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

PART FOUR: RUDDERS AND STEERING SYSTEMS

On metal hulls, the entire rudder port is

usually built into the hull, and the rudder-port

compression tube is tightened down with

four bolts through flanges on the upper and

lower rudder-port tubes. The bolts should be

of 316 stainless. Diameter should be 0. 13

times rudder-stock diameter or larger, but

never less than^3 / 16 in. (4.7 mm).

The flange thickness is

Aluminum: 0. 18 ×shaft diameter, or more

not less than^1 / 4 in. (6.3 mm)

Steel: 10. 12 ×shaft diameter, or more

not less than^3 / 16 in. (4.7 mm)

Roller Bearings

Delrin and other plastics that expand and

degrade when immersed in water used to be

used for rudder bearings. Since UHMWPE has

become available, I’ve found solid UHMWPE

has few problems and gives good service if

installed as described above. The ultimate in

rudder bearings, however, are stainless or

aluminum roller bearings. Such rudder bear-

ings have closer clearances for less “chatter”

in the bearing and less “backlash.” They also

have the lowest friction for even better helm

feel (Figure 11-3).

Initial dimensions for the bearing height

can be estimated by using the allowable bearing

loads for roller bearings (Formula 11-7), but

manufacturers provide tables giving the design

loads for their bearings in different sizes and

constructions.

Another advantage of manufactured

roller bearings is that they are available in

self-aligning configurations. This means

that the bearing is free to rotate slightly to

align with any flex of the rudder stock,

thereby preventing binding, uneven wear,

and excess friction. For performance sail-

boats with high-aspect spade rudders, self-

aligning roller bearings are strongly

recommended.

Premanufactured roller bearings are

available with or without built-in bearing

seals and in configurations suitable for instal-

lations in wood, fiberglass, aluminum, or steel

hulls.

Rudder Bearing

Loads—Rudder Bearings

Top and Bottom of Rudder

All of the preceding applies to spade rudders

with no bearing at the bottom of the rudder

blade. On many single-screw boats (and some

twin-screw vessels), the rudder is supported

with a bearing in the hull just above the rudder

blade, and with a lower bearing at the bottom

of the rudder fastened to the keel or rudder

skeg. In this case, the rudder stock experi-

ences only modest bending loads; however,

Figure 11-3.
Rudder bearing
(Courtesy Edson)

Figure 11-4. Cross section of self-aligning

rudder bearing (Courtesy Jefa)