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

PART FOUR: RUDDERS AND STEERING SYSTEMS

Find bearings that have enough area to fall

below the allowable stress (see Table 11-3).

Lignum vitae is a very hard, dense, oily

wood traditionally used for bearings in marine

applications (including propeller-shaft bear-

ings). Though practical, lignum vitae is

largely obsolete today. Babbitt bearing metal

(or white bearing metal) should be oil lubri-

cated and has been largely abandoned for use

in rudder bearings. Of the hard, tough, slippery

(low coefficients of friction) plastics, ultra-high-

molecular-weight polyethylene (UHMWPE) is

probably best for nonroller rudder bearings.

Not only is it very strong and resistant to attack

by chemicals, but it has virtually zero water

absorption, so it doesn’t expand after immer-

sion, which would squeeze or bind the shaft.

Bearing area is shaft diameter times bearing

height.

Maximum bearing height (length) is 1. 75

times shaft diameter.

Minimum bearing height (length) is 1.0 times

shaft diameter.

Example:For the lower bearing (of

UHMWPE) we find

15,310 lb. ÷800 psi = 19 .14 sq. in. required

19 .14 sq. in. ÷ 3. 75 in. shaft dia. = 5. 1 in. high

Max. bearing height is 1. 75 × 3. 75 in. dia.

= 6. 562 in.

Min. bearing height is 1. 0 × 3. 75 in. dia.

= 3. 75 in.

Accordingly, a 5.1-inch-high bearing will do,

but a bit more will reduce bearing stress; use

6 inches high.

For the upper bearing (of UHMWPE) we

find

6,789 lb. ÷800 psi = 8 .50 sq. in. required

8 .50 sq. in. ÷ 3. 75 in. shaft dia. = 2. 27 in. high

Max. bearing height is 1. 75 × 3. 75 in. dia.

= 6. 562 in.

Min. bearing height is 1. 0 × 3. 75 in. dia.

= 3. 75 in.

Accordingly, a 3.75-inch-high bearing will

do.

Or

For the lower bearing (of UHMWPE) we

find

67,685 N ÷ 5 .51 N/mm^2 =12,284 mm^2

required

12,284 mm^2 ÷100 mm shaft dia. = 122 .8 mm

high

Max. bearing height is 1. 75 ×100 mm dia.

=175 mm

Min. bearing height is 1. 0 ×100 mm dia.

=100 mm

Accordingly, a 122.8 mm high bearing will do,

but a bit more will reduce bearing stress; use

150 mm high.

For the upper bearing (of UHMWPE) we

find

30,037 N ÷N/mm^2 =5,451 mm^2 required

5,451 mm^2 ÷100 mm shaft dia. = 54 .5 mm

high

Max. bearing height is 1. 75 ×100 mm dia.

=175 mm

Min. bearing height is 1. 0 ×100 mm dia.

=100 mm

Accordingly, a 100 mm high bearing will do.

If the rudder stock has been tapered at

the upper bearing, the upper rudder bearing

would have to be calculated, using the smaller

(tapered) diameter.

The structure supporting the upper rud-

der bearing must be able to withstand the

6,798 lb. (3,063 kg) side load with a safety fac-

tor of 4. Similarly, the lower rudder-bearing

structure must withstand 15,310 lb. (6,902 kg)

with a safety factor of 4. When the lower rud-

der bearing is built into the keel structure, this

isn’t usually difficult to achieve, but keep the

large loads in mind.

TABLE 11-3. ALLOWABLE BEARING

STRESS

Material psi MPa*

Lignum Vitae 360 2.48

Babbitt Bearing Metal

(oil lubricated) 650 4.48

Plastic Bearing Material

(UHMWPE, etc.) 800 5.51

Roller Bearings

(alum. or stainless) 800 5.51

Roller Bearings (plastic) 350 2.41

*MPa =N/mm^2 (megapascals =newtons per

millimeter squared)