Re: sliding riggers

Charles Carroll wrote:
Carl,

Charles -

Although i do think this is a dead horse, for you I am happy to flog it a bit longer ;)

All Richard Tonks says is that "From the moment the blades touch the water the boat must accelerate."

Then he is wrong.

Please see Walter's very clear graphical depiction of velocities of boat, sculler & total system:
http://www.rowingnz.com/Resource.aspx?ID=56
This parallels other such data recorded by other researchers, yet rowers at large ignore it & its significance. For the 1st ~1/4 of the power stroke the boat's velocity is falling. That is real deceleration. Period.

In contrast, for the 1st 2/3 of the stroke the rower's CofG is increasing its velocity - it is accelerating. And it is accelerating more than the total system (which has to be true, since the system's accelerations will be close to the mass-weighted mean of those of boat & crew).


It seems to me that it might be simpler if we, like Richard Tonks, focused on just one thing at a time. Would it be amiss to limit our discussion - if only for the moment - to what the boat is doing without involving ankle bones, hands, bum, oars, blades, sliding seats, complicated levers, etc. etc, etc?

I would be amiss to do so.

The bowball is rigidly connected to the rest of the boat, so it accelerates just as the boat does. Your toes are also fixed to the boat, so they too move in lockstep with the boat. And your ankle bones are not that much disconnected from the boat, so their accelerations closely match those of boat, bowball, etc. In short, you cannot disconnect those items since they are all bound to move in synch. They therefore represent that one thing of which you speak.


Let's consider just the bow ball as it travels towards the finish line. Let's assume that all other variables remain constant. The stroke rate remains constant. The pressure applied against the pins remains constant. The ratio between the recovery and leg drive remains constant.

Now what happens to the bow ball as it moves towards the finish line? If you measure the bow ball at different points in the stroke, do you get a constant velocity? Or does its velocity vary according to where you measure it? If the latter, then at what point in the stroke is it moving the fastest? And at what point is it moving the slowest?

Hasn't simple observation yielded answers to these questions? Doesn't the bow ball reach its highest velocity right after the release?

Depends entirely on recovery technique. The body is so much heavier than the boat that rather slight changes in the tension you apply through your legs during recovery (which is necessary to overcome fluid drag on the hull in order to bring frontstops towards you) will have marked consequences for transient boat velocity. Walter's graph shows a lull in boat velocity at the finish, followed by a subsequent surge. This results from one particular style of finish & recovery, whereas other styles produce other effects including that which you describe. So there are no absolutes - which is why understanding technique really is so important, & goes so much further than mere appearances.

And
doesn't it reach its lowest velocity at the catch?

Clearly not.


Now how does a bow ball move from a lower velocity to a higher velocity without accelerating?

Is it "unscientific" to think that an object moving from a higher velocity to a lower velocity is decelerating? Is it "unscientific" to think that the reverse is equally true - if an object is moving from a lower velocity to a higher velocity it is accelerating? After all, isn't acceleration defined as the rate of increase in velocity per unit of time?

So why is it incorrect to say that a bow ball during the leg drive is accelerating? This is really all I am asking.

Because, as Walter's graphs confirm, if first decelerates & only later accelerates.


I cannot believe that we are all that far apart on this idea. It seems to me that a bow ball decelerates during the recovery and accelerates during the leg drive. Moreover to produce an average mean velocity per stroke of the bow ball with respect to the finish line, any acceleration during the leg drive must compensate for any deceleration during the recovery.

Aren't you saying as much when you write that "In every stroke there are accelerations & compensating decelerations?"

Yes.


Or when you write: ". . . you return to much the same speed at the same point on every stroke." Doesn't the very idea of "returning" to much the same speed on every stroke presuppose acceleration and deceleration?

Yes.

Isn't the idea of rowing well to find a rhythm that keeps the amplitude between these velocities as small as possible, and that allows us to reproduce the same mean average velocity per stroke for every stroke in a piece? As you write, "no net acceleration there, just the exact cancellation of the integrals over time of the positive & negative accelerations."

By the way, what is "net acceleration?" I am not clear on the term at all. The OED defines acceleration as ". . . in Nat. Phil. the rate of increase in velocity per unit of time." So by "net acceleration" do you mean the net the rate of increase in velocity per unit of time? I do not mean to be such a dunderhead but for the life of me I cannot think of an example.

Nt acceleration occurs when you increase your mean velocity, as when going from cruising pace A to sprint pace B.


And another question: why is it of such great consequence to say that Mr. Tonks "is in the business of accelerating (= kinetically energising) his personal C of G from the instant of the catch?"

Because that's the correct description of what's happening. If your mental model is fundamentally false, the lessons you hope to learn from it will lead to a dead end. If I believe that the chisel's handle does the cutting, so I hold it by the blade, I'm going to find, suddenly & painfully, that my model is wrong

The rowing stroke is an intermittent application of power to overcome a continuing if varying fluid resistance. The only way it can work even half usefully is for surplus energy to be stored, during the power application, in the increased speed of progression of the heaviest component (the body) in the system's general direction of motion - you increase its kinetic energy.


Please understand that I am not disagreeing with what you are saying. I, too, think it is the rower's business to accelerate his personal C of G. Nevertheless I wonder if this isn't subordinate to a higher end - namely, to rowing your bow ball across the finish line ahead of the other guy's. How he or she manages to accomplish this certainly involves accelerating his C of G. But doesn't the fact remain that the entelechy of rowing is a bow ball moving forward?

Unfortunately any close consideration of the bow ball amounts ergonomically & in every other relevant sense to putting cart before horse, or counting angels on the head of a pin. Nothing you can do in considering the bowball, beyond ensuring that what you do does not needlessly increase the cyclic fluctuations in its velocity (which is also the boat's velocity, & that of your toes, etc.) offers useful insights or models through which to improve your rowing

Warmest regards,

Charles

And mine to you -
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: carl@xxxxxxxxxxxxxxxxx Tel: +44(0)1932-570946 Fax: -563682
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
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