Re: [Paddlewise] A new way to teach the forward stroke?

rebyl_kayak_at_energysustained.com wrote:


 > 1. Arm mass 4kg (roughly 6% of my body weight)
 > 2. Paddle mass 800g
 > 3. Paddle height displacement 35cm
 > 4. Fraction of paddle stroke time that potential energy is being 
dissipated
 > 0.5
 > 5. Average crusing power 100W
(numbered for convenience)

Hmmm...

1: Is that the weight of BOTH your arms?
2: Nice paddle! Mine is 1300.
3: Is that at one hand, or in the middle?
4: What's the need for that? It only matters how many times you store 
and release that potential energy, not how long it takes to release it.
5: That sounds VERY high. Wasn't that what a trained biker could deliver 
for one hour? I'd say 50 is more likely.

Assuming that both your arms and paddle together weight 8.800 grams, you 
measured the height at the center of the paddle, and you make about 1 
stroke per second, you'd get about 31 watts.

 > This gives me a power contribution of 17 watts while the paddle is
 > falling through the water and an average power contribution of 8 watts
 > being 8% of my paddling power.

With my more optimistic numbers 50%.

 > I'd suggest that even in a relaxed arm there will still be work done as
 > muscle fibers retain some tension in balancing the arm and paddle
 > weight, also as Matt says the energy to forward conversion efficiency
 > would be an issue too. So as a guesstimate if a third of the energy was
 > converted then 2% to 3% of ones paddling power might come from the
 > potential energy.

A conversion of potential energy into kinetic energy (as is done is the 
forward-resting-stroke) is 100% efficient. It's as efficient as a 
pendulum, and see how long it can keep going, converting kinetic energy 
into potential energy and vise versa.
Yes: There will be losses in muscle tension - but those doesn't change 
the energy _output_. It only changes the _input_ of the system. It will 
dissipate as heat.
If 30 watts come out of dropping your paddle, then 30 watts will be 
transferred to the water, some way or another. There is no where else to 
go, apart from some joint friction and air resistance.


 > I'm not sure that this would be true for a an arm moving in a typical
 > 'vertical' or 45 degree paddling stroke. The tension in an arm as it
 > controls paddle position and balance might counter the downward movement
 > of gravity.

There will be some tension in your upper elbow, keeping it straight - 
but that won't interfere with upward or downward motion from the 
shoulder. There will be some tension in the upper shoulder:You have to 
keep drawing that arm over your chest, to keep the paddle from falling 
over your lap. These muscles shouldn't interfere with moving the paddle 
up or down - or not much.

The lower arm has no tension at all, except in the fingers. That arm 
could be replaced by a rope. It just hangs from the paddle.

 > This would also dissipate energy as the muscle fibres are
 > repetitively triggered faster than their relaxation time to maintain
 > tension.

We'd need number on that: Oxygen use at rest, during exercise and at 
static tension. Does anyone have those numbers? I'd be interesting to 
include that in this discussion.

 > Is it possible to paddle efficiently with relaxed arms during
 > the downward phase of the stroke? If I've understood, then that is one
 > of the points you are making

Yep! Mostly relaxed _shoulders_ though, and at least one completely limp 
arm.

 > and the principal benefit could be as much
 > in minimising energy dissipated in a tensioned arm as in the potential
 > energy contribution.

That's actually why I designed my paddle-carrier: Simply to relieve 
muscles that (as I thought) add nothing to forward motion. Only much 
later did it occur to me that there might be more to it.

Niels
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