Chapter 10: Rudder Geometry, Shape, and Size
Spade rudders should have their leading
edges angled slightly aft to shed debris—line,
plastic bags, kelp, seaweed, nets, and so on.
The forward lower corner should be some-
what rounded, not only to help shed debris
but to reduce damage from minor impacts.
Antifouling Skeg
Note that the sailboat spade rudders shown
all have a small antifouling skeg fastened to
the hull just forward of the rudder’s leading
edge. This helps avoid line, plastic bags, sea-
weed, and so on from catching and jamming
between the top of the rudder and the hull.
Traditional Sailboat
Rudders
You can see a traditional sailboat rudder—
hung on the back of the keel—in Figure 12-22,
which shows rudder apertures. Another ver-
sion is shown in the sailboat outboard rudder
with skeg (Figure 10-3). On traditional or trail-
erable small craft, an outboard rudder is often
made to kick up (as shown in the kick-up
rudder drawing in Figure 10-3). The kick-up
rudder blade is usually aluminum plate.
Powerboat Rudder
Planforms
Single-screw displacement powerboats usu-
ally have balanced rudders on skegs immedi-
ately aft of the propeller. You can see one of
good proportions in Figure 12-21. (Note: The
skegin this usage is the support for the lower
rudder bearing projecting aft from the bot-
tom of the keel, not a lateral-plane appendage
forward of the rudder, as on sailboats.)
Generally, aspect ratios between 0.8 and
2 .3 work well. A higher aspect ratio—as with
sailboat rudders—improves response, but
this is controlled by draft restrictions and
propeller diameter. On some particularly
shoal boats I’ve used rudders with aspect ra-
tios as low as 0. 6.
Usually, such rudders are better made
roughly rectangular in planform. Over the
years, though, almost every imaginable shape
has been used, and all seem to work accept-
ably, so long as the area and balance are cor-
rect and the aspect ratio is reasonable.
Sometimes single-screw powerboat rud-
ders are spade rudders (with no bearing at
the bottom of the rudder blade). In this case,
the rudder is better formed like a planing
powerboat rudder, but no consideration has
to be made for cropping the top corners for
deadrise angle.
Planing powerboat rudders are a bit
different. Such rudders are almost always
spade rudders. Figure 10-13 shows stan-
dard, stock, cast-bronze powerboat boat
rudders. And Figure 11-1 shows a custom-
fabricated one in stainless. Current practice
is to use trapezoidal-shaped rudders with
sharp corners on high-speed boats, and
rounded-corner, more-square rudders on
medium- to low-speed boats. It’s not clear
that there’s a great deal of difference in real-
world steering response between these two
planforms. Both will work well as long as the
area and balance are correct and the rudder
section shape is appropriate for the boat’s
speed.
Spade rudders for twin-screw boats,
with out-turning props, are installed out-
board of the shaft, aft of each propeller (see
Figure12-20). Since almost all planing hulls
have considerable deadrise, the tops of the
rudders (both forward and aft of the rudder
stock) are cropped down at an angle so that
the top corners don’t hit the underside of the
V-bottom hull when swung over. Out-turning
propellers (viewed from aft, starboard props
turns clockwise, port props counterclock-
wise) are standard. If the props turn in theFigure 10-13.
Stock cast rudders
(Courtesy Marine
Hardware, Inc.)