This is from an interesting article in the Nov.2003 issue of Sailing World; an interview with Juan Kouyoumdjian:
SW:What are morphing foils?
JK:When you get to a certain speed and you’re asking a foil to create lift–one side has high pressure and one side has low pressure the pressures get so low on the windward side the water might boil at 60 degrees. When you get to the cetain speed, the water simply boils and the foil cavitates and suddenly luffs the boat up or it bears away. This has been known before with propellors. But we’re now in an era where we’re seeing sailing boats hitting that wall. You can shape a foil to cavitate a little later ,but the problem is those shapes are very bad for going at lower speeds. If you start with that shape you don’t get to the high speed in the first place.Therefore, the idea of having to start with something else. By using morphing foils you can gain 4,5,6 knots extra. It’s not something that will help you to keep high averages but when you’re sailing at 25 or 30 knots, if you can have that thing that allows you to go four or five knots extra on occaision,its a good idea.
SW: So how do you make the foil change shape? Is it mechanical or material?
JK: It’s a little of both. Fortunately, nature is on our side here. The overall pressure of the water on the foil is pushing things in the direction you want.There is a combination of elastic material,like a soft rubber with a mechanical device that simply,when instructed to do so,stops resisting pressure and lets itself be deformed by the pressure around it.
SW: This is some pretty radical stuff.How sure are you it will work?
JK: There are a lot of question marks.The fact of cavitation is more than theory ,but how to make things like this work is purely theory. I’m sure the first, second and third attempts will fail and by the fourth we’ll get it right. But for sure somebody has to start doing it.
Juan Kouyoumdjian has designed a 116’ 25 ton speed machine for a client in Asia. The boat, Maiden Hong Kong uses what at first glance appears to be CBTF but according to the designer since the front foil is not connected to the rear rudder it is not CBTF. OK. I wonder if CBTF,Inc will buy that logic on the F100CBTF because the same thing is true there…hmmmm
The concept of morphing foils is interesting and could conceivably have applications on very fast models like hydrofoils that sail at speeds from zero to over 20mph using pretty much the same rudder fin thruout ; even the hydrofoils operate in a range of boatspeed from 5 to 20 +…
This sounds like the guy is using variable camber airfoils for the rudder and keel; foils that will change their camber in response to increased forces acting on them.
First off, I think he is full of crap if he honestly believes that his foils are cavitating. More likely they are ventalating. You only need to reduce the pressure on the suction surface of the foils by a small amount to get a pressure lower than atmospheric pressure. At that point (given that the foils peirce the surface) you will begin to suck air bubbles down the suction side of the foil and you will create a cavity of air where the water should be. This effect will kick in at much lower speeds than true cavitation. It is virtually impossible with a surface piercing foil to get close to the cavitation speed without ventilating first.
But lets assume for a moment that he meant to say ventilation. Windsurfers have been wrestling with this problem for years given their high wing loading and high speeds. Sailboats see this phenomenon only rarely when they try to push things too hard (oversteering can cause the rudder to ventilate for example).
Based on the small amount of detail presented in the interview, it sounds like he is employing some sort of elastic and mechanical mechanism that allows the foils to camber under load. So what will that do?
Well, a cambered airfoil has a much smoother pressure distribution on the suction side than a symmetric airfoil. A symmetric airfoil will generate an area of extremely low pressure near the leading edge and then the pressure will recover rather quickly. So you get all your lift from that small area. A cambered airfoil generates a more spread out area of low pressure. There is less of a spike near the leading edge and the pressure recovery is delayed until near the trailing edge. So therefore, the lowerst pressure on a cambered airfoil will be higher than on a symmetric airfoil and it will be less likely to suck down airbubbles and ventilate. Good idea.
Wish I had thought of that…
Oh, wait! I did think of that and I have the patent to prove it… Hmmmmm. Maybe I need to give Juan a call…
Having the hull to block the ingress of bubbles definitely helps.
I was mainly dealing with windsurfers in my time and they tended to get a lot of bubbles travelling along the bottom of the board.
I have seen ventalation happen most on rudders when the boat was heeled to the point where the rudder root was right near the surface. It would be hard to get a keel into a similar attitude, but not impossible…
BTW, for those of you who have been emailing me asking about my patent, here is a scan of the coverpage:
pat5181678_2.gif