Re: [Paddlewise] wing theory

On Sunday, June 8, 2003, at 09:50  PM, Michael Edelman wrote:

> Nick Shade wrote: -
>>>> Moving water indicates kinetic energy in the
>>>> water that could have been kinetic energy for
>>>> the kayak.
>  I replied:
>>> Not necessarily.  Water moving in the opposite direction of the 
>>> movement of the kayak indicates that energy was transferred to the 
>>> kayak. We had a lot of discussion some time ago about votex shedding 
>>> off the blade and how that contributes to the lift generated by the 
>>> paddle. And so on.
>  Nick replied:
>> Yes, absolutely. Any water motion is energy that in a 100% efficient 
>> system would have been kayak motion. In real life water motion is 
>> inevitable consequence of applying propulsive force against a fluid, 
>> but that does not mean it is not lost energy. Vortex shedding may add 
>> to lift, but the vortex is still lost energy. This not to say that a 
>> paddle that creates a vortex is not more efficient than one that 
>> somehow avoids making the vortex, but lost energy is lost energy 
>> regardless of the form it takes. 100% efficiency is only possible 
>> when pushing against an absolutely stiff object of infinite mass, for 
>> our purposes the solid ground of the earth is a reasonable 
>> approximation. Poling your kayak would be as close to 100% efficient 
>> as is possible.
>
> You say "Any water motion is energy that in a 100% efficient system 
> would have been kayak motion", but as you note, water in not a 
> perfectly rigid medium. The only way you can generate motion with a 
> paddle is to move water backwards. Water moving in the opposite 
> direction to the kayak's travel indicates energy that was transferred 
> to the kayak. Newton's second law.
>
> Vorticies don't necessarily indicate wasted energy either. We talk 
> about vortices "shedding" but the vorticies indicate a low pressure 
> zone behind the paddle which generates lift. If you had perfectly 
> laminar flow around the paddle you'd have no lift, and hence no 
> propulsion.


You are confusing a "necessary" process with an "efficient" process. 
Consider the internal combustion engine. When you car engine is cold it 
runs inefficiently. It needs to heat up before it really runs well. But 
the biggest source of inefficiency in a car engine is the fact that 
much of the energy that is contained in the gas is turn into heat. The 
engine creates so much heat that you actually need a radiator to get 
rid of the excess. That heat is energy that could have made your car go 
farther or faster. However, if you were successful in keeping the 
engine stone cold it would never run as efficiently as it could. It is 
necessary for the efficient running of the engine that it heat up, and 
it is also true that the most efficient source of that heat is from the 
prior combustion of fuel in the engine, but that by-no-means implies 
that the heat the engine produces is not lost energy and thus 
inefficient. A source of inefficiency may be harnessed to improve 
efficiency. But that doesn't eliminate the inefficiency, it just limits 
it.

It is inevitable and necessary that you move water to propel a boat. It 
is not a contradiction that the most efficient way to propel the boat 
has built in inefficiency. If you take a snap shot of the vortex system 
made by a paddle and analyze it you must see that there is energy in 
that vortex system due to the kinetic energy of the water. That energy 
had to come from somewhere. You and your breakfast of warm beer and 
cold pizza is the source of that energy. If you are creating energy 
systems outside of adding to the energy of the boat itself, those 
energy systems are inefficiencies.

The goal is to minimize the non-kayak-forward-motion-energy-systems. 
The vortex contributes to creating a forward force on the kayak, it 
does this by expending a small amount of energy into the water. You 
improve efficiency by reducing the amount of energy expended to create 
that force. Just because it is necessary that you expend this energy 
does not mean it is not lost energy and thus a source of inefficiency. 
Some inefficiency is necessary, the goal is to limit it.

Nick Schade

Guillemot Kayaks
824 Thompson St
Glastonbury, CT 06033
USA
Ph/Fx: (860) 659-8847
http://www.guillemot-kayaks.com/

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