Sopa is correct that the pipe/foil comparison is apples to oranges in most cases. I just used it to show that turbulent and laminar flow can exist proportionally together. it's not just one or the the other. You are right that there is a point of separation, a bubble starts to form indicating the transition from laminar to turbulent flow.Senor Sopa, you might be right on all those points - I am not an expert so cannot argue with any certainty. I didn't mention the boundary layer - I think I need to do more reading and how it relates to flow separation. However the vortices I mentioned are a type of turbulence - or aren't they?
The Reynolds number equation for a pipe is different from a foil from what I can see - foil being a fin, wing, hydrofoil - I assume that's what you call flat plate? I realise now my mention of surfboard hull is completely wrong for the foil comparison (it has no low pressure surface).
Stephen Hawking said in one of his popular books on physics dumbed down for the masses, that there is no difference between throwing a ball inside the carriage of a moving train or a stationary train - it is still travelling at the same speed relative to the train and all speed is relative to other objects so a flat plate travelling in a stationary body of water would be the same as the body of water moving and the flat plate stationary.
Yes I have had fin hum from badly made home made fins - I didn't know they were turning the water to steam. I have experienced what is thought to be ventilation on a sailboard - when trying to sail too close to the wind and say hitting a lump of chop which would lift the board a bit - air from the surface is sucked down the length of the fin on the low pressure side and hold is lost - the board could go sideways for some considerable distance. Someone I followed on instagram posted quite a convincing picture of his foil board mast ventilating - sucking down air from the water surface - the air is not steam.
Fin cavitation is an odd one but I feel it does occur to some degree but it is not the same type of cavitation we experience with a pump impeller inside of a casing where you have bubbles collapsing during phase change at low pressure. To be honest, I have no idea as to what makes the hum, whistle or in a couple of instances a popping sound with some odd foiled fins but I feel it is some sort of cavitation.