Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Chapter 10: Rudder Geometry, Shape, and Size

speed of 8.72 knots, the rudder laminate shell
thickness should be as follows:

or

The foam core at the central portion of the
rudder blade—where the foam is between the
rudder-frame support arms coming off the rud-
der stock and the inside of the FRP shell—
should be least 8.0 lb./cu. ft. (128 kg/m^3 ) density.

Rudder Profile or Planform

The rudder profile or planform is the shape of
the rudder viewed from the side. There has
been an incredible variation in opinion
regarding the planform shape that works best
for different types of boats. To this day, there
isn’t wide agreement; however, certain funda-
mentals are important.

Sailboat-Rudder Planform

Earlier we saw what aspect ratio is and how
it’s calculated. On sailboats, the higher the

Thickness,in.

8 .72 kts 27. 7 in.

1,290

= 0. 187

×

= in.

Thickness, mm

8 .72 kts 703.6mm

1,290

4 .76 m

=

×

= mm

aspect ratio (i.e., the longer/deeper and nar-

rower the rudder), the more effective it is,

within reason.

Rudder A in Figure 10-12is a 20 sq. ft.

(1.86 m^2 ) rudder with a span of 9.73 ft.

(2.96 m). It has an aspect ratio of 4. 73. The

trade-off for a higher aspect ratio is that the

bending moment on the rudder is greater and

the stock is longer. This requires heavier,

stronger stock and bearings, but the rudder

blade itself is rather slender—hard to make

really strong.

• Aspect ratios of 4 and over are high,
  and aspect ratios over 5 should gener-
  ally be avoided.


• Aspect ratios between 3 and 3.5 are mod-
  erately high and are a good choice for
  most performance-oriented boats. Rud-
  ders B and C have aspect ratios of 3. 2.


• Aspect ratios between 2.4 and 3 are
  moderate and work well on almost all
  normal sailboats. Rudder D has an
  aspect ratio of 2. 6.

For the same rudder area and rudder-

stock location, the higher the aspect ratio,

the greater the turning force for a given helm

angle. However, the difference isn’t large.

Identical boats fitted with rudder A or rud-

der B would have nearly the same steering

response. Also, high-aspect rudders tend to

stall more easily at high rudder angles.

Very high-aspect rudders, such as A, are

only appropriate for sailboats with very high-

aspect-ratio fin keels, which have been reduced

to the minimum acceptable keel area. The

deep, high-aspect-ratio rudder provides some-

what more resistance to leeway—it will be

more effective as lateral plane generating lift.

Such very high-aspect rudders thus effectively

do double duty as additional keel area.

Rudders B and C have the same area and

aspect ratio; however, B is curved and formed

to have minimum area in the tip. This is to ap-

proximate the theoretical optimum elliptical

planform—for minimum induced drag at the

rudder tip. Rudder B would thus have slightly

less drag than rudder C. Otherwise, the steering

response of the two will be virtually identical.

Rudder C is simpler and less expensive to build.

Rudder D will work quite well even

though it has the lowest aspect ratio of all

four and is roughly rectangular in section.

Figure 10-11. Rudder width, or chord