[Paddlewise] paddles, etc...

Dave Carlson says:

(I said)
> > There's still an error here. You can't assume all force applied goes
> > into forward movement of the boat. No paddle or propellor is 100%
> > efficient at turning effort into forward motion, and indeed,  John in
> > the following paragraph quotes a study stating that racing paddles are
> > 87% efficient which contradicts the prior claim.
> 
> Nick would have been perfectly correct, and I think he meant to say that
> "all the force
> that is in the direction of the motion desired will  go directly to
> propelling the
> boat."  This is required to satisfy Newton's  "theories" of motion. 
 Of
> course, force
> is not the same thing as power.  While the force will go into propelling
> the boat, some
> of the power used to generate that force will be lost.  This is where the
> inefficiencies come in.

There's still an error lurking in there, I think. If Nick is talking
about the total force applied by the paddler in that direction, then
he's making the error I noted above, i.e., neglecting paddle efficiency
which we know is significantly less than 100% from John's figures. If
Nick is talking about that component of the force that produces useful
work/thrust/propulsion in the desired direction, it's a tautology, i.e.,
the statement becomes that the component of force that produces thrust,
produces thrust.

Much of the confusion in this debate does stem from a confusion of
terms. Originally I posited that the utility of the Greenland blade
comes from its biomechanical efficiency; humans work more efficiently-
that is, they can produce a greater total work output, by working at low
force and high rates than at high force and low rates. This is the
bicycle gearing analogy, and I think those that objected to it were
confusing efficiency with maximum power output. I had a nice off-list
exchange with one poster in which we cleared that up.

There are actually *three* different kinds of efficiency that have been
discussed here without it always having been made clear what was being
referred to. One is the biomechanical efficiency: The amount of work
done by the paddler divided by the amount of useful work produced.
Second is the efficiency of the paddle in turning force into thrust. And
third is one that has been particularly confusing- the efficiency with
which the paddle couples to the water. That is easily confused with #2;
although they're strongly correlated, they're still different things.
Two paddles might offer the same resistance per unit of force applied
and yet produce significantly different amounts of thrust. This doesn't
violate Newton or the second law of thermodynamics.

I think John and Nick may be confusing biomechanical efficiency with the
efficiency with which the paddle couples to the water. It is certainly
true that larger paddles couple more efficiently to the water. As paddle
size increases, geometry shows us that the area increases faster than
the perimeter so less water escapes around the paddle relative to the
amount moved by the paddle. 

Ideally we'd use a paddle of infinite area, which would have zero slip,
since the ratio of the are to the perimeter would be infinite ;-)
Racers, who are maximizing power output and not total energy conversion
efficiency do tend to use larger paddles- but in longer races, they may
use smaller paddles for increased efficiency in total energy conversion.
Larry Edwards told me some time ago that his small-bladed Ultra Swift
paddles had been used successfully in a number of long open-sea races
for that reason.

-- mike
---------------------------------------------------------------
Michael Edelman   mje_at_spamcop.net
http://www.foldingkayaks.org (nomadics)
http://www.findascope.com (choosing a telescope)
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