What is cavitation and why it limits the speed of flying boats
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Our America’s Cup analyst Federico Albano explains why foil cavitation poses a limit to the development of Cup flying boats
Cavitation, the limit to the speed of flying boats
An increasingly significant problem in AC75 design that has emerged in this America’s Cup is that of foil cavitation, which determines the boats’ critical speed and thus their theoretical limits of technological development.
The heat of fire provides thermal energy to water molecules, putting them in an ever-increasing state of agitation, to the point where they are able to overcome the force of pressure and “escape” away as vapor. This explains that to bring water to a boil one can either add heat energy or decrease pressure. Similarly, the pressure differential created around the foils of America’s Cup AC75s causes the water around the appendage to boil.
What is cavitation
To understand what this is all about, let us start with a simple example: a pot full of water on the stove. We can (with some approximation) imagine that water molecules are somehow forced to remain inside the pot in the liquid state by the action of (in this case mostly external) pressure. The heat of the fire provides thermal energy to the molecules, putting them in an ever-increasing state of agitation, to the point where they are able to overcome the force of pressure and “escape” away in the form of vapor. This explains that in order to bring water to a boil, one can either add heat energy or decrease the pressure so that water can boil at room temperature, as demonstrated by simple laboratory experiments.
And on foils, how does it act
But how do foils work? By generating an area of low pressure at the top, which goes to make up the lift. The magnitude of the depression increases as speed increases, and this causes that above a certain threshold (50÷55 knots for AC75s) the water above the foil begins to form vapor bubbles, precisely generating cavitation. The risks range from sudden loss of foil efficiency to possible structural damage due to the subsequent implosion of the vapor bubbles once they leave the area of greatest depression. This is why, in the last America’s Cup, boats “self-limited” in the stern. The evolution of AC75s is directed toward faster and faster boats with smaller and smaller foils (to decrease hydrodynamic drag), but a small foil must generate even more depression per unit area and thus has an even higher risk of incurring cavitation. Federico Albano
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