On the Rudder

By Volker Nolte

Hammersmith, GREATER LONDON. United Kingdom Cambridge University Boat Club, Trial Eights for the 2017 Boat Race The Championship Course, Putney to Mortlake on the River Thames. Photo by Peter Spurrier.

With rudders, we are always trying to strike a balance.  The size of a rudder is directly correlated with its drag, which we try to avoid at all costs. And yet its size also determines its turning impact. Where we position the rudder matters as well, but like size and shape, it all depends on the situation. It’s why, for example, we choose bigger rudders over smaller ones when racing on winding head-style courses. The force that a rudder generates on the shell has two components with different directions relative to the boat: parallel and perpendicular to the longitudinal axis of the boat. The parallel component of the rudder force is the resistance that slows the boat down. The perpendicular component creates a torque on the boat that makes it turn. The goal is to reduce the resistance component while increasing the turning component, but the two are strongly connected. You can only generate a turning force by setting the rudder at a certain angle to the longitudinal direction of the boat, which consequently causes a resisting force. In general, the turning force increases more than the resisting force between 20 and 30 degrees. This is the sweet spot, so to speak. Using rudder angles of more than 30 degrees will only increase the turning force a small amount, while slowing the boat dramatically.