Managing the Run of the Boat
MagnusBurbanks
manage the run of the boat will only negligibly affect the distance
travelled from finish to catch. However I had doubts, and decided to
try to work it out for myself.
It turns out that there are significant differences between recovery
strategies, and it can be worthwhile trying to manage the recovery:
start sliding slow, and save as much of the sliding action until the
end of the recovery, if you can.
I have posted in full on my site here:
http://www.slidingseat.net/howtorow/howtorow.html#therecovery_quant
Cheers, Magnus
Tinus
> manage the run of the boat will only negligibly affect the distance
> travelled from finish to catch.
>
> strategies, and it can be worthwhile trying to manage the recovery:
Those who stated that the strategy is not much important included two
other parameters which have opposite effect to the effect which you
found (late loaded non flat profile is better because it increases
distance travelled):
- The flat profile could be seen as an advantage because the maximal
kinetic energy of the recovery motion is smaller and hence less energy
is lost in this motion allowing the rower to increase speed (and
traveled distance in the recovery) by means of a better efficiency.
(http://www.atkinsopht.com/row/freertrn.htm)
- The front loaded recovery could be seen as less of a disadvantage
(it still is) because, while distance traveled is less, total energy
loss is less as well. Instead of the 8m advantage you would have
something like 6m advantage. (http://home.hccnet.nl/m.holst/
recover1.html)
One could also add two other parameters but these are complicated:
- The effect of angular displacement of the boat, which could be
affected by recovery strategy. Checking the boat might not so much be
a bad thing because of the large momentum exchange and coupled loss of
boat speed but more because it's effect on the stern moving more
downwards as the force of the feet on the stretcher has a vertical
component.
- Physiological effects. I can think of advantages of a front loaded
recovery because the arms already apply a high force allowing this to
use it as a force to accelerate the body relative to the boat (or vice
versa). It is also an advantage to generate a high momentum in the
body using the arms and the abs allowing the kinetic energy to move
the legs in the early phase of the recovery. (when they are fully
stretched it is less advantageous to use them to apply force on the
stretcher as it requires a high torque on the knees). And also a late
loaded curve could be less advantageous because it requires a fast
change of direction of force in the legs changing pulling into pushing
very quickly.
MagnusBurbanks
I visited Marius’ results in the link you supplied. His numbers and
simulation results are consistent with mine, however I disagree with
his logic and his conclusions.
His concluding words:
“the difference with the most unfavourable mode 2 is only 0.033m, or
8...9m advantage on a 2000m course. This advantage is not free of
charge, because the outflow of energy is 1.2J (a very small quantity
indeed) more than in mode 2”
…
“In practice such sharp differences in seat motion as in the
simulation can never be obtained. It will be a smooth curve sometimes
closer to mode 3, sometimes closer to mode 2 and the difference in
covered distance will diminish.
It seems useless to coach on a very pronounced mode of seat motion.”
First the logic: He starts off by simulating a sliding pattern that he
admits is not physically possible, establishes that it yields
significant benefit over a race distance, then concludes that because
it’s not physically possible the line of investigation is not
worthwhile. His reasoning is that because an impossible profile
theoretically yields a benefit, all beneficial profiles must be
impossible!
In contrast, I start off with a perfectly possible profile and
demonstrate that it too yields worthwhile benefit, and a few metres
benefit over a 2000m course certainly seems worthwhile to me!
He also seems to imply that a strategy which gains 8-9m over a 2000m
course is not worth pursuing! And you are implying that a different
strategy which might “only” yield 5-6m extra is even less worth
pursuing. Tell that to Alan Campbell, the silver medallist at the
recent World Championships!
I do agree with you that a late-loaded curve requires a very quick
change in direction of force, hence my caveat that this should
probably only be attempted by very skilled rowers.
Cheers, Magnus
Tinus
> admits is not physically possible, establishes that it yields
> significant benefit over a race distance, then concludes that because
> it’s not physically possible the line of investigation is not
> worthwhile. His reasoning is that because an impossible profile
> theoretically yields a benefit, all beneficial profiles must be
> impossible!
I don't believe that Marius argued that managing the run of the boat
isn't worthwhile because the profiles in his investigation aren't
physically possible. He claims that in practice curves will look less
pronounced and also the magnitude of the effect will be smaller. I add
that this investigation, of the run of the boat, only uses a
mechanical point of view. Physiology considerations might seem to
counter the positive effect of a late loaded curve.
It is especially important to take into consideration the need for a
higher maximal slide speed (which involves higher loss of internal
kinetic energy) in order to get a more pronounced late loading.Your
own investigation also included a practically impossible profile with
an extreme late high peak. This profile resulted in the highest amount
of distance travelled (even if you'd take into account the energy cost
or the change in speed it would be better).Why should one regard it as
not feasible?
The same argument why the extreme late loaded curve is not a good
curve should apply to any profile which lies in between the flat curve
and the extreme late loaded curve. Intuition should tell that the
positive effect of changing the curve towards extreme late loading
should be bounded/balanced by an opposing effect (otherwise the
extreme late loaded curve should be the best curve which we know from
practice to be false). Without quantifying those opposing effects the
investigation might show a false result of a positive effect which
grows unlimited and indicate a profile which would not be a good
profile in reality. I agree with you that there is a positive effect
from a late loaded curve but I believe it is strongly balanced by
other effects. This is not clearly visible in your investigation.
Carl Douglas
>> First the logic: He starts off by simulating a sliding pattern that he
>> admits is not physically possible, establishes that it yields
>> significant benefit over a race distance, then concludes that because
Just a question, Tinus:
When you write of higher maximal slide speeds, are you thinking that the
body has to move at that speed, or recognising that the boat always
moves more than the body?
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)
Tinus
>
> When you write of higher maximal slide speeds, are you thinking that the
> body has to move at that speed, or recognising that the boat always
> moves more than the body?
Both boat and body move relative to each other. I recognise that the
boat mostly moves at a higher speed from the perspective of the centre
of mass of the system.
Although, at the start of the recovery, when the upper body starts to
move towards stern and the seat is not yet moving, the mass of the
part of the body which moves is small. At that point the boat moves at
smaller speeds relative to the centre of the mass of the system than
the moving body mass does.
Viewing it in that way (using a variable moving body mass), one
already has a relative late loaded curve naturally when a person first
starts the recovery by using mostly the upper body and have the seat
and the legs move later. When making the recovery in this way a large
part of the body mass will be initially at the same speed of the boat
and the speed fluctuation of the boat starts of small just like the
late loaded curve.
Mike De Petris
> (otherwise the
> extreme late loaded curve should be the best curve which we know from
> practice to be false)
?? YOU know from your practice to be false? Or what? Dogma?
Tinus
I have to admit it is not well founded.
But I hate dogma (except for the movies). Do you have a set of twins
available to test this hypothesis?
Mike De Petris
anyway IMHO most of this discussing is useless if targeting at fast 2k
races, where athletes row in the high 30es all the way, there's not
much you can modulate in your recovery
Carl Douglas
Wouldn't that be another of those unjustified assumptions?
Rowing at 38 still leaves you nearly 1 second of recovery time. I
re-iterate that what is being moved is not your whole mass but that of
the boat, plus part of your legs. Even the accelerations are quite
small, so the loads remain relatively light. Now consider how rapidly
athletes in other sports can & do make repeated movements under much
greater loads.
It depresses me that, in rowing, we refuse to think new thoughts. We
find it easier to tell ourselves that they are unthinkable. To avoid
serious engagement with interesting but knotty problems we arbitrarily
decide that there are physical limits, that there are tricks we can't be
taught. Yet we still want to go faster.
I think we can usefully explore the very relevant insights that Magnus
has provided. But in rowing we prove, time after time, the old adage
that, while you can take a horse to water, you can't make it drink.
Most rowing is done in crews, & crews have a relatively short time
together as a unit. If they don't row together, that slows the crew.
So coaches have to work all the time for togetherness & in the time
that's left they work for technical improvement. That's bound to
restrict scope and enthusiasm for research into better ways to row.
It's always a safer for coaches to try to get crews to do the same
things more powerfully than to seek better ways, because to lose with
orthodox technique is less likely to cost you your job than to have your
crews go better by adjusting technique, but still to lose (although by
smaller margins). The latter is always seen as having thrown away the
chance of success.
MagnusBurbanks
extremes the principle of late-sliding "works" from the point of view
of increasing distance travelled. However, it is not feasible I would
suggest for two reasons. First, the magnitude of the force required to
be exerted and borne by the hamstrings will likely be too large (up to
+/- 3000N compared to 180N in typical sliding case), and second the
energy cost of this proifile vs typical will be around 30-40J, as
opposed to 1-2J for the smooth late-loaded.
As for the practicality of trying to change a technique to something
potentially more effective: if this is not worth experimenting with,
why bother coaching at all? The argument that at rate 38 it's all
happening too fast for it to be feasible to change can equally be
applied to say that it's not worth ever coaching novices, because if
they're rubbish at rate 20 they'll never even get to rate 38, let
alone be able to try two subtly different variations at 38. And yet
even Mahe Drysdale was an ungainly novice once.
Cheers, Magnus
Walter Martindale
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
Hmm. Interesting discussions. Some discussion of recovery speed and
entry speed was given in the article cited:
Towards optimizing rowing technique
SANDERSON, BRIAN; MARTINDALE, WALTER
Medicine & Science in Sports & Exercise. 18(4):454-468, August 1986.
It's a long time since Brian wrote this and graciously included me as
co-author but I recall (and am not in a hurry to dig out my remaining
copy of the article) that a slow start to the recovery followed by a
quick end of the recovery with a fast, well timed blade entry was
theoretically more effective than the opposite velocity profile. That
was discussion occupying about 1/3 of a column of a 14 page article.
W
Tinus
> find it easier to tell ourselves that they are unthinkable.
> As for the practicality of trying to change a technique to something
> potentially more effective: if this is not worth experimenting with,
> why bother coaching at all?
I hope I am not misunderstood. I am not against unorthodox rowing
style. It is just that I want to get to the bottom of the theory.
I am not against strange recoveries. As a proof: I recently changed my
technique to erg above 30spm for marathon distance and at 45spm for
2km (I have yet to make it work on the water).
Carl Douglas
In which case, Tinus, I am happy dissociate you from my reference to
innate technical conservatism in rowing.
Since there are limits (I suppose) to the levels of fitness and strength
attainable by the average rower, if we want to go faster we have to look
in the only other place where improvement might be found - in how we
row. I think that, in that box, is a veritable treasure chest of ways
to adapt our actions to gain modest improvements.
Gain just 1 metre per 2k each from 8 small technical adjustments - each
affecting speed by only 0.05% - & you'll be 1 length of a 1x faster.
That 0.4% speed improvement would otherwise have required >1.2% higher
work rate. And for those who say they can do that extra work anyway,
well then they can do that as well - & be 2 lengths faster.
How many of us wouldn't (or wouldn't have) given our eye teeth for just
few metres' improvement in speed? We have brains, so let's use them &
not just be galley slaves.
Charles Carroll
You write that "there are significant differences between recovery
strategies, and it can be worthwhile trying to manage the recovery: start
sliding slow, and save as much of the sliding action until the end of the
recovery."
But what about what Bill Atkinson wrote: "Insofar as the rower's momentum in
relation to the boat is concerned it is my conclusion that nothing the rower
can do, one way or another, on the slide can change the average velocity of
the center-of-mass of the system.
(See:<http://www.atkinsopht.com/row/freertrn.htm> )"
Does your research disagree with Bill's?
Cordially,
Charles
Charles Carroll
Have you seen the slow motion video of Vyascheslav Ivanov on YouTube?
After looking at Ivanov in the vido, do you really believe that "there's not
much you can modulate in your recovery"?
I don't know. I don't have the best eyes in the world. But I can tell you
that Vyashcheslav Ivanov's recovery is probably the smoothest and most
relaxed I have ever studied.
Cordially,
Charles
Mike De Petris
wrote:
What about AUS pair in Bejing?
MagnusBurbanks
wrote:
Put as simply as that, yes I do disagree with Bill.
Cheers, Magnus
Charles Carroll
My only point is that even at very high rates at full pressure you can teach
yourself to be smooth and relaxed on the recovery.
But will this result in moving a boat faster?
Bill Atkinson's mathematical model suggests no. Magnus Burbanks' experiments
with accelerometers suggest yes.
And thus I find myself exiled from the clear light of day - forced once
again into a familiar habitation, the murky cave of confusion.
But I concede that I am predisposed in favor of Magnus's experiments. I see
no harm in trying to be smooth, relaxed and in control during the recovery.
I cannot demonstrate that I am right. But I can tell you what leads me to my
predisposition.
A few days ago I was experimenting with 1 minute intervals on the erg. The
goal was to erg for 1 minute at the highest rate and pressure while
maintaining good technique. During one of the intervals I switched focus
from technique to split time.
The result was disastrous. I crashed against the back of the slides, then
the front of the slides. I felt my brain ricochet inside my cranium. The
mirror at my side showed that I was all over the place.
So I calmed down and re-focused on technique. My goal was to stay as still
as I could and pull the erg back and forth underneath me. It is not that
hard to do if you brace and diligently maintain "the connection," if you
keep your feet firmly in contact with the stretcher, and if you keep your
heels firmly planted in the heel cups during the recovery. You can use your
legs and heels to pull the erg underneath. You don't even have to pull very
hard.
And what was the result? I moved much less in the mirror. I stayed pretty
still and managed to pull the erg back and forth underneath me. Eventually I
increased my rate to where it had been when I switched focus and started to
concentrate on split times. And lastly, I dropped my splits almost 5
seconds.
So I think there may be something to be said for being smooth, relaxed, and
in control during the recovery.
Cordially,
Charles
Charles Carroll
> Put as simply as that, yes I do disagree with Bill.
Magnus,
Not only simple, but eloquent. And I for one find your research heartening.
As I mentioned to Mike De Petris, I confess that I am predisposed in favor
of being smooth, relaxed and in control during the recovery. I cannot help
marveling at how Vyascheslav Ivanov sculls.
Cordially,
Charles
MagnusBurbanks
> with accelerometers suggest yes.
> And thus I find myself exiled from the clear light of day - forced once
> again into a familiar habitation, the murky cave of confusion.
> But I concede that I am predisposed in favor of Magnus's experiments.
Charles,
Thank you for your kind comments, but allow me to correct a slight
misunderstanding: although some of what I have documented on my site
is based on my accelerometer setup, this particular piece about the
recovery is based on a (relatively simple) mathematical model, albeit
having estimated the "drag factor" in the formula for water drag on
the shell from experimental accelerometer data. If you wish to see or
check the maths of the model, there is a link in the second last
paragrraph of the article directing you to an appendix.
All I can say is that, each of us having used a mathematical model to
simulate the recovery, I come up with a different conclusion from
Bill.
Cheers, Magnus
Mike De Petris
wrote:
I just wanted to underline that "being smooth, relaxed and in control
during the recovery" is a different thing then adjusting the recovery
speed curve, and that is even different then moving body segments with
different timings.
Carl Douglas
Everyone is right in this.
Smooth & relaxed means that each little movement progresses smoothly
into the next, as in a smooth curve. Rough & awkward, or whatever we
care to call it, means that the flow of motion is more jerky - either
like a collection of short straight moves connected together (imagine a
long bend in the road made up from a succession of 100-metre straights
joined by abrupt 5-degree corners) or even with local abrupt
accelerations & decelerations, like saw-teeth, steps or ripples imposed
on a smoother what might otherwise have been a smoother curve.
A smooth recovery is obviously what is required, since there's a high
energy cost in a juddering motion or in any sudden changes in velocity.
However, there's an infinite variety of time/velocity profiles that even
a smooth recovery can follow. If we revert to the popular but quite
unreal view of what happens in the boat, here are 3 alternatives:
1. Faster away from the finish, peaking early in relative velocity
between boat & sculler & then a gentle deceleration between body & boat:
_
/ \
/ \
/ \
/ \
/ \
/ \ _
Finish Catch
2. The exact reverse of that:
_
/ \
/ \
/ \
/ \
/ \
_ / \
Finish Catch
3. Something in between:
________________
/ \
/ \
/ \
Finish Catch
4. Or something rather erratic:
__ __
/ \ / \
/ \ / \
/ \____/ \
/ \
/ \
Finish Catch
Or you can add in your own variants. My point being that each of these
recovery patterns is entirely feasible. But which of them is the most
efficient?
Each recovery curve (& all should be much smoother than my poor ASCII
art indicates) shows the position of, say, sculler's CofG WRT stretcher.
Each therefore signifies quite different patterns of energy exchange
between sculler & boat, resulting in quite different boat speed vs time
curves during those recoveries. Fluid drag varies as the square (or
rather higher) of boat speed, so every 1 % velocity increase above the
mean costs increasingly more than is saved by every corresponding 1 %
point by which speed must then be below the mean to maintain that
average velocity, & hence the same distance of run during the same
recovery period (i.e. at the same rating).
By this reasoning, whichever recovery curve best reduces the integral of
boat speed^2 over time will give the least hull drag during recovery.
While we're at it, when you read that part of Bill's work to which
Charles pointed us, you find this statement:
"ROWING shows that it is better to accomplish both the acceleration and
deceleration quickly leaving as much time as possible in the middle for
a constant velocity, zero-force, "float" to frontstops. The steeper the
accelerations on either end of the float the higher the rating and the
faster the boat (for equal total rower power expended)."
The flaw in this scenario is that there can be _no_ such thing as Bill's
"constant velocity, zero-force, "float" to frontstops". You do _not_
move at constant velocity WRT the boat without expending energy. At all
times, just to at constant velocity WRT the boat you must pull against
the stretcher to draw your body mass against the fluid drag on the hull
which is sucking energy & velocity out of the system. Otherwise you
will find yourself sliding back into backstops as you continue (obeying
Newton's 1st law) & the boat decelerates (obeying Newton's 2nd law).
Willi
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)- Hide quoted text -
>
> - Show quoted text -
As has already been mentioned, this is not a new concept. This was
being talked about back in the mid 90s i.e. the most efficient
recovery was slow start and fast approach to the catch. Something to
do with Newtons second law and boat drag. However, to do it needed
very good catch technique - most club rowers do not have a good catch
technique and after 1500m of a race most people’s will be
deteriorating. The opinion back then was that the gain would be
smaller than the loss from a poor catch. Better to get a good catch,
which is easier if you are moving slower.
Paul.
Carl Douglas
> deteriorating. The opinion back then was that the gain would be
> smaller than the loss from a poor catch. Better to get a good catch,
> which is easier if you are moving slower.
>
> Paul.
I would strongly agree that catch technique is often extremely poor.
But that deficiency does not make a case for therefore doing nothing to
improve recovery technique.
Catches are too often poor & disrupt the flow of the stroke because
rowers were told the catch was something big that had to be to hit hard
& for which they had to gather themselves. They were also told to slow
down on the slide - to supposedly "control the slide". With all that
mental burden, they stop just before the catch & then seek to restart
the stroke from that tense & dead position. The result is not good.
Sort out the catches & then you can proceed to model a better recovery.
Even if you stoppped at just sorting out the catches you'd be rowing
so much better. Good catches are quick & light. They can't be hard,
because the oar has to be bent to have any load & be bent more to have
more load which, like stretching an elastic cord, takes swift movement
wih growing force, not stasis & then fierce impact.
If we are willing to let crews race with oviously poor catch technique,
while at the same time training the pants off them, aren't we at risk of
putting the cart before the horse & ensuring that they will never learn
to row well enough to benefit properly from their growing fitness?
Cheers -
Carl
--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Walter Martindale
>
> >> - Show quoted text -
>
> > As has already been mentioned, this is not a new concept. This was
> > being talked about back in the mid 90s i.e. the most efficient
> > recovery was slow start and fast approach to the catch. Something to
> > do with Newtons second law and boat drag. However, to do it needed
> > very good catch technique - most club rowers do not have a good catch
> URLs: www.carldouglas.co.uk(boats) &www.aerowing.co.uk(riggers)
This isn't just replying to Carl... Richard Smith, a researcher in
biomechanics in Sydney, AUS, has results of force measurements on foot
stretchers indicating that the horizontal component of force on the
foot stretchers during the recovery is approximately 100 N in a
single. I'm on the wrong computer (it's on the PC and I'm currently
on the Mac) to pull up his graphs, but when you think about it... 100
N is approximately the force required to raise 10 kg one metre (yes?)
in 1 second. Or, about 50 N per foot... When you think about it
another way, when a person runs, he or she pushes on the ground with a
force somewhere near 2-3 x his or her body weight. Each stride. So,
really, 50 N feels like not a heck of a lot.
Then... All of us (well, not people I've been coaching for the last
few years - I apologise to the ones I coached before this for not
figuring it out earlier) have been coached to thing about moving our
body mass towards the stern of the boat - or - towards
"frontstops"..
So, we don't think about manipulating a light racing shell under our
bodies; we think about manipulating our (big clumsy mushy mostly
water) bodies on top of this shell, and our mental image of the
recovery is of us sliding towards the stern, instead of us sliding the
stern forwards under us with our feet. When we think about sliding
towards the stern, we also think - OK, I have to stop my body sliding,
and we try to control our torso, thighs, and arms as they come
crashing to frontstops (or gently changing direction, or whatever).
According to my memory of Dempster's cadaver studies, about 20% body
mass is in our thighs (10% per thigh), and about 65% of our body mass
is above our hips. What's easier to control, 85% of our body mass, or
15% of our mass plus a boat that is about 1/6 of our mass? I'd go for
controlling my lower legs and the boat. Set the upper body up early
in the recovery, draw the boat forward with the feet, and let the
hands follow the oar handle(s) out as the gates carry the blades past
you to the catch..
If you don't believe that the boat travels faster than the crew during
the recovery, there are three ways to relieve yourself of this
delusion - 1: instrumentation. Expensive, but more credible -
perhaps... 2: if you're rowing, watch the water going past you during
the recovery.... You (your eyeballs) continue to move in the
direction of travel over the water no matter how fast you try to swing
your body 'forward'. If you're still moving towards the finish line
relative to the water, how the )(*^(&% is it possible to think that
you're moving towards the stern of the boat? The stern of the boat is
moving towards the finish line faster than you are, so, while
relatively, I suppose, you are moving sternward, YOU are still moving
bow-ward, and the boat is moving bow-ward even faster.... 3: If you're
watching from the shore - and not moving at the mean speed of the crew
(i.e., you're standing on shore, not on the bike path or whatever) -
watch the crew when the blades are out of the water on the
recovery... the crew on the recovery continues moving in the
direction of travel - NOT back towards the start line - and the boat
moves past, underneath them until they get to the end of recovery and
have to catch again...
Here's an idea - coaches - especially if you have a NK speed coach XL
or some other way to measure boat speed. Record a crew travelling at
whatever stroke rate you want. Then talk to them about the recovery,
where they sit, and set up, and let their feet come towards them
(paraphrased from Tonks's discussion in Rowing Faster, edited by
Nolte). If they buy into the idea - then record their boat speed over
a stretch and see if it makes any difference in their boat speed.
If you do this, please report back to this forum..
Cheers,
Walter
Mike De Petris
> I'm on the wrong computer (it's on the PC and I'm currently
> on the Mac) to pull up his graphs
this should be read "Graphs are on the wrong computer, I'm currently
on the rigth one"
:-)
Walter Martindale
But... What about the Linux box I'm using now? (Fedora 11)
Neil.W.James
> deteriorating. The opinion back then was that the gain would be
> smaller than the loss from a poor catch. Better to get a good catch,
> which is easier if you are moving slower.
>
> Paul.
As you say, it is not a new concept. It goes back further than the mid
90s and certainly to the mid-60s where it was used by the Ratzeburg
crews coached by Karl Adam.
Neil
Bill Atkinson
wrote:
Magnus:
It is easy to define accurately the area (slide distance traveled)
under a sine function of a given amplitude and, likewise, under the
trapezoidal function defined in the "flat" mode.
I note that their two "results" are practically identical, possibly
because their distances traveled have been defined as virtually
identical in well understood geometrical terms.
I wonder, though, about the calculation of the precise distances
traveled (unpublished here) in the more complex early and late slide
modes. How accurately are they defined in assuring them equal to the
other two modes?
A small discrepancy here could easily explain differences both between
the late-early results and between these and the identical sine-flat
results.
Best regards,
Bill
MagnusBurbanks
>
> - Show quoted text -
Bill sorry about the late response I am abroad.
The more complex early and late slide models are using "x^2sin x"
function, with the coefficients scaled to allow the integral to return
exactly L in time T, in this case of course 0.45m in 0.60s. The actual
formulae I have used can be found in the more technical appendix which
is actually referenced in the article, in the second last paragraph,
but I admit I have not made it easy to find! It's referenced in the
sentence "A more formal presentation of how I have modelled the
situation, including the maths, can be found _here_".
By all means check the maths, but I am sure I have not made a mistake,
and have even checked the integral mechanically by summing each of the
6,000 cells of the spreadsheet simulation to ensure that the distance
travelled is 0.45m.
Cheers, Magnus
PS I did receive your direct emails but as I mentioned I am in South
africa at the moment and mt ability to communicate is intermittent.
William Atkinson
wrote:
Magnus:
Yours is a nice piece of work.
Thanks for your considered reply.
It would be an asset to add the five calculated slide distances
(0.4xxxm) to your results
table in order to enhance confidence in the differences found between
the shell speed outcomes.
I am interested in your work in determining values of shell resistance
factor, as I find a great dearth of published values for various
hulls.
My best regards,
Bill
p.s. It must be clear to all that I have not mastered the "hide/show
quoted text" business.
Tinus
> (0.4xxxm) to your results
I expect that there may be a considerable difference because the
equation used for the velocity change of the boat is not entirely
correct. It was assumed that the velocity of the boat changes due to
drag and the acceleration of the rower with respect to the boat. This
latter term is not entirely correctly described by f'(t).
∆vb/∆t = (mr f'(t) − K vb^2)/(mr+mb)
The term f'(t) describes the change of velocity between the boat and
the rower. However this change of velocity f'(t) is also partly due to
the drag on the boat which does not act directly on the rower. So
because of that the above equation does not hold. f'(t) can be unequal
to zero while ∆vb/∆t = (K vb^2)/(mr+mb) is true. (e.g. the rower is
moving at constant speed while the boat decelerates due to drag. No
force is applied between rower and boat in this situation but still
f'(t) is not equal to zero)
This effect would be in favour of an early loaded curve which has a
long period during which the boat decelerates in relation to the
rower. The amount of this deceleration is overestimated. Also, a late
loaded curve requires a work done by the rower in order to decelerate
before the catch. An early loaded curve is able to use the water drag
and in this way the early loaded curve is able to use this negative
thing, water drag, as a functional force.
Tinus
>
> The term f'(t) describes the change of velocity between the boat and
> the rower. However this change of velocity f'(t) is also partly due to
> the drag on the boat which does not act directly on the rower. So
> because of that the above equation does not hold. f'(t) can be unequal
> moving at constant speed while the boat decelerates due to drag. No
> force is applied between rower and boat in this situation but still
> f'(t) is not equal to zero)
The correct formula should be
∆vb/∆t = (mr f'(t) − mb K vb^2)/(mr+mb)
Such that ∆vr/∆t = (-mb f'(t) − mb K vb^2)/(mr+mb) = 0 means that
f'(t)=-K vb^2 and ∆vb/∆t = (-K vb^2)/(mr+mb)
Tinus
Oops I was wrong. If ∆vr/∆t = 0 then ∆vb/∆t = (-K vb^2)/(mb) and not
∆vb/∆t = (-K vb^2)/(mr+mb).
MagnusBurbanks
understanding the hide/show logic of posting here.
Thanks, Magnus
MagnusBurbanks
including energy considerations and more informative tables, so your
suggestions are welcome.
I did also originally intend to include a graphically illustrated
method of my estimation of the drag term - I'll also include that.
Cheers, Magnus
MagnusBurbanks
Amongst other things, you write:
> The term f'(t) describes the change of velocity between the boat and
> the rower.
The term f (t) DEFINES the relative velocity (call it u) between the
boat and rower, as a function of ONLY t and without reference to the
forces involved. In fact the expression for the force F depends on how
I have defined u = f (t).
i.e. u = f (t)
Thus du = f ‘ (t) dt. [equation 1]
Furthermore u = vb –vr
so du = dvb – dvr.
thus dvb – dvr = f ‘ (t) [call this equation 2]
Momentum considerations of the free body diagram give us two further
equations
(F – K.v^2).dt = mb.dvb [equation 3]
-F.dt = mr.dvr [equation 4]
Eliminate F.dt between equations 3 & 4 and use equation 2 to
substitute dvb – f ‘ (t).dt for dvr and we get
dvb = (mr.f ‘ (t) – K.vb^2)/(mb+mr).dt
which is the equation I have published and used and just re-re-
checked.
Furthermore, you also qualitatively write “e.g. the rower is
moving at constant speed while the boat decelerates due to drag. No
force is applied between rower and boat in this situation but still
f'(t) is not equal to zero.”
This cannot be correct. If the boat is decelerating due to drag, and
the rower is moving at constant speed, there is a relative
acceleration occurring. Because the two bodies are connected by the
legs there MUST be a force between rower and boat in this situation
(leg connection is modelled by ensuring the forces F and –F on the two
bodies are always equal and opposite).
I stand by my analysis.
Cheers, Magnus
MagnusBurbanks
thus dvb – dvr = f ‘ (t).dt [call this equation 2]
everything else is fine
Magnus
MagnusBurbanks
wrote:
>
> Furthermore, you also qualitatively write “e.g. the rower is
> moving at constant speed while the boat decelerates due to drag. No
> force is applied between rower and boat in this situation but still
> f'(t) is not equal to zero.”
>
> This cannot be correct. If the boat is decelerating due to drag, and
> the rower is moving at constant speed, there is a relative
> acceleration occurring. Because the two bodies are connected by the
> legs there MUST be a force between rower and boat in this situation
> (leg connection is modelled by ensuring the forces F and –F on the two
> bodies are always equal and opposite).
>
> Cheers, Magnus
Actually this specific situation can be correct my apologies, but only
momentarily and it is consistent with my analysis which is still fine.
If you put your second statement above first, i.e. that the rower-boat
force is zero, then you can have the situation where the boat is
decelerating due to drag and the rower is moving at constant velocity.
However, this cannot happen during the recovery, because the boat is
moving TOWARDS from the rower so it's academic. There is one
transitional moment during a recovery where the rower-boat force is
zero though, and that is where the rower goes from pulling the boat
towards him, to decelerating the boat to a standstill for the catch.
The rower-boat force then passes through zero momentarily on its way
to changing sign.
Cheers, Magnus
Carl Douglas
>> moving at constant speed while the boat decelerates due to drag. No
>> force is applied between rower and boat in this situation but still
>>
>> This cannot be correct. If the boat is decelerating due to drag, and
>> the rower is moving at constant speed, there is a relative
>> acceleration occurring. Because the two bodies are connected by the
>> legs there MUST be a force between rower and boat in this situation
>> bodies are always equal and opposite).
>> ...
>>
>> Cheers, Magnus
>
> Actually this specific situation can be correct my apologies, but only
> momentarily and it is consistent with my analysis which is still fine.
> If you put your second statement above first, i.e. that the rower-boat
> force is zero, then you can have the situation where the boat is
> decelerating due to drag and the rower is moving at constant velocity.
> However, this cannot happen during the recovery, because the boat is
> moving TOWARDS from the rower so it's academic. There is one
> transitional moment during a recovery where the rower-boat force is
> zero though, and that is where the rower goes from pulling the boat
> towards him, to decelerating the boat to a standstill for the catch.
> The rower-boat force then passes through zero momentarily on its way
> to changing sign.
> Cheers, Magnus
I'm sure you don't mean that the boat is decelerated to a standstill,
Magnus ;) but that boat & rower will for an instant stand still WRT
each other?
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
No, my apologies. We agree on the equation. I corrected my hasty
examination of the equation but you still responded to this wrong
statement. Somewhere in the equations I used a factor (mb) instead of
(mr+mb).
I am currently working on an analytic approach to these equations. It
might add nicely to the computational methods.