pressure 4. 32 inches aft of the rudder-stock
pivot point, the torque is 3,650 in.-lb. or 304
ft.-lb.—845 lb. × 4. 32 in.= 3,650 in.-lb., and
3,650 in.-lb. ÷ 12 = 304 ft.-lb.
Or
Assuming that this is a balanced spade
rudder, with the center of water force
or pressure 110 mm aft of the rudder-stock
pivot point, the torque is 42.1 kgm or
413 Nm—383 kg × 0 .11 m= 42 .1 kgm, and
42 .1 kgm× 9. 8066 = 413 Nm.The 30-Pound (13.5 kg)
Helm Limit
The greatest force the average person can
exert comfortably on a wheel is about
30 pounds (13.5 kg, 122 N). This is really an
upper limit, as holding the wheel against a
continuous load like this will tire you out
fast. However, maximum force occurs at
hard over, and you’ll seldom have the wheel
over that far for more than an instant or two.
To find how long the tiller would have to
be to keep the maximum hand force—the
force you’ll need to steer with—to under 30
pounds (13.5 kg), simply divide the maximum
rudder torque by 30 pounds (13.5 kg). In our
preceding example, we would get 10.13 feet
(3.1 m). This is too long for many boats. In fact,on most boats longer than 40 feet (12.2 m)
LOA, it becomes difficult to arrange a tiller
long enough to give sufficient leverage or me-
chanical advantage—assuming that you
didn’t want to move the helm station some-
where else anyway.Wheel-and-Cable Steering
System
The solution is simple: use a steering wheel.
Wheel steering systems have built-in mechan-
ical advantage, and even better, they can be
located wherever you happen to need them,
rather than right aft at the rudder. In the
proverbial good old days, there was only one
way to get the power from the wheel to the
rudder: rope or wire cables. This is still a
simple, inexpensive, and reliable system; it
is the standard for sailboats, and until quite
recently it was common on powerboats.
Cable steering systems, in fact, also offer the
best helm feel.Advantage Mechanical
The required component sizes to keep
steering-wheel loads under 30 pounds
(13.5 kg) can be found from the following
formula.PART FOUR:RUDDERS AND STEERING SYSTEMS
Figure 12-3. Cable
steering (Courtesy
Edson Corp.)