BOATYARD
crisper steering and more speed. Only thing is, he added, “there’s lots
of stuff going on under the boat, and nobody totally understands it.”
How much toe is enough? Both Critchett and Gerr agree that the
best way to set the rudders is to run the boat in a straight line, at
moderate speed, and disconnect one rudder. Let it free-trail and
mark the angle it takes. Repeat with the other rudder and set the
tie bar to lock in those angles. “It’s a game of just mess with it and
see what works best,” said Critchett. “Do your sea trials to set up the
steering. Once it’s dialed in, it doesn’t have to change.”
Ackermann’s Angle
That’s not all, if you want your rudders to be tuned even better. Think
about your car for a minute: When you go around a curve, the inner
front wheel traces a circle of less radius than the outer wheel, the
difference being the distance between the wheels. The correct steer-
ing angle is therefore different for each wheel, the inner one need-
ing more angle to follow the tighter radius. Same with a twin-screw
boat—in turns, the inboard rudder should be cranked over just a
little bit more than the outer rudder. But the wheels on a car, and the
rudders on most multi-engine boats, are connected with a tie bar, so
how can the steering angles be different for each wheel/rudder?
Way back in 1817, Rudolph Ackermann, an English agent for Ger-
man carriage-maker Georg Lankensperger, came up with a simple
arrangement of steering arms and tie bars that solved the geometry
problem. Both Lankensperger and Ackermann patented the setup,
in Germany and England respectively. According to the Royal So-
ciety of London however, the mechanism was actually invented by
Erasmus Darwin, grandfather of Charles, in 1759.
Whatever. The basic principle of Ackermann steering is to toe-in
the steering arms on a carriage or a car, or the tillers on a boat (in-Adjusting rudders often means following a sea trial with mathematics.