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

Wood Outboard Rudder

Thickness

Some traditional sailboats have what is

essentially a wooden flat-plate rudder hung

on gudgeons and pintles on the transom or

behind the keel. Such rudders, as we’ve

seen, are called outboard rudders. Particu-

larly large outboard rudders on traditional

craft are referred to asbarn-door rudders

(Figure 10-10). Traditionally, such rudders

were of bolted-up solid timber. Modern

wooden construction is of epoxy-laminated

plywood sheathed in glass. In either case,

the standard thickness is

Thickness, in.= 0. 12 × immersed

area, sq. ft.

or

Thickness, mm.= 32. 8 × immersed area, m^2

or

Use whichever is largest.

Example:Thus if we had a 26-foot

(7.9 m) catboat, with 9.5-foot (2.89 m) beam,

and a barn-door rudder hung on the transom

of 5.5 sq. ft. (0.51 m^2 ) immersed area, the

thickness would be:

Thickness, in.= 0. 12 × 5 .5 sq. ft.= 0. 66 in.

Thickness, mm= 32. 8 × 0 .51 m^2 = 16 .8 mm

Use the thicker measurement, and round

to a standard size. In this case, use 1^1 / 4 inches

(30 mm).

Foam-Core Fiberglass

Rudder-Blade Shell

Thickness

Many modern fiberglass rudders are hollow

surfaced with a fiberglass shell and filled with

a foam core. (In metal construction, this is of-

ten termed a double-platerudder, though such

rudders are empty inside except for metal

stiffeners, and not filled with foam.) The FRP

shell must be strong enough to resist water

pressure and must also resist impact from

debris. For standard fiberglass polyester lam-

inates of woven roving and mat, with a ten-

sile strength of 26,000 psi (180 MPa) or

greater, use the following formula to deter-

mine the FRP shell thickness.

Formula 10-5. FRP Rudder Shell

Thickness for Standard Fiberglass

Polyester Laminates

Where

kts= boat speed in knots, but never less

than 5 knots

Width= max. chord, or fore-and-aft

width of the rudder blade, in. or mm

But not less than 0. 09 in. (2.28 mm)

thick.

Example: If we had a spade rudder with

a maximum chord of 27. 7 inches (703.6 mm)

(Figure 10-11), on a boat with a maximum

Thickness,in.

LOA, ft. Beam, ft.

12. 66

=

+

PART FOUR: RUDDERS AND STEERING SYSTEMS

Figure 10-10. Barn-door rudder on a

Marshall Catboat

Thickness, mm=+× 3 .63 LOA.m6().58 Beam, m

Thickness,in.

26 ft.LOA 9 .5ft.Beam

12. 66

=

+

= 11. 15 in.

Thickness,mm 3 .63 7.9mLOA

6 .58 2.89 m Bea

=+

(m× m29)= .2mm

Thickness,in.or mm

kts Width,in.or mm

1,

=

×

2290

Formula 10-5.