RE: [Paddlewise] paddle sizing and Technology guides design

"Peter A. Chopelas" wrote:
} Subject: RE: [Paddlewise] paddle sizing and Technology guides design
>  Bob Myers wrote:
> > Slippage and efficiency are related, and what is more, it can be
> > mathematically related to power required.
> 
> I do not know on what basis you say this, but with regards to my last 
> posting I suppose this could be argued but it is not nearly enough 
> information by itself to determine efficiency, and all of the other 
> complicated relationships that would have to be included to the equations. 
>  By itself it is meaningless, see my last posting.

See *my* last posting.  I understand efficiency the same way you do -
power output / power input.  Your somewhat condescending previous 
posting tried to imply that we do not understand you or the concept
of efficiency, but that is not so.

> There is no reason to assume a high aspect ratio paddle will slip more than 
> a low aspect ratio paddle.  It will not in fact.

I've actually made no assertion about whether a high aspect ratio paddle
is more or less efficient.  Others have, including John Winters; I would
suggest you bring that up with him.  I believe that John has claimed to have
done some controlled experiments concerning this, something I don't
believe you haved claimed as yet.

What I've been primarily pointing out is that slippage is directly
related to efficiency, which is something you have been denying.

> And your arguments are not true anyway, the most efficient aircraft wing 
> (with the least amount of drag) is the SMALLEST one that generates enough 
> lift to stay in the air.  And given wings of the same size, the lowest drag 
> will occur with the highest aspect ratio.  There is a complex interaction 
> between tip vortex, skin friction, form drag, induced drag, and a bunch of 
> other stuff that affects both paddles and wings (and propellers too).

Again, what exactly does an efficient aircraft wing have to do with a 
paddle?  You have not shown any direct relationship between the 
efficiencies of an aircraft wing and a paddle. 

Just saying that it's all fluid mechanics doesn't cut it.

Paddle drag that is parallel to the direction of motion is actually
useful for thrust - airplane wing drag (and propeller drag) is not.

> True enough.  But the force applied, by itself, tells you nothing about 
> efficiency.  And much of the ENERGY you apply to the water does not go to 
> propelling the boat forward.

Right, that's what slippage tells you - how much of that energy is not
propelling the boat - the efficiency of your power usage.

> If the object was just to move water backward why not use a tennis racket, 
> all that turbulence would absorb a lot of energy, but not a lot of it would 
> go into forward motion of the kayak.

Because it would have horrible slippage rates, as I've discussed.  
You could move the tennis racket backwards very quickly, but all you'd 
be doing is moving very small amounts of water very quickly - that 
is very inefficient because too much of your power goes into 
moving water instead of counteracting boat drag - yes, moving it 
turbulently, but primary point is moving at high velocity.

The object, as I've said all along, is to move as large a mass of water,
as slowly as possible, as directly aft as possible.  Your tennis racket
comment seems to show that you have not been paying any attention to my
argument.


> > I think you're confusing paddles with propellers & wings again.
> >
> Nope, not at all.  ANY surface, that produces thrust in ANY fluid works the 
> same way: wings, pumps, propellers, turbines, insect wings, bird wings, 
> paddles, swim fins, fans, etc. etc. 

Assuming similar angles of attack, yes.  Most of those surfaces operate
at fairly low angles of attack, less than 15 degrees or so.

But what is the angle of attack of a kayak paddle?  Pretty high,
especially if you use a low blade angle as is usually recommended for
Greenland-style paddling.  How much separation and turbulence are you
going to get on the back side, no matter what its shape?  As you go to
very high angles of attack, is it not true that the upper surface of a wing
loses effectiveness faster than the lower surface?

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