Living with cavitation

Sail-powered boats rely on fins or ‘foils’ to counteract the side-force of the wind, and to stop the boat slipping sideways. When travelling through the water at speeds around 60 knots, virtually all foils in the real world experience a phenomenon called ‘cavitation’. Cavitation will happen in a fluid when you reduce the pressure so much that it effectively boils at room temperature. The liquid turns to vapour. Cavitation happens everywhere where liquids are subject to very low pressure. It can happen on boat propellers, hydraulic pumps or on the fins of high speed fish like Tuna. It is very damaging and when the bubbles collapse, because they can do so with tiny pin pricks of huge force that will erode away the hardest materials.

The foils we have all been using so far have relied on the water passing over both of their sides just like air on the wing of an aircraft. On one side there is high pressure creating lift and on the other side there is low pressure which is actually sucking the wing up (or foil sideways). Underwater, it is typically when this suction becomes too great that we experience cavitation. The foil loses grip on the water. The pressure side of the foil now has to do all the work so the foil skids sideways until it is at double the angle (to do double the work). The trouble is that this now also doubles the drag. Imagine going for a speed record in your car and right when you are approaching maximum speed, you open all the doors wide open and pop the bonnet! You now need double the power to go any faster. This can also lead to a big loss of control.

If you design foils specifically for confronting cavitation, you step away from traditional tear drop profiles, as one whole side of the foil is now travelling in a vapour ‘bubble’. The shapes are more like sharp wedges where you are only using one surface at a greater angle. There are many high speed power boats that use propellers with this profile... but it has never been done effectively on a sailing boat.