Re: [Paddlewise] paddle sizing and Technology guides design

From: "Peter A. Chopelas" <pac_at_premier1.net>


> 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.
> 

In "The Shape of the Canoe", John Winters writes:

"... my tests show that the lower aspect ratio paddles are somewhat more efficient
  (Fig 50 - he shows a graph with thrust decrease vs aspect ratio).  This appears
  to fly in the face of aeronautical research [...]  A possible explanation is that wave
  making resistance is higher for wide shallow paddles. "

He goes on to say that the paddle is used at the interface of two fluids of different
properties and that there is an impulse effect and not just a steady-state flow, both 
of which can affect the results relative to the aerodynamic theory.

Experiment beats theory any day, especially if the experiment doesn't leave out
what the theory does.
  
> I also have hundreds of miles in canoes, much less in kayaks, but both with 
> Euro paddles and only recently with native style blades.  The high aspect 
> ratio blade is more efficient.

Your experimental background should make you less inclined to go with a 
subjective conclusion.  Have you actually _measured_ this efficiency or have 
you just decided the paddle makes you feel better?  

>  Larger also gets heavier, and must be stiffer.

And longer doesn't?  

Don't forget you are comparing a thick wood paddle to a thin composite paddle.  
Most woods have stiffness-to-weight ratios that can't be beat by other materials 
without going to extremes.  Unless you go to foam-cored graphite/boron/whatever 
(to get the volume up), wood almost always wins.  

> For both airplane wings and propeller (and paddles), the component of drag 
> that goes down with increasing AR is called induced drag.  

Given that the chord length of a paddle is next to zero, wouldn't that imply that
induced drag is negligible?  Assuming here a composite blade.

Or to put it another way, the drag on the blade as a flat plate swamps the 
induced drag.

Besides, why would you want _less_ drag?  Drag is what makes the paddle work!

> You must use a paddle with a smooth low pressure face.  Most commercial 
> paddles have very poor low pressure faces, they attach the handle on the 
> "back" (side opposite from the "thrust" face) and make no attempt to smooth 
> the attachment.  These already have spoiled flow over the low pressure side 
> and the test would not be valid.
> 
My expectation is separation of the flow at the edge of the paddle with eddies
coming off both sides, possibly with Karman vortex trails causing flutter.  How
does a wide, flat(ish) blade result in a flow on the back face?

Mike




***************************************************************************
PaddleWise Paddling Mailing List - Any opinions or suggestions expressed
here are solely those of the writer(s). You must assume the entire
responsibility for reliance upon them. All postings copyright the author.
Submissions:     PaddleWise_at_PaddleWise.net
Subscriptions:   PaddleWise-request_at_PaddleWise.net
Website:         http://www.paddlewise.net/
***************************************************************************