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 *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
The easy way to recognize ventilation is if the bubbles stay in the water and rise to the surface, it is ventilation because cavitation bubbles would have collapsed and disappeared shortly after they appeared. With ventilation the bubbles contain surface air, cavitation bubbles contain almost nothing (vacuum). One of the most useful aspects of paddle technique to work on is the elimination of ventilation, splashing or anything that makes noise. These all involve bringing air down into the water which will decrease the possible efficiency of the stroke. Just getting a clean entry with the paddle stroke will go a long way to improving overall efficiency. Generally this requires getting the paddle blade fully submerged before applying power to the blade. Nick On Wednesday, June 11, 2003, at 08:21 PM, Michael Daly wrote: > 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. Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 USA Ph/Fx: (860) 659-8847 http://www.guillemot-kayaks.com/ *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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