Chapter 12:Rudder-Stock Angle, Control, and Installation Considerations
too much friction and play in these cables for
larger boats or more complex runs, however.
Still other mechanical steering system
linkages are available from companies such
as Edson, Lewmar/Whitlock, and Jefa, among
others.
Steering Refinements and Installation Considerations
We’ve reviewed the basics of sizing and se-
lecting rudders, of the required strength of
rudder stocks and bearings, and of selecting
the steering gear itself. There are still other
considerations for a good steering response.
A Problem with Buggies
Over a hundred years ago there was a prob-
lem with buggies—yep, as in horse and buggy.
When one of these horse-drawn contraptions
turned a corner, the two front wheels both
pivoted to the same angle. So what’s the prob-
lem? Well, since the buggy has width, the out-
side wheel was turning along a larger-radius
circle than the inside wheel. To keep the
wheels from skidding during a turn, the inside
wheel really should pivot to a greater
angle (to fit a tighter turning circle) than the
outside wheel. Rising to the challenge, a fel-
low named Akermann realized that if the
wheels were mounted on independent axles
connected with a drag link to two arms with
some in-angle (toe-in), you would get a buggy
with front wheels that automatically adjust
themselves. The Akermann steering geome-
try—in complex modern variants—is still
used on all cars and trucks. Less well-known
is that it’s useful on twin-screw boats.
Akermann Steering
Figure 12-18 shows the Akermann steering
setup for an average twin-engine vessel. The
dotted lines show the two rudders with the
helm midships. You can see that the tiller
arms (connecting the two rudders through
the drag link and also connected to the steer-
ing mechanism—not shown) are angled in-
board at 10 degrees on both sides. When you
put the helm over, the rudder on the outside
of the turn will pivot to less of an angle than
the inboard rudder. The solid lines show the
steering system at hard over in a turn to
starboard. Akermann’s geometry puts the in-
board rudder at 40 degrees, while the out-
board rudder is only at 35. The whole thing’s
automatic and works the same—mirror im-
age, of course—both port and starboard.
“AKERMANNIZE” YOURBOAT?Of course, most
boats—unlike all cars—don’t have Akermann
steering. Akermann steering is, however, worth
considering. You don’t notice the “skidding” of
a boat’s rudders (without Akermann geometry),
because they’re out of sight underwater. Nev-
ertheless, when both rudders turn to the same
angle, the outboard rudder’s actually too far
over and stalling somewhat (imposing excess
drag), while the inboard rudder isn’t over far
enough (insufficient steering response).
If you retrofit or design a boat with
Akermann steering, you’ll find it will have a
slightly quicker, more positive helm feel and
higher speeds during maneuvering. Indeed,
the principal reason most boats aren’t fitted
this way is that it slightly complicates initial
machining and setup. Except for the initial
machining, Akermann steering doesn’t cost
any more than the standard setup. The
builder simply has to specify that the key-
ways on the rudder stocks be milled at theFigure 12-18.
Akermann
steering for
twin-screw boats