[Paddlewise] Vortex and thrust!

Hi folks,





It just so happens that I had designed a diving fin once for a very large and
well known manfacturer, and know quite a lot of about the hydrodyamics of this
subject.





I will only say it is way more complex than even the people in the industry
ever relize.  It is not intuitively obvious how anything that creates thrust
in a fluid works, so scientifically proper explainations only confuse most
people.  To make matters worse,  the mechanics of the human leg motion are not
very well adapted to efficent water propulstion and any fin design at best is
a very severe compromize.





I have examined these new style fins and find them interesing attemps to solve
a number of problems.  They are a big improvement in some ways, but still are
not very effiecent compared to what I think is possible.  and it is not likly
they would improve a paddle, the stroke mechanics are very different than the
motion of a leg.





Vortex lift can be used to generate thrust on both sim fins and paddles,
assuing the designer know enough about the phenominon to take the best
advantave of it.  Vortex generated thrust has the advantage of being very
forgiving of techique and turbulence, which is great for WW paddles, but it is
not very effeicent compared to what is known as potiencial flow lift, which
requires much more precise shapes, cleaner flow and more controled technique.
All which are possible in sea kayaking.





Needless to say, a high aspect ratio [long narrow blade], with careful
techique and a proper foil profile will be way more efficient for long
distance low speed crusing that a low aspec ratio paddle blade designed to
take advage of vortex lift.  But the high aspect ratio cursing paddle will not
perform well in turbulent water, or in rapid high thrust manuvers since it
will eaisly stall and lose thrust.





But a low aspect ratio blade designed to take advantage of vortex lift would
be well suited to rapid thrust manuvers, and in turbulent waters.  As in WW
and surf kayaking.





However, a well designed low aspect ratio paddle would way out perform a high
aspect ratio blade, so you have to have compareable shaped blades to even
relize this.





I have never seen a paddle optimally designed to take full advantage of either
situations since likely these principles are not very well understood by the
industry (this is also true of swim fins as well).  





But OTOH, we always ask more of a paddle in either type of usage than simply
moving strait forward.  So even the best design is only a compromise.  The
wing paddles for example attempts to optimize the design for one conditon,
high powered strait ahead trust, and it makes using them for skulls or
stearing or rolls diffcult.





though, no doubt, VERY significant improments to paddle design are possible
for recreational paddlers, if anyone would bother to learn something about the
fluid mechanics of a paddle stroke.





For now, I perfer to stick with the high aspect ratio native designs that have
evolved over a long period of time by people who develped the kayak because
their lives depended on it.  And they work quite well.





Peter Chopelas





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