Chapter 10: Rudder Geometry, Shape, and Size
all hydrofoils and of airfoils as well.) Accord-
ingly, the center of water force doesn’t fall at
the geometric center of the rudder blade as
viewed from the side, but forward of this. In
fact, at a helm angle of 35 degrees, it usually
falls somewhere between 30 and 40 percent of
the fore-and-aft length of the rudder aft of the
leading edge (see the sidebar to estimate the
center of force at other rudder angles).
If a boat’s rudder has a span—vertical
height—of 2.1 feet (0.64 m) and a mean chord
of 0.9 feet (274 mm), the center of water
force will fall about 35 percent of the chord
aft of the leading edge, or 0.31 feet (96 mm)
aft. The unbalanced rudder—at a water force
of 985 pounds (447 kg)—would thus generate
a torque (a force multiplied by a lever arm) of
305 foot-pounds: 0 .31 ft.× 985 lb.= 305 ft. lb.,
or 0.096 m× 447 kg= 42 .9 kgm.
Fit this boat with a rudder of the same
area and proportions but with a 17 percent
balance, and the pivot point will be farther aft
and closer to the center of water force—in
this case, about 0.07 feet (21 mm) away. The
balanced rudder will generate a torque of just
69 foot-pounds (9.4 kgm) less than a quarter
as much! This results in much easier steering
and lighter loads on the steering gear, auto-
pilot, and not incidentally, on the helmsman.
Balance is determined by the percentage
of rudder area forward and aft of the rudder-
post axis. Figure 10-4 shows a rudder with
17 percent balance, which years of trial and
error have demonstrated to be about ideal.
Twenty percent balance is usually the absolute
maximum. More balance than this can move
the center of water force ahead of the pivot
axis, making the rudder sheer wildly or lock up
at hard over, causing uncomfortable steering.
Balanced rudders should not be installed
directly behind a skeg or keel. Since the lead-
ing edge projects out from the hull centerline
when the rudder is turned, such a balanced
rudder would catch the water flow on the
leading-edge side. This creates unwanted
turbulence. For powerboats and racing sail-
boats with spade rudders, the advantage of a
lighter and more responsive helm makes the
balanced rudder hard to beat. For cruising
sailboats, however, an unbalanced rudder
with a skeg or keel immediately ahead of it
offers better protection for the rudder and—
depending on configuration—better tracking,
particularly in a following sea.
Figure 10-4. Spade
rudder with a
17 percent balance
and a 2.32:1
aspect ratioCenter of Water Force at
Rudder Angles Other
Than 35 DegreesIf a design project requires you to esti-
mate the location of the center of water
force on a rudder at helm angles other
than the maximum 35 degrees, you can use
the following approximate formula:WFL%= 0.195+ (0.305× sin
(rudder angle))Where
WFL%= water-force location as
percent of mean chord aft of
leading edge
rudder angle= rudder angle, in
degreesThis will give you a passable approxi-
mation for rudders of normal airfoil section
shape and for angles up to 40 degrees. For
wedge-section and parabolic rudders, add
14 percent to the formula result.