Subject: RE: [baidarka] Superiority of Native paddles
From: Peter A. Chopelas (pac@premier1.net)
Date: Fri Jun 22 2001 - 23:48:30 EDT
On Wednesday, June 20, 2001 7:59 PM, Michael Daly
[SMTP:michaeldaly@home.com] wrote:
>
> Let's try again. The flow is normal to the paddle power face. The chord
is not
> normal to the flow. You are using the width of the power face instead of
the
> thickness in determining the AR.
>
This statement is impossible unless the water can flow through the face of
the paddle. Even if you pull the paddle strait back (which is seldom done
in actual practice, just watch carefully next time you are on the water),
the flow over the surfaces of the blade will move chord wise as the water
moves either left or right (or sometimes splitting down the middle) to get
around the blade. So the flow over the surface is not normal, but parallel
to the chord. The idea of using the thickness of the blade to determine
the AR is non-sensical, there is no "area" for the thrust to act upon, and
the flow over the surface does not move parallel to the thickness of the
blade.
As far as each particle (or "finite element") of water is concerned it just
flowed over the surface cordwise, when it accelerates around the edge
(forming a vortex), the reaction is thrust against the blade. There will
be a lot of turbulence and a large vortex as this happens, meaning there is
a lot of wasted energy. But it is this large vortex that allows you to
"push" against the water, it is just an inefficient way to do it, but it
does work. A high AR blade reduces the size of the vortex, and reduces the
energy input requirement.
And a properly shaped, high AR surface swept through the water in a arc
(rather than pulled strait back) is the most efficient of all be cause the
vortex and the turbulence becomes almost (but not quite) non-existent. Go
out and try it.
Peter
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