Excellent demo of ventilation.
https://www.youtube.com/watch?v=0J0jaMIRMR0&index=36&list=WLcompared to cavitation:
https://www.youtube.com/watch?v=DijdU0rmDdc&index=12&list=FLCqf4CrtfoxFcfAyVh0wMkgMagnus talks about it around 11:45 and the fences they used to minimize ventilating the foil.
http://vimeo.com/72619588#There is quite a bit behind the reasons and why/when ventilation happens. We see it on the CFR a lot b/c as a uni with a LOT of sail area, the mast is raked WAY back which of course loads the rudders more. The pressure differential is the reason that you get ventilation and it becomes much worse as you heel b/c of additional vertical flow along the foil. Those rudders are also more prone to stalling b/c they are a higher aspect shape with a thinner section. The stall can initiate a worse ventilation event at which time they sort of enter into a "chicken and the egg" situation; one can initiate the other, but they both are part and parcel to the event when you lose lift (steerage) b/c of how relatively shallow beach cat rudders are.
I think the situation that people confuse is that a stall can start at the tip of a foil and get progressively worse, or depending on the foil shape, happen along the entire foil at the same time. Most boats today have elliptical rudders (more efficient), but they will stall all at once while a square tip (old P-19 and 18-2) will stall progressively starting at the tip. This is where the L-foil boats run into a lot of trouble; those things are typically elliptical b/c of the performance envelope, so when it stalls, night night....
Like Jake said though, you make a big course correction (think bear away to duck) and you significantly increase the angle of attack while simultaneously increasing the load due to drag from AoA and changing wind angle and therefore sail pressure. There are some good crashes in the VX40's and AC45's from this exact thing happening.
Interesting enough though is that Oracle had so much trouble with cavitation that they were having to repair their stainless rudder foils at the rudder/foil intersection daily until they changed the shape mid way through the finals.
http://biekerboats.blogspot.com/2013/09/oracle-team-usa-wins-americas-cup.html
