Chapter 10: Rudder Geometry, Shape, and Size
The greater the distance the rudder is aft of
this combined center, the greater the lever
arm it has with which to twist the boat
around, and the more effective it can be. For
the same steering effect, a smaller rudder can
be used if it’s farther aft. Similarly, a boat with
a deep forefoot or a steering fin forward will
respond more quickly to the helm than a
boat without. This isn’t of much importance
on average craft, which generally have fairly
good proportions for this purpose; however,
long, shallow hulls or very high-speed,
shallow-bodied planing hulls sometimes
benefit from the addition of a small fixed steer-
ing or skid fin forward—around midships.
Sizing and Locating
a Skid Fin
You have to be careful not to overdo a skid
fin, as too much area forward makes a boat
too quick on the helm. In rough seas, such a
fin could broach a boat or cause it to trip over
the forward fin and capsize. A rule of thumb
is that the skid fin—when one is required at
all—should be about 10 to 15 percent of the
length of the waterline forward of the center
of gravity, and that the area of the skid fin
should be about 80 percent of the rudder
area. (If you have no exact information, you
can assume that the center of gravity of a
high-speed planing hull is about 60 percent of
the waterline length aft of the bow at the
waterline.)
Aspect Ratio, Rudder Area,
and Your Boat’s Bank
A less-well-known consideration of rudder
design is that the rudder’s aspect ratio affects
how your boat banks in a turn. When you put
the helm over for a hard turn at high speed,
centrifugal force acts on the boat’s center of
gravity, pulling outward. Since the center of
gravity of almost all planing hulls—in fact,
of most boats of any type—is well above the
waterline, this tends to make the boat heel
outward in a turn. The outward bank is
uncomfortable, and decreasing it or getting
an inward bank is ideal. The force of water
pressure on the underside of a planing hull’s
outboard chine counteracts this outward
heel considerably, but the water force acting
on her rudder also helps. The deeper the rud-
der (i.e., the greater its aspect ratio) and the
more area it has, the more effective it is in
generating a sure-footed inward bank.Types of Rudders
Rudders come in two basic types—inboard
rudders, or rudders that are mounted entirely
under the hull, and outboard rudders, or rud-
ders that are hung on the transom and thus
project aft of the hull proper. These may each
be divided again into balancedand unbal-
ancedrudders. A balanced rudder has some
of its area forward of its pivot or turning axis,
whereas an unbalanced rudder pivots orFigure 10-2.
Forces from a turn
to port