Why Do Rudders Fail?
We've already seen that even in the easiest of steering situations, the rudder is subject to quite high loads. Because the load on the rudder is proportional to the square of the boat's speed, it doesn't take a radical change in conditions for that load to get markedly higher. An increase in boat speed from 5 to 7 knots doubles the dynamic pressure on the rudder, and at 10 knots it's quadrupled. These loads are cyclical, and they sometimes create vibrations, both of which contribute to fatigue--inducing strains on the blade, the stock, and the bearings and the structure supporting them within the boat.