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/ ***************************************************************************Received on Wed Jun 11 2003 - 19:23:07 PDT
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