and forth, back and forth, over and over. This
is like having an NFL linebacker jump up and
down on it over and over for hours and days
at a time! Hydraulics are much easier to in-
stall than cable systems, but the cylinder had
better be mounted strongly.Tiller-Arm Strength
The tiller or tiller arm (Figure 12-8) is usually
fabricated and supplied by the hydraulic-
steering manufacturer. If you need to design
a custom tiller arm, the tiller is a cantilever
in pure bending. The load at the cylinder-pin
end is the rudder torque (as we found earlier)divided by the length of the tiller arm. You
can then use standard beam theory to calcu-
late the required section modulus and
moment of inertia for the tiller. The safety
factor should be 4 over ultimate tensile
strength with an allowable deflection of 1: 300
or less. The keyway and other details can be
taken from the standard propeller-shaft
dimensions for shafts of the same size.Drag-Link Size
Tiller arms must be custom-designed only in
special circumstances, but you will always
need to specify the drag link that connects
twin rudders inside the boat (Figure 12-9).
The drag link is in pure compression with
two pin ends. With the hydraulic cylinder or
cylinders attached directly to the tiller arm or
arms, the drag link is designed to withstand
60 percent of the total rudder torque (both
rudders combined), with a safety factor of 4.
Take our example twin-engine boat from
the last chapter. We foundTM= 8,512 lb.× 3. 64 in.= 30,984 in.-lb.orTM= 3,839 kg× 0 .092 m= 353 kgmThis is the torque from one rudder. The two
rudders combined produce 61,968 in.-lb.PART FOUR:RUDDERS AND STEERING SYSTEMS
Figure 12-8. Bronze tiller arm for a drag
link or hydraulic cylinder (Courtesy
Marine Hardware, Inc.)Figure 12-9. Drag-link assembly with jaws for
clevis pins (Courtesy Marine Hardware, Inc.)Figure 12-7.
Hydraulic
cylinder layout
Figure 12-6.
Hydraulic steer-
ing: 2 helms,
2 rudders
(Courtesy Edson
Corp.)