Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

(Rick Simeone) #1

PART FOUR:RUDDERS AND STEERING SYSTEMS


On planing powerboats, twin rudders
must always be dead vertical athwartships.
It might look correct on a drawing to have the
two rudders projecting out from the hull
underbody (behind the propellers) at right
angles to the hull shell, but on a deep-V boat,
this would mean an athwartships rudder an-
gle of 21 degrees or so to the vertical. Though
it may lookright, it is wrong. The angled rud-
ders will act as diving planes or trim tabs—an
effect that is aggravated further (and unpre-
dictably) when the boat rolls.

Planing-Boat Rudder
Configuration
Figure 12-1 shows the best proportions for
rudders in relation to the hull, propeller, and
shaft on twin-screw planing hulls. The leading
edge of the rudder should be about 15 percent
of propeller diameter aft of the propeller
blades. Less is too close and can cause vibra-
tion or difficulty pulling the prop. A bit more
is acceptable, but much more and the rudder
is less effective, being farther aft in the slip-
stream. The aft edge of the rudder should
ideally be 10 percent of propeller diameter
forward of the transom’s bottom edge, but a
bit farther forward is OK if you meet all other
criteria. Siting the prop and rudder farther aft
allows a shallower prop shaft angle, but as the
rudder blade nears the transom edge it expe-
riences an increased chance of ventilation,
being no longer shielded from surface air by

the hull above. Of course, many boats do have
their rudders farther aft, so the aft edge of the
rudder is nearly at the transom. Most of the
time, including on several of my designs, it
works OK.

Steering Systems: Controlling the Rudder


Rudder Steering Torque
Having found the stock diameter, we have
also found the maximum torque, or twisting
moment (TM).
In our twin-screw planing boat example
in the previous chapter, we found that the TM
was 30,984 in.-lb. or 2,582 ft.-lb. (353 kgm or
3,462 Nm). IMPORTANT: Don’t forget to mul-
tiply total torque by the number of rudders.
This steering system, with two rudders,
needs to handle 5,164 ft.-lb. (706 kgm, 6,924
Nm). This torque governs the size of the
steering system, which we’ll examine later.
So far, we’ve examined the rudder size,
shape, and strength, but you need more than
a properly sized and shaped strong rudder;
it’s every bit as important to be able to make
it do what you want it to—all the time, every
time. A modern wheel should have nearly
fingertip control. Improperly designed steer-
ing systems and steering gear problems can

Figure 12-1.
Planing boat
rudder and prop
configuration

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