Re: [Paddlewise] Paddle flutter

From: "Michael Vandamm" <mvandamm_at_hotmail.com>

> I don't
> pretend to be an expert on this, but here's what I heard about paddle
> flutter from a U.S. national whitewater slalom kayak team member:
> (Assuming the paddle is not defective in design or manufacture,) the
> primary cause of paddle flutter is cavitation, i.e. the formation and
> separation of bubbles at the back of the blade as it is drawn through the
> water. 

This isn't correct.  Cavitation is not easy to create with a paddle, as it 
requires a significant pressure drop across the blade.  He's confusing it 
with ventilation, where air is dragged into the water when the blade is 
inserted.  But he's still not completely correct.  The flutter is caused by 
Karman vortex shedding.  These vortices can form regardless of whether 
or not they are ventilated, so that bit is irrelevant.

Karman vortex shedding is a form of instability where a vortex is spilled 
alternatively off of one and then the other side of the blade.  My own 
experience is that these _seem_ to be independent of the formation 
of large vortices on both sides of the paddle.  There is no visual indication
that the large, roughly symmetric vortices change in sync with the period
of the Karman vortices.  However, I think that this may simply be due
to the fact that I can't visually detect the changes.  It seems true that
the Karman vortices are superimposed on the ventilated vortices and 
they occur independantly.  Getting rid of the ventilation doesn't guarantee
that either vortex is eliminated, just that they become harder to see.
Tilting the blade enough will eliminate the symmetry and reduce the problem, 
as some have suggested.  This, however, sounds like the wrong approach
IMHO.  Good paddles don't have objectionable flutter even if drawn straight
through the water.  There is something else happening to John's paddle.
Since others have reported that their Eddyline MidSwift paddles don't have 
this problem, either the paddle is defective, or John is doing something to 
aggrevate the problem.  Sitting here, I don't know what the answer is.

Mike

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