we are swimming in it. The bigger the applied
load, the more you can benefit from this rather transient, quasi-solid
condition.
When I wrote earlier I was trying to address what happens around a blade
if you load it less hard than you could do. And I wrote because Zibi
was implying that this was advisable as water, he supposed, was a soft
thing. It is a nest of difficult & hard-to-explain concepts, & I
somehow doubt I've been able to cast much light on it at this late hour.
Especially after having driven right around London today on our
infamous M25 circular car-park. But I did try ;)
Zbigniew A.
> Carl,
>
> So the faster you engage with water, the less it moves.
>
> Am I right then to infer from this that the faster you engage with
> water, the less opportunity you will give the blade to tear and slip
> through the water?
>
Guys, i am so glad we are on the same page! :-)
If there is any difference in a wording we use to describe this
theoretically, blame it on my bad English. Most importantly, we agree on
a practical conclusion.
--
Yours Virtually, Zibi
Tink
>
> Have any of you try to push a car that get stuck in a snow? (Charles you
> are excused, you live in California.) When you find your feet standing
> on ice covered with a slippery snow, you need to apply the force in a
> special way... I think that's the way!
>
> --
> Yours Virtually, Zibi
Nice description :o)
Walter Martindale
>
> Near the catch you have highly non-steady-state flows. The water didn't
> know what was about to hit it & the pressure & consequent flow fields
> have had no time to establish themselves. So the blade is cutting into
> & working within something much more like concrete & much less compliant
> that water feels when we are swimming in it. The bigger the applied
> load, the more you can benefit from this rather transient, quasi-solid
> condition.
>
um.. Air is just sitting there, too, until a wing hits it. A
hydrofoil that's been going for a while still keeps hitting fresh
water that didn't know it was coming, too. In a steady state
situation (flying or 'foiling' across the channel, for example) the
wing/foil tend to have a fairly constant angle of attack and
velocity. But they're both hitting air or water that doesn't know
what's coming...
Do you perhaps mean that it's non-steady state because the blade
changes depth through the entire stroke from first contact with water
until extraction, changes attack angle through the entire stroke, and
it's not really doing anything for a long enough period to let flow
fields get well established?
Cheers,
Walter
who lives only a km away from Canada's favourite parking lot - the
401. sometimes it's 3 lanes each way parked around here, but in
Toronto (a mere hour away) it can be 8 lanes each way, parked.
Carl Douglas
> On May 24, 7:32 pm, Carl Douglas<c...@carldouglas.co.uk> wrote:
>
>>
>> Near the catch you have highly non-steady-state flows. The water didn't
>> have had no time to establish themselves. So the blade is cutting into
>> that water feels when we are swimming in it. The bigger the applied
>> load, the more you can benefit from this rather transient, quasi-solid
>> condition.
>>
>
> um.. Air is just sitting there, too, until a wing hits it. A
> hydrofoil that's been going for a while still keeps hitting fresh
> water that didn't know it was coming, too. In a steady state
> situation (flying or 'foiling' across the channel, for example) the
> wing/foil tend to have a fairly constant angle of attack and
> velocity. But they're both hitting air or water that doesn't know
> what's coming...
> Do you perhaps mean that it's non-steady state because the blade
> changes depth through the entire stroke from first contact with water
> until extraction, changes attack angle through the entire stroke, and
> it's not really doing anything for a long enough period to let flow
> fields get well established?
>
> Cheers,
> Walter
> who lives only a km away from Canada's favourite parking lot - the
> 401. sometimes it's 3 lanes each way parked around here, but in
> Toronto (a mere hour away) it can be 8 lanes each way, parked.
I'll try to explain better:
When a wing (or anything else, for that matter) passes through the air a
pressure field precedes & envelopes for a long (theoretically infinite)
distance all around. If the plane is in steady flight, the
characteristics of that field, both near & far, are steady &
predictable. And that pressure field moves with & at the speed of the
plane. All the time the molecules of air will move, imperceptibly at a
distance, & much more close up, under the influence of that pressure
field. The flow & pressure field close to & far from the wing & the
plane in general maintain the same characteristics.
That is a steady state situation. The flow around the wing is
predictable & the flow patterns are said to be fully developed. But it
takes time for such a situation to establish itself.
It takes time to establish steady state flows because you go from having
no flows or pressure variation in the air surrounding the parked plane
to having large masses of air within a quasi-infinite space responding
sequentially & identically to the approaching, passing & retreating plane.
Only when the plane is moving at close to the speed of sound do things
get more complex. The pressure responses to the oncoming plane cannot
travel faster than the speed of sound, & when it reaches the speed of
sound they cannot move faster than the plane. So the air ahead has no
warning. When the leading edge hits it, it feels hard. This results in
a shock wave radiating off the leading edge of the wing, the nose of the
plane, etc. Within that very thin sheet of shocked air occur step
changes of pressure, velocity & temperature. Instead of the long,
gentle pressure wave travelling far ahead of the plane to constantly
re-establish the steady flow regime through which the wing will pass,
any observer or nearby object is hit by a sonic bang. The surrounding
flow & pressure fields of, & the design rules for, transonic &
supersonic wings are thus very different from those for comfortably
subsonic wings.
The blade entry is rather similar. The water knew nothing of the
blade's existence & had no chance to redistribute its local velocities,
pressures or levels (surface level reflects local pressure) to what's
about to hit it. And, because water is pretty dense (800 x as dense as
air), it takes a lot of energy, therefore some time, & hence some
distance of movement, to establish what could be that steady state
situation. So you have "unsteady-state" or transient flows which don't
get time to fully evolve & this provides a much harder resistance to the
blade.
We can't give precise values, because flows always take time to fully
evolve, but you will appreciate that, if a plane hits an updraught,
quite a lot of air must pass over its wings before the new pressure &
velocity field gets fully established. Things settle soon enough, but
the speed of the plane means that in that short time the plane flies a
fair distance before the new transients disappear & it flies steadily on
its new alignment.
Did that help?
Walter Martindale
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Hmm. Still - how far ahead of a 777 wing flying at 900 km/h does the
air get to realize that there's a pressure split coming at 10,000 m
altitude? How much "warning" does the air get? Millimetres?
Centimetres? Meters?
I get that there's nothing at all simple about the fluid dynamics of
an oar in water - I'm helping a Learn to Row program at the moment -
one of the participants is thinking of how to assign a fluid dynamics
exercise to her students around the oar - she's completing a fluid
dynamics PhD at U Waterloo.
Anyway - for a coaching perspective, I don't need this much detail but
understanding that there's lift, having water in contact with the back
of the blade, inelasticity of water, etc., all help me explain the
basics of making sure the blade's deep enough to move boat more than
move blade.
W
sully
> On May 25, 2:05 pm, Carl Douglas <c...@carldouglas.co.uk> wrote:
snip
> Anyway - for a coaching perspective, I don't need this much detail but
> understanding that there's lift, having water in contact with the back
> of the blade, inelasticity of water, etc., all help me explain the
> basics of making sure the blade's deep enough to move boat more than
> move blade.
from a teaching perspective, I like to sit someone down on the dock
with only
a scull in their hand, put one hand at the collar and other on or
near handle,
put the blade in the water buried, and half buried and have them push
blade thru
water each time. Don't have to do much explaining that way. :^)
Carl Douglas
>> distance all around. If the plane is in steady flight, the
>> characteristics of that field, both near& far, are steady&
>> plane. All the time the molecules of air will move, imperceptibly at a
>> field. The flow& pressure field close to& far from the wing& the
>> plane in general maintain the same characteristics.
>>
>> That is a steady state situation. The flow around the wing is
>> takes time for such a situation to establish itself.
>>
>> It takes time to establish steady state flows because you go from having
>> no flows or pressure variation in the air surrounding the parked plane
>> to having large masses of air within a quasi-infinite space responding
>>
>> Only when the plane is moving at close to the speed of sound do things
>> get more complex. The pressure responses to the oncoming plane cannot
>> sound they cannot move faster than the plane. So the air ahead has no
>> warning. When the leading edge hits it, it feels hard. This results in
>> a shock wave radiating off the leading edge of the wing, the nose of the
>> plane, etc. Within that very thin sheet of shocked air occur step
>> gentle pressure wave travelling far ahead of the plane to constantly
>> re-establish the steady flow regime through which the wing will pass,
>> any observer or nearby object is hit by a sonic bang. The surrounding
>> flow& pressure fields of,& the design rules for, transonic&
>> supersonic wings are thus very different from those for comfortably
>> subsonic wings.
>>
>> The blade entry is rather similar. The water knew nothing of the
>> pressures or levels (surface level reflects local pressure) to what's
>> about to hit it. And, because water is pretty dense (800 x as dense as
>> distance of movement, to establish what could be that steady state
>> situation. So you have "unsteady-state" or transient flows which don't
>> blade.
>>
>> We can't give precise values, because flows always take time to fully
>> evolve, but you will appreciate that, if a plane hits an updraught,
>> quite a lot of air must pass over its wings before the new pressure&
>> velocity field gets fully established. Things settle soon enough, but
>> the speed of the plane means that in that short time the plane flies a
>> its new alignment.
>>
>> Did that help?
>>
>> Cheers -
>> Carl
>> --
>
> Hmm. Still - how far ahead of a 777 wing flying at 900 km/h does the
> air get to realize that there's a pressure split coming at 10,000 m
> altitude? How much "warning" does the air get? Millimetres?
> Centimetres? Meters?
Very many metres ahead there are significant effects, but you can't
measure exactly something which tapers out asymptotically. There's a
large flow field ahead of & around the advancing wing within which air
is moving under the wing's influence.
And on the wing itself, each part of the surface feels pressure
redistributions whenever there are changes, e.g. in aileron settings
which, in effect, alter the wing geometry to, say, increase lift.
Now take something like our own AeRowFin (shameless plug!). Applying a
small amount of rudder (equivalent to an aileron adjustment on a wing)
generates flow changes way _ahead_ of the fin, & the pressure changes
which generate the lift which pulls the stern sideways (which is how
steering works) are greatest at ~25% of the way from the leading to the
trailing edges - i.e. way ahead of the rudder component. And that's how
well-designed foils work.
>
> I get that there's nothing at all simple about the fluid dynamics of
> an oar in water - I'm helping a Learn to Row program at the moment -
> one of the participants is thinking of how to assign a fluid dynamics
> exercise to her students around the oar - she's completing a fluid
> dynamics PhD at U Waterloo.
I wish her every success. Indeed, I'd be happy to correspond privately
should she wish to discuss this interesting but often counter-intuitive
matter.
>
> Anyway - for a coaching perspective, I don't need this much detail but
> understanding that there's lift, having water in contact with the back
> of the blade, inelasticity of water, etc., all help me explain the
> basics of making sure the blade's deep enough to move boat more than
> move blade.
> W
There we do agree :)
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
Charles Carroll
My observation from experimenting with macons a year or two ago led me
to conclude that macons could offer resistance just as well my
hatchets.
The important thing was to establish the essential tensile connection
between the mass of water behind the backs of the blades and the backs
of the blades themselves. Once this connection was established, I
could put equal loads on the blades.
I found, however, that try as I might I could never manage to
establish this essential connection as quickly with the macons as I
could with hatchets. Nevertheless, as I acquired skill, I found I
could scull as fast with macons as I could with hatchets.
For this reason it seems to me that my experiment with macons calls
into question the idea that in the rowing stroke we are interested in
what gets useful power on soonest.
If we are concerned about what makes us fast, two things seem to me
much more essential:
1) That we reproduce the average net velocity of the previous stroke;
2) And that we curtail as much as we possibly can the amplitude
between peak velocity and lowest velocity in any given stroke.
Now consider what the Dreissigacker brothers have to say about the
Science behind blade design:
"As a rower applies force to an oar, the motion of the blade through
the water resists the athlete's effort and generates the load the
rower feels.
"Different blade designs typically have different loading profiles.
Some blades find more resistance early in the drive while others have
more resistance later in the drive. These differences can also be
described in terms of the speed of the blade through the water. Or
more accurately, the apparent speed of the handle as the oar moves in
response to force applied by the rower. The greater the catch and
finish angles, the more apparent these differences become.
"Loading profile refers to the relative loading at different phases as
the blade progresses through the drive. Loading profile should not be
confused with the terms used to describe a rig that is too heavy or
too light for a crew. The overall load of the system should be
considered a function of rigging and oar length.
"In Conclusion:
"This brings us back to the three main ideas to take away from this
presentation.
1. There ARE performance differences between different blade shapes.
2. These performance differences may be DEPENDENT on various factors
such as rigging, catch angles, power application, and so on.
3. Crews need to determine for THEMSELVES what works best for them."
As I ponder this discussion on Kernschlag or Schubschlag, I confess
that I am a little uneasy with what I see as a tendency to offer a
"one size fits all" solution. Yes, it is interesting to discuss of how
we can get the load on quickly at the catch. But in the grand scheme
of all things sculling and rowing, is this really as essential as
people are claiming?
Let me quote again Tim McLaren, "There are many ways to [scull/row] -
whether you are a pusher or a puller do
it to high level and you have a chance of being competitive."
Warmest regards,
Charles
Carl Douglas
Charles -
As ever, solid facts seem to dissolve before one's eyes in a miasma of
imprecise verbiage.
I sell no oars & have no new blade shapes to offer. My interest is in
what happens at the end of an oar & how best to use it. The various
claims of finely nuanced differences appear somewhat wishful & some of
rowers' wonderful yarns about supposed differences look rather flimsy.
A typical oarblade seems unlikely (from my experience & discussions with
such folk) to earn many plaudits from designers of turbomachinery. Your
experience in finding scant difference between macons & cleavers is
pretty normal. So I wonder at the enlistment of 'Science' within your
quote above.
Compare the effort & intellect invested in the fine details of the wings
of an open-wheel race car, where real differences between designs &
performances are self-evident, with all the wooly stuff spouted about
virtually identical oars.
Then consider the inescapable fact that, while rowers all want a more
efficient oar, they are unwilling to accept that the inevitale
consequence of this is that the stroke _will_ take longer to execute or,
as it is commonly put, "the load will feel too heavy". For rowers, if
it doesn't "feel right" they won't be adjusting their technique or
sttitudes to see if they can make it work. Puttng it another way, I'm
sure it'd take an impossibly costly advertising campaign to bring this
about. This is plain irrational - if we are in the business of winning
races - since the load is only what you apply. But it is such relative
naivete which sets the bar for most discussions on oars. After all,
most rowers think that oars work by cupping & pushing water, & that the
rower's job is to pull the blade "through" the water as quickly as they
can. :(
Let's deal with reality rather than perception? I've said that the path
of a well-buried oar through the water in the first 1/4 t 1/3 (which
follows a tip-first loop) is barely affected by the load applied. This
is demonstrable, yet has led some into the trap of claiming that 'lift'
is irrelevant to oar function - a fundamental misunderstanding of the
flow processes around an oarblade & of hydrodynamic lift on any foil or
wing.
You are bound to use different blade somewhat differently, & this makes
for unreliable conclusions. If there is a meaningful difference between
macons & cleavers, it might be from these 2 causes (or from others?):
1. In the mid-stroke stall, if that's where you concentrate your
effort, there is no doubt that a larger blade area (_if_ well buried
below the surface) will move less through the water. That gives it a
higher propulsive efficiency there, which might be advantageous - if you
continue to major your effort where the reurn is lowest. But a larger
blade does not enhance performance at the ends of the stroke, where area
loadings are very low for a hydrofoil & you don't want the extra drag
you get from the extra area.
2. At the catch, if you ensure your macons are well buried & you load
rapidly, I'd expect no difference between types. But it depends on how
quickly you choose to bury & load your blade.
Much is made of speed of catch, without recognising that the hands move
quite slowly towards the bow there. In fact, the vertical sped of the
blades at the catch far exceeds that of their movement into the bow,
while the necessary horizontal blade speed is way below boat speed. Yet
we find no difficulty in burying the blades - except when we strive for
speed in the horizontal plane.
In describing the catch, we like to separate it into separate,
supposedly independent actions. As a result, we find it hard to see
several actions occuring simultaneously & merging smoothly, yet that's
how a good stroke works.
Now, please someone explain the supposed difference between push & pull
in taking a stroke? Is there a way to push the handles towards the bow?
Can you apply load to the handles without pulling? Can you sustain
that load without pushing the feet away? As I said, I do worry about
the words we use. ;)
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Carl Douglas
> On 25/05/2011 20:00, Walter Martindale wrote:
FWIW, you here's just one attractive example of how intelligent
designers apply the same rules as in aeronautical fluid dynamics to
design for underwater flight:
http://deepflight.com/subs/Super_Falcon_Specs.pdf
Best of all, have a play with this delightful tool:
http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html
View it zoomed-out to see how the flow is affected far ahead of the
wing. Change the shape from foil to plate to cylinder to ball. See how
lift results from the fall in pressure over one surface. See how
changing the foil camber generates lift, even at zero or negative angles
of attack.
It is in some senses over-simple. It doesn't show stall, it doesn't let
you put a simulated oar-blade (use a flat plate with camber) at 90-deg
to the flow (= mid-stroke), it doesn't deal with the boundary layer or
flow separation at thin leading edges & higher angles of attack. But
what a lovely applet for those wanting a feel for fluid dynamics!
Charles Carroll
If you can produce the average net velocity of the previous stroke,
and if you can curtail as much as possible the amplitude between peak
velocity and lowest velocity within any given stroke, why do need to
concern yourself with producing force increase as early as possible in
the drive?
Another way of asking this is that assuming you are sculling smoothly
and keeping boat check to an absolute minimum, why fret about where
you increase force to its maximal value during the drive? Should you
increase it in the early phases of the drive, or the mid phase, or
even end phase? If you are sculling well, why does it matter where you
increase force to its maximal value?
Let me see if I can be a little more specific.
Elsewhere you have written, "In a generating entry, there is no
pussyfooting or 'waiting for the blade to lock in or engage'. Water
unprepared for the entering blade is as hard as brick & doesn't need
you to hang around. All it requires is that you engage swiftly, load
immediately & neither backwater nor scratch at the surface."
I understand the need to "engage swiftly," that is, to establish
swiftly the essential tensile connection between the mass of water
behind the backs of the blades and the backs of the blades
themselves."
I also understand why you don't want to backwater or miss water.
What I don't understand is what you mean by "load immediately." What
percentage of your maximal value of force increase should you try to
load? And why is it important to produce it as immediately as
possible?
One of my coaches is ten inches taller than I am. He has a long trunk
and relatively short legs. He says that his body forces him to scull
much differently than I scull. My anatomy, according to him, makes me
strongest in the early phases of the drive. And, accordingly, he
coaches me to use my strength in these phases. On the other hand, he
says that he is strongest in the later phases of the drive, so that's
where he tries to use his strength. As a consequence I use almost no
layback, while he uses a lot of layback.
His point is that we simply need to appreciate our differences and
adjust our sculling accordingly.
He always tells his young crews, "There are many ways to move a boat."
Warmest regards,
Charles
Carl Douglas
Yes, Charles, there are many ways to scull. And for the very reasons
you state - that we're all different.
Now to where we must disagree:
You have, say, 0.8 seconds in a stroke available for you to do work
against the water. If you miss doing as much work as you might have
donein the 1st 0.1 seconds, because you were slow to apply load, but
pull as hard as you can for the rest of the stroke, you have irrevocably
lost the portion of work you missed in that 1st 0.1 seconds. You
couldn't pull harder in the rest of the stroke - it just can't be done -
& had you actually done so you'd have expended a part of that extra
effort in the least propulsively efficient parts of the stroke - around
the middle.
So, by failing to get the load on sooner, your potential to maximise
your useful work in that stroke has been irrevocably lost. And you will
go slower than you might otherwise have done.
It's a bit like the coach who asked me how much time his eight might
save over 2k by using a well-designed steering foil. I suggested that
the straighter course he would get from the boat & cox, which meant his
boat would spend less time going slightly sideways (a high drag mode)
between sucessive course corrections, might save him 3 seconds on an
average day, He responded that his crew could row 3 seconds faster
anyway. I replied that they should certainly try to do that, but they'd
still gain a further 3 seconds from the steering foil.
The interesting issue of who can pull hardest at which part of the
stroke is an entirely separate matter. Except for the person who cannot
pull at all at the catch, being slower at tloading the blade than you
need be is always going to diminish the useful work you are able to
accomplish in the stroke.
It's back to the old problem of nice words massaging the senses into
believing that what you wish were true has been talked into becoming
fact. And this is exacerbated by the actual fact that you don't have to
be the technically best sculler to be champion - if you are that best
sculler, it takes only a sculler who can do enough more work than you to
compensate, & a bit more, for his lesser skills to win the race. Life
really is not fair, but if one wishes to maximise one's performance than
it is best to accept that it is not possible to argue one's way across
the unbridgable gap between wishful thinking & incontrovertible fact.
We let kids & politicians do this to us, & we are free to believe what
we ourselves want to beieve, but we should not try to batter down the
weight of established science with the intensity of our own desires.
Any help?
Charles Carroll
You write that we must disagree. Thus I am compelled to ask wherein is
our disagreement?
I am not trying to promote a specific technique for taking the catch.
I am not advocating anything. I am only trying to understand how to
load the blade and complete the stroke.
Should you use a solid stroke with a hard beginning?
Or should you use a thrust-stroke?
You state with unmistakable clarity that by failing to get the load on
sooner, the potential to maximize useful work in a stroke is
irrevocably lost. To me this suggests that we should set a goal for
ourselves - the goal being that we should try to maximize useful work
every stroke. And this suggests that you are advocating that we should
try to increase boat velocity with each stroke.
Elsewhere, however, you have written that it is a mistake to try to
accelerate boat velocity each stroke. And from this you have inferred
a different goal, namely, that we should only try to accelerate boat
velocity enough to reproduce the average net velocity of the previous
stroke. So with this goal the definition of useful becomes doing only
enough work to compensate for the loss of velocity during the
recovery.
Am I the only one to see a contradiction in these two statements?
In the first statement the goal is to maximize useful work. In the
second statement the goal is to limit useful work. My difficulty comes
with trying to reconcile "maximize" with "limit."
But thinking about this over the weekend and reviewing observations of
my own sculling, I have come up with a possible reconciliation.
It seems to me that a solid stroke with a hard beginning (i.e. getting
the load on sooner in order to maximize your work) is very useful when
you want to increase velocity with each stroke. So I would think that
you would use a solid stroke with a hard beginning for racing starts
in which you are trying to increase boat velocity with each stroke.
But once you have attained a boat velocity that you can sustain, then
wouldn't it be time to switch to a thrust-stroke, which I think of as
a much less demanding stroke.
In other words, use Kernschlag for racing starts and at times when you
want to increase boat velocity. Use Schubschlag for when you want to
maintain velocity.
Consider Peter Mallory's video of Vyacheslav Ivanov. For the first
three 500m pieces everyone finds their position and rows at the same
rate and maintains the same velocity. Then Ivanov in a burst of quick
strokes (Kernschlag) increases his velocity, and then maintains the
increased velocity (Schubschlag) to the finish and gold.
http://www.youtube.com/watch?v=7hhVNRonWf0&playnext=1&list=PL4D4F6C4439A61C24
I don't know. I am not advocating anything. I am just asking.
Warmest regards,
Charles
Carl Douglas
> Carl,
>
> You write that we must disagree. Thus I am compelled to ask wherein is
> our disagreement?
>
> I am not trying to promote a specific technique for taking the catch. I
> am not advocating anything. I am only trying to understand how to load
> the blade and complete the stroke.
>
> Should you use a solid stroke with a hard beginning?
>
> Or should you use a thrust-stroke?
Charles, you are a wicked old stirrer! Why would anyone not use a
"solid stroke with a hard beginning"? I am truly puzzled by your
proffered alternative of pouring out the load over the length of the
stroke which, sadly amounts to rowing the stroke with an average of
half-pressure.
For simple maths, let's suppose we have one unit of stroke length and
can apply one unit of force? And suppose, too, for the simplest case,
that we have the ability to sustain that 1 force unit for the whole 1
length unit of the stroke? Then the maximum work that we can do in the
stroke is:
Total work = force x length = 1 x 1 = 1 work unit.
Now suppose, instead, that we build up the force uniformly over the full
length of the stroke? At the catch the load is zero. At the finish the
load is 1 force unit. That gives us:
Average load = (0+1)/2 = 0.5 force units
So the work we can do in this nicely linear assumed stroke is now:
Total work = 0.5 x 1 = 0.5 work units
What happened to the other 0.5 units of work? Simple - you denied
yourself the opportunity to do that work because you chose to start
soft, & because you could not pull harder elsewhere in the stroke than
your allotted 1 force unit. In fact, to do the same amount of work with
that force profile you'd have to be able to pull all teh way up to 2
force units at the end of the stroke - and that would require twice your
present muscle cross-sectional area.
That was a simple example. In reality, every stroke starts a bit soft,
because it takes time to get the load on for these & other reasons:
1. You have first to bend the oar, which takes time, as without a bent
oar you have no load & the load can only be proportional to how far you
have bent the oar.
2. At the catch your load capacity is probably reduced by the position
you have to adopt
If you were to factor in the real droop in propulsive efficiency that
occurs in the mid-stroke, that would only impair further the outcome of
the soft-start stroke.
>
> You state with unmistakable clarity that by failing to get the load on
> sooner, the potential to maximize useful work in a stroke is irrevocably
> lost. To me this suggests that we should set a goal for ourselves - the
> goal being that we should try to maximize useful work every stroke. And
> this suggests that you are advocating that we should try to increase
> boat velocity with each stroke.
I'm not interested in increasing boat velocity, which sounds a brutal
thing to say. Mean boat speed depends only - if we ignore secondary
effects due to the way boat speed does or does not fluctuate about the
mean in each stroke - on the rate at which we do useful work - i.e. the
total work we do minus the part of that output which then gets
dissipated in moving water (any motion induced in the water, which we
can't avoid BTW, is permanently lost from that remaining to move the
boat). Any technique which reduces our total useful work will slow us
down, while one which maximises our useful work must make us faster.
>
> Elsewhere, however, you have written that it is a mistake to try to
> accelerate boat velocity each stroke. And from this you have inferred a
> different goal, namely, that we should only try to accelerate boat
> velocity enough to reproduce the average net velocity of the previous
> stroke. So with this goal the definition of useful becomes doing only
> enough work to compensate for the loss of velocity during the recovery.
>
> Am I the only one to see a contradiction in these two statements?
This attention to acceleration is not relevant. What we are concerned
with is mean system velocity - that of boat, blades & crew - since all
will arrive at the finish of the race at the same instant. We hear so
much waffle from the launch & the towpath about "accelerate the boat
every stroke", but it is meaningless. Especially since at the end of
each stroke the boat is still going no faster! Our only job is to
deliver most power we can in the most efficient way & then let boat &
system velocities reap the benefit as they must. Our job is not to
accelerate the boat but to provide the continuous flow of energy needed
to overcome the fluid drag which is constantly applying the brake to our
progress. Always that energy flow needs to be the greatest you can deliver.
>
> In the first statement the goal is to maximize useful work. In the
> second statement the goal is to limit useful work. My difficulty comes
> with trying to reconcile "maximize" with "limit."
That is your interpretation, but it is incorrect.
>
> But thinking about this over the weekend and reviewing observations of
> my own sculling, I have come up with a possible reconciliation.
>
> It seems to me that a solid stroke with a hard beginning (i.e. getting
> the load on sooner in order to maximize your work) is very useful when
> you want to increase velocity with each stroke. So I would think that
> you would use a solid stroke with a hard beginning for racing starts in
> which you are trying to increase boat velocity with each stroke.
>
> But once you have attained a boat velocity that you can sustain, then
> wouldn't it be time to switch to a thrust-stroke, which I think of as a
> much less demanding stroke.
>
> In other words, use Kernschlag for racing starts and at times when you
> want to increase boat velocity. Use Schubschlag for when you want to
> maintain velocity.
>
> Consider Peter Mallory's video of Vyacheslav Ivanov. For the first three
> 500m pieces everyone finds their position and rows at the same rate and
> maintains the same velocity. Then Ivanov in a burst of quick strokes
> (Kernschlag) increases his velocity, and then maintains the increased
> velocity (Schubschlag) to the finish and gold.
>
> http://www.youtube.com/watch?v=7hhVNRonWf0&playnext=1&list=PL4D4F6C4439A61C24
>
>
> I don't know. I am not advocating anything. I am just asking.
>
> Warmest regards,
>
> Charles
I fear, Charles, that you have persuaded yourself that there are
touchy-feely options for how to do work, but I have to say that I think
your argument cannot succeed.
There's only 1 way to move a boat - do the most work in the most
(propulsively) efficient way. It does not matter whether you wish to
sprint or to cruise. If you're sprinting, that's because you see the
need to do even more work in order to win. It might seem that the work
is being done differently, but that is only because we can't help doing
things differently when right at our limit as opposed to cruising almost
at our limit. In other words, I doubt that the force profiles change
all that much, but to the extent that they do change that will be due to
the limits on force applicable by our different muscle sets & to the
necessary changes in rhythm accompanying a higher rating (e.g. reduced
recovery to stroke ratio).
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
Charles Carroll
I think Zibi was right when in an earlier post he suggested that we
really don't disagree - we just misinterpret. And I, especially, am
guilty of this.
When you write about "maximizing useful work" I hear "pull harder,"
i.e. a solid stroke with a "hard" beginning.
But you are right to say that this is a misinterpretation of what you
mean. Here is something that you wrote a couple of years ago.
"In rowing, work has to be done & there's no getting away from that.
If all else is equal, within the normal human capacity range pulling
harder at constant rate will make you go faster since you are thereby
doing more work. Similarly, if your technique remains consistent,
rating higher at the same pressure will enhance speed because that's
another way of working harder."
So when you write, "maximizing useful work" it doesn't necessarily
mean pulling harder. It could also mean increasing the rate or
improving timing or any other of a number of things.
But even so, when I read "a solid stroke with a hard beginning" I
cannot help thinking "pull harder" in the initial phases of the
stroke. The technique brings to mind that old, Thames waterman, Jimmy
Hastie, who told Steve Fairbairn, "Take hold of it as hard as you can,
row it through harder, and finish it out hardest." ("Steve Fairbairn
on Rowing," p.399)
In any event, I now have a much clearer understanding both of what you
are saying and of the rowing stroke itself. Thank you.
Warmest regards,
Charles
Carl Douglas
of some of the confusion too?