with 10 to 15 percent more area than indicated
in the formula. Sportfishing boats and ferries
fall into this category. There’s some penalty in
extra drag, but the quicker helm response at
low speed will be worth this for such vessels.High-Speed Displacement
Power Catamaran
Rudder Area
Many power catamarans reach high speeds
without planing. Such craft are usually round-
bilged with quite slender hulls and can be
powered to go much faster than comparable
displacement monohulls. Rather than deter-
mine rudder area directly, a good rule for each
rudder (a rudder behind each hull, with a pro-
peller directly ahead of each rudder) is that
the rudder be entirely under the hull (not pro-
ject beyond the transom), with clearance be-
tween the top of the rudder and the underside
of the hull as close as practical. The rudder-
blade span or height should be 90 to 95 per-
cent of the maximum hull draft. The mean
width of the rudder blade is then 60 percent of
the height. It’s common for such rudders to be
nearly perfect rectangles; however, they can
be trapezoidal, in which case the chord at the
tip is usually roughly 60 to 65 percent of the
chord at the root. Balance is 17 percent. For
further rudder calculations (for the rudder
stock, steering gear, etc.), you then calculate
the area of the rudder you have drawn by
multiplying the span times the mean chord.Sailboat Rudder Area
For sailboats, rudder area should be between
8 and 10 percent of total lateral plane. The
higher the aspect ratio and the farther aft the
rudder, the lower the required area. Thus, if
the rudder post is right at station 10 (at the aft
end of the waterline) and the rudder is deep
with a high aspect, you could even get suffi-
cient steering response with 6 or 7 percent
of lateral plane area (on a fin-keel sailboat).Sailboat Rudders as
Directional Stabilizing Fins
Though reducing the wetted surface with
such a high-aspect rudder well-aft would
seem to make sense, in practice it’s seldom agood idea. This is because rudders on fin-keel
sailboats perform a second function: they act
as directional stabilizer fins, like the feathers
on the trailing end of an arrow.
Common belief is that sailboats with deep
and narrow (high-aspect) fin keels don’t hold
course well, and longer keels are needed for
good directional stability. In fact, a high-aspect
balanced spade rudder of sufficient area
damps out or stabilizes directional oscillations
well and can make a high-aspect fin-keel boat
quite steady on the helm (though not as steady
as a traditional long-keel design would be).
For this reason, I usually use rudder areas of
9 percent of total lateral plane on all sailboats.
Indeed, with very high-aspect fin keels,
the rudder additionally plays a larger role in
resisting leeway, and a rudder area of 10 per-
cent of lateral plane can give better upwind
performance combined with better direc-
tional stability—a steadier helm.Skegs on Sailboat Rudders
Sailboats with separate fin keels can have bal-
anced spade rudders or unbalanced rudders
on skegs. If the rudder is on a skeg, up to 12
percent of the rudder area can be assumed
to be in the skeg. Thus if you need 10 sq. ft.
(0.92 m2)of rudder area (to get 9 percent of
lateral plane), 12 percent of that 10 square
feet (1.2 sq. ft. [1,115 cm^2 ]) could be counted
as being in the skeg. If the skeg is larger, that’s
fine. The skeg will improve directional stabil-
ity and further protect and strengthen the rud-
der, but the additional area (over 12 percent
of the desired rudder area) should not be
counted toward the effective rudder area.
If the skeg gets very large—if the skeg
area is 80 percent of rudder area or more—
this is closer to a rudder hung off the back of
a traditional long keel, and none of the skeg
should be counted as rudder area.THE RUDDER IN
CROSS SECTION, AND
BASIC SCANTLINGS
There’s a tendency to think of the rudder cross
section (rudder sections) as being a classic
airfoil shape: a rounded entry, maximum
thickness roughly at 30 percent of chord aftPART FOUR: RUDDERS AND STEERING SYSTEMS