[Paddlewise] Cavitation or Ventilation - which term is correct?

Someone contacted me backchannel and asked me to clarify my comments 
on cavitation vs ventilation - specifically wrt my statement that you 
need a one atmosphere pressure drop across the blade to cause 
cavitation.  I responded and thought I'd post part of my response 
here (with a few modifications and an additional bit about the 
velocities required).

Cavitation is well documented and I learned about it formally at 
university  pursuing an engineering degree.  My now ancient "Fluid 
Mechanics With  Engineering Applications" by Daugherty and Franzini 
(ISBN 07-015420-1) describes cavitations roughly as follows (most 
fluid mechanics texts will be similar):
 
According to the Bernoulli Theorem, if at any point in a fluid the 
velocity of the fluid increases, there must be a corresponding 
decrease in the local pressure.  If the pressure decreases to a point 
where it is less than the vapour pressure of the fluid, the fluid 
will vapourize.

The vapour pressure of water at, say, 50F is around 0.2 psi. 
Atmospheric pressure is typically around 14.7 psi.  Hence you need a 
drop of very nearly 1 atmosphere to cause cavitation.  I'm ignoring 
the pressure increase due to the depth of the water by assuming the 
paddle is used near the water surface.  This is conservative, since 
the increase in pressure with depth makes it more difficult to cause 
cavitation..

To get a 14.5psi drop on a paddle requires considerable speed. The 
water does speed up to get around the paddle, but by how much to 
cavitate?  If you use Bernoulli's theorem to calculate the required 
velocity, it works out to be around 47 feet per second (almost 8 
times the average paddle blade speed I assumed [6 feet per second, 
which is probably, but conservatively, too high!]).  That's a pretty 
steep velocity gradient across a paddle blade.

As well, one can expect that the distribution of pressure (and 
velocity) must be smooth - you won't see high pressure along the 
paddle and then a spike of low pressure with a pressure drop of 14.5 
psi.  That would require a lot of energy to change in a small space.  
Hence, to get a peak difference of 14.5 psi, you need an average 
that's an appreciable fraction of 14.5 psi.

If the average pressure drop is only 1psi and the paddle is 100 sq. 
in., that's a force on the paddle of 100 lb.  Even a 0.25 psi average 
with a 14.5 psi spike results in a 25 lb force.  Clearly this isn't 
happening.

Another phenomena associated with cavitation is the collapse of the 
bubbles as they move from the low pressure area back into the high 
pressure zone. This collapse is quite rapid and the dynamic pressure 
associated with this is so great that it causes damage to adjacent 
structures. Cavitation can cause severe pitting in boat propellers 
(hence its name - it causes cavities to form).  If cavitation was 
common in paddles, being plastic and not bronze, they'd show severe 
damage as well.  This, too, is not seen and is another indication 
that cavitation is not occuring.

The only text I have that discusses ventilation is John Winters' self-
published "The Shape Of The Canoe".  He describes ventilation as "the 
sucking of air down the face [of the paddle] from the surface". This 
has the effect of changing the effective shape of the paddle and 
limiting the opportunity for a pressure drop on the paddle.  This in 
turn limits the force that the paddle can generate, reducing the 
efficiency.  If the paddle is ventilating, you cannot get as much 
paddle force as if it were not ventilating.

Since the occurrence of bubbles on submerged paddles is coincident 
with relatively low paddle forces, this suggests that ventilation and 
not cavitation is the culprit.

Mike

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