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

Chapter 12:Rudder-Stock Angle, Control, and Installation Considerations

lead to dangerous accidents, even in smooth
seas. In rough conditions or at high speeds,
such problems can be catastrophic.

Stick It to ’Em: The Tiller

Small and medium-size sailboats have the
simplest rudder control system there is—the
tiller. Not much can go wrong: either the
stick’s strong enough or it isn’t. Figure 12-2
gives the minimum height and width of
square-section tillers at their widest part—at
the rudder—for wood, handheld tillers of var-
ious lengths. The tiller should gradually
taper from this square section to a round end
11 / 4 inches (30 mm) in diameter, the ideal size
for a firm handgrip. A tiller with these propor-
tions will not break even if a 200-pound
(90 kg) man falls on it.

Tiller Limits

Tillers, unfortunately, have their limitations.
You can use a tiller only from a location di-
rectly adjacent to the rudder, and there’s also
a limit to how large a rudder the tiller can
manage without becoming too long.
Example:Say your sloop has a 10-square-
foot (0.93 m^2 ), airfoil-section, balanced

rudder and cruises at about 7.7 knots. Using

our rudder force formula (Formula 11-2), we

find:

7 .7 kts × 1. 69 = 13 ft./sec.

or

7 .7 knots × 0. 514 = 3 .96 m/sec.

The maximum water force on this rudder

would thus be about 845 pounds (383 kg). As

we saw in the previous chapter, this water

force effectively acts through a center of pres-

sure, which falls somewhere aft of the rudder

post, creating a lever arm. The lever arm for

the center of water force on a rudder—relative

to the rudder-post axis at hard-over 35-degree

helm—is found as described earlier.

Assuming that this is a balanced spade

rudder with the center of water force or

Water Force 0 .5(13ft./sec.)10sq.ft.

84

=×^2 ×

= 5 5lb.

Figure 12-2.

Required tiller

section

Water Force 0 .5(3.96 m/sec.

0 .93 m 52.

)^2

2

= ××

× 555 =383 kg