Rudder stock

The rudder stock is the forged steel shaft that transmits the steering-gear torque to the rudder blade and, in a spade rudder, carries the full bending moment from the hydrodynamic side force. Its diameter is set by class rules from the rudder torque and the bending moment, with the rule formula scaling the stock diameter to the rudder force, the area, and the lever arm. The upper stock runs through the rudder trunk to the tiller or rotary-vane actuator; a coupling flange or cone-and-key joins it to the blade. For a spade rudder the stock is the only support, so its diameter is larger than for a semi-spade or horn-supported rudder.