Chapter 10: Rudder Geometry, Shape, and Size
We started with a simple wedge-shaped,
transom-hung rudder from Marine Hard-
ware. It is their stock rudder (OBRI 1.375 -
B). We simply added fairing putty to one set
of blades to achieve your Parabolic Section.
On a second section, we ground down the
trailing edge slightly and used fairing putty
to achieve your Intermediate Section. The
boat (our new 341-ft. inboard powered by
twin 380 Cummins) topped out at 34.5 mph.
There may have been a very slight improve-
ment in speed with the Parabolic Section
(0.1 mph). The Intermediate Section
was equal to the Parabolic Section or per-
haps slightly better (0.1 mph again). I say
“perhaps” because I have found that even
with a GPS, a 0.1 mph change in speed is
usually not worth noting since so many
variables can alter speed on each run. Load
conditions, wind, and wave height were
similar on each run. Most interesting to me
was that I could not detect any difference in
handling. The only difference was that the
intermediate-speed rudder did have a very,
very slight tendency to burble or cavitate
at high angles at high speed, but this was
negligible.This indicates to me that though the
intermediate-section rudder seems to be of-
ten overlooked, it is preferable at speeds up
to about 30 or 32 knots.
Rudders for High-Speed
Multihull Sailboats
Wedge-section and parabolic-section rudders
are for use on powerboats. Such rudders
create too much drag for racing or high-
performance multihulls under sail, yet these
boats can reach 30 knots or more. For such
craft, airfoil-section rudders—as thin as prac-
tically achievable and with their maximum
thickness farther aft—are best.Skeg- or Keel-Hung Sailboat
Rudders
On traditional sailboats, the rudder is fas-
tened to the trailing edge of the keel. More
modern sailboats may have the rudder run-
ning along the aft end of a large, full-height
skeg. In either case, the optimum rudder sec-
tion is the trailing portion of the standard
airfoil section, starting with maximum thick-
ness at the rudder stock, just aft of the keel or
skeg. You can see this in Figure 10-8.
As with airfoil-section spade rudders,
such skeg- or keel-hung rudders will be
nearly as effective and with nearly as little
drag if made flat with a rounded taper to
the trailing edge as shown.
RUDDER-SECTIONOFFSETS Table 10-1, “Foil-
Thickness Form Dimensions,” gives the offsetsFigure 10-8.
Skeg- or keel-hung
sailboat rudder
sections