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 Post subject: learning question
PostPosted: Mon Jul 21, 2014 3:49 am 
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I had my second session behind my jet ski. There has been Nooooo freaking wind in Florida for a week so I still haven't had a chance to try it with a kite.
As I was riding about 18 mph foil suddenly succumbed underneath my feet. I just happened so fast i thought that front wing broke off. It was as if I fell into a giant pothole? It happened twice. What was that? How to avoid it? Also at times foil made funny whistling like sound. Could anybody explain please.
Thanks, Kris


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 Post subject: Re: learning question
PostPosted: Mon Jul 21, 2014 7:01 am 
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Welcome to the club, it could be several things, cavitation, ventilation or just for riding too far out of the water. What foil were you riding?


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 Post subject: Re: learning question
PostPosted: Mon Jul 21, 2014 8:19 am 
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If you were on a short rope behind the ski, you can lose lift in the aerated water behind the boat.

Did you hear a spraying/splashing sound before you went down? If so, you came too far out of the water. It is easier with the kite, so hopefully you will be some wind soon.


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 Post subject: Re: learning question
PostPosted: Wed Jul 23, 2014 3:55 am 
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I ride Zeeko Makaira Freeride wing. I wasn't to high. and I wasn't crossing wake. I noticed that when you cross it water has less density but when you cross wake it does not happen so abruptly. It was as if I literally fell nose down into a hole. I did not hit anything either. What is the difference between cavitation and ventilation?


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 Post subject: Re: learning question
PostPosted: Wed Jul 23, 2014 7:21 am 
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From: http://lancet.mit.edu/decavitator/Basics.html

There are two particularly persistent problems faced by designers of hydrofoils: cavitation and ventilation. Ventilation occurs when part of a hydrofoil pierces the surface of the water and air gets sucked down the lifting surface of the foil. Since air is much less dense than water, the foil generates much less lift and the boat crashes down. Ventilation can occur at any air-water interface.

Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.
Cavitation occurs when the water pressure is lowered to the point where the water starts to boil. This frequently happens with propellors. When a propellor is turned fast enough, the blades generate so much lift (i.e. the pressure on the lifting surface of the blades goes down) that the water flowing over the propellor blades begins to boil. When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water.


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 Post subject: Re: learning question
PostPosted: Wed Jul 23, 2014 3:02 pm 
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From the description it seems like cavitation because wing was far from surface and I was riding vertically at the moment. Again the board just suddenly nosedived. It was fun anyway :D
Thanks Hawaiis for the explanation.


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 Post subject: Re: learning question
PostPosted: Wed Jul 23, 2014 4:03 pm 
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Location: Florida--JACKSONVILLE
not trying to make a smart remark, but if you hit a big fish, would that have cause the crash?


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 Post subject: Re: learning question
PostPosted: Wed Jul 23, 2014 6:11 pm 
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We're usually dealing with ventilation. Cavitation is, generally, a function of high speed. If you get to around 45 knots you really have an issue with it, but nobody is quite there yet with kite foils


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 Post subject: Re: learning question
PostPosted: Wed Jul 23, 2014 10:31 pm 
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I agree, it's definitively ventilation. For cavitation we'll be waiting for faster foils. Well, maybe just a few of us :D


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