When boatspeed approaches 40 knots, very low pressure areas form on the daggerboards and rudders, causing the water literally to boil and form cavitation bubbles. This causes severe vibration and increased drag. Almost halfway through the America’s Cup Match in 2013, Oracle’s Paul Bieker devised fillets and torpedo shapes for the rudder wings.
We can see similar shapes on the rudders of the one design AC45F cats racing in the America’s Cup World Series, and on the experimental boats that the teams use for testing
Artemis skipper Nathan Outteridge with an AC45F rudder. Notice the bulb where the vertical section and the wing meet.
Oracle’s capsize on March 2 gave us a good look at the rudder wing and anticavitation shapes.