As a foil moves through water, it pushes the water; accelerates it, so that it can squeeze around the profile. When the water accelerates, the pressure of the water drops. This local surface pressure is referred to as dynamic pressure.

Cavitation is the formation of gas bubbles on the fin surface due to the dynamic pressure being low enough for it to form. It’s a bit like when you open a bottle of beer or soft drink- the high pressure of liquid in the sealed bottle drops suddenly as the bottle is opened, and dissolved gas comes out of the liquid as bubbles. You can see this just with unpressurised water if you are lucky enough to fly in planes. In windsurfing, the gas can be simply air that’s naturally dissolved or increased by wave action on the water, or in extreme cases when a near vacuum is created can be formed from low pressure “boiling” of water. In windsurfing, it’s the former that occurs first.The bubbles can sit on the surface, or can be shed off in a sheet or in “blobs” (collapsing as it enters the free stream where the pressure is back to normal).

Cavitation can trigger ventilation by its effect on lift. Generally, cavitation reduces the amount of lift for a given angle of attack (with the same speed). This is known as the lift gradient, or lift slope. Cavitation has the effect of unexpectedly higher drag and loss of lift slope with increasing speed. If load is increased at speed, it can cause the fin to start cavitating due to the lower dynamic pressure, then the angle of attack increases more than a non-cavitating foil for the same load increase.

This increase in angle of attack makes the cavitation worse so it is a sudden onset effect that appears as ’hitting a wall’ with speed. A cavitating foil will have higher turbulence in the wake and a more intense trailing vortex, so this makes it more prone to a “spontaneous” spin out once it is cavitating. If the bubbles are shed, it takes time for the bubble to collapse (water has to be accelerated to fill in the void), so the low pressure in that volume of gas assists in creating the low pressure channel that can trigger ventilation. All this moving around of water uses up a lot of energy- and that energy can only come from whatever is pushing the fin through the water- so its adding to the drag.

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