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

Chapter 10: Rudder Geometry, Shape, and Size

Formula 10-2. Rudder Area for
Displacement and Semidisplacement
Powerboats

RA%= 5. 33 − (1. 18 × SL)

Rudder Area= RA%× WL× draft

Where

RA%= rudder area percentage of

WL length× draft

SL =speed-length ratio at cruising

speed (not maximum boat speed)

Rudder Area= rudder area, sq. ft. or m^2

WL= waterline length, ft. or m

Draft= draft, ft. or m

SL or speed-length ratio is

or

Formula 10-2 should be used up to
speed-length ratio speed 2.8 (cruising speed);
higher than that you should switch to using
Formula 10-3.
Example: If you had a boat that was 48
feet (14.6 m) WL and 5-foot (1.52 m) draft,
with a top cruisingspeed of 9.5 knots, the
rudder area would be

or

RA%= 5. 33 − (1. 18 × 1 .37)= 3. 71

so

Rudder Area= 0. 0371 ×48 ft. WL× 5 ft.

draft= 8 .9 sq. ft.

or

Rudder Area= 0. 0371 × 14 .6 m WL × 1 .52 m

draft= 0 .82 m^2

Planing Powerboat Rudder-

Area Formula

For planing powerboats, rudder area is found

from a variation of Skene’s formula:

Formula 10-3. Rudder Area for Planing

Powerboats

K= (WL, ft.)^1.^5 ÷ 50

or

Km= (WL, m)^1.^5 ÷ 90. 56

Where

kts= max. speed in knots

A twin-screw planing 75-foot (22.8 m)

motoryacht, 66.34 feet (20.22 m) WL and

77 .85 tons, would work out as follows:

K= (66.34 ft. WL)^1.^5 ÷ 50 = 10. 81

or

Km= (20.22 m WL)^1.^5 ÷ 90. 56 = 1. 004

Since this is a twin-screw boat, it has two

rudders (one behind each prop) and each

rudder would have 4.76 sq. ft. (0.442 m^2 ) area,

or a bit more.

Note that additional rudder area doesn’t

harm maneuvering, but it does add extra ap-

pendage drag and extra cost in larger steering

gear. It doesn’t make sense to use rudders

much larger than required; however, boats

that do a lot of low-speed maneuvering do well

Area Km

Disp., tons

55 .2kts 0.0328 WL

WL, m

=

×

××( ,, m )^3

Area K

WL, ft. Disp., tons

100 kts 0 .01WL, f

=

×

××( tt.)^3

SL ratio0. 552

9 .5kts

14 .6m

=× = 1. 37

SL ratio

9 .5kts

48 ft.

== 1. 37

SL ratio0. 552

Knots

WL, m

=×

SL ratio

Knots

WL, ft.

=

Area 10. 81

66 .34 ft.WL 77 .85 tons, Disp.

100

=

×

××××

=

28 kts 0 .01 66.34 ft.WLL)

3 9 .52 sq.ft.

(

Area 1. 004

20 .22 m WL 77.85 tons, Disp.

55. 2

=

×

× 2 28 kts 0 .0328 20.22 m WLL)

3 0 .885 m)^3

××

=

(

Formula 10-2.

Formula 10-3.