Cavitation consists of a liquid turning into vapour at ambiant temperature, due to a drop in pressure. Then, an aircraft propeller
, working in air, can't cavitate.
The stall angle of a rudder depends on the aspect ratio (height/chord): high aspect ratio -> stall at about 15-20° angle of attack. But when turning, the water
already comes at an agle to the rudder, reducing the AoA. For this reason, rudders on ships generally go to 35°. On sailing boats, 45° is useful for turning tight at low speed, e.g. in port.
On a sailing boat, the main reason for losing rudder effectiveness is ventilation: air from the surface is sucked on the suction side of the rudder. This can happen when sailing fast (suction is proportional to the speed squared), as soon as the rudder top is out of the water
: very common on transom-hung rudders, slightly less on under-hull ones (needs some heeling).
When the rudder is ventilating, you can see a cloud of white bubbles of air in the wake.
To reduce ventilation, there are 3 possibilites:
- reduce the AoA: reduce rudder angle
- reduce the speed
- put the hull
at the rudder top back in the water: reduce the heel