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

Chapter 11: Rudder-Stock Size, Construction, and Bearing Specification

So

Bearing OD = 5. 000 in., + 0. 000 in., –0. 002 in.

Rudder-Port Tube ID = 5. 004 in., + 0. 002 in.,

–0. 000 in.

or

Bearing OD = 130 .000 mm, + 0 .000 mm,

–0.053 mm

Rudder-Port Tube ID = 130 .107 mm,

+ 0 .053 mm, –0.000 mm

The opening at the bottom of the rudder
port should never make contact with the
shaft, so this opening needs to be larger than the
bearing diameter. Use twice the shaft allowance
for the bearing ID, or a bit more. Thus for our
3 .75-inch (100 mm) shaft, the ID of the opening
in the rudder port (below the bearing) should
be

0. 006 in.× 2 = 0. 012 in., and 0. 012 in. +

3. 75 in. = 3. 762 in.

or

0 .159 mm× 2 = 0 .318 mm, and 0.318 mm +

100 mm = 100 .318 mm

Rudder-Port Tube Wall

Thickness

Aluminum = 0. 13 ×shaft dia., minimum

not less than^1 / 4 in. (6.5 mm)

Steel, Stainless, Bronze = 0. 08 ×shaft dia.,

minimum

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

Example:If we were building this rudder
port into an aluminum boat, the rudder port
would be of aluminum pipe or tube. Minimum
diameter would be

Wall min. = 0. 13 × 3. 75 in. dia. shaft = 0. 4875 in.
or
Wall min. = 0. 13 ×100 mm dia. shaft =13 mm

We selected a 5-inch (130 mm) OD rudder
bearing, so the OD of the rudder port tube
would be

0. 4875 in. wall× 2 = 0. 975 in. + 5. 00 in. bearing

= 5. 975 in. OD; use 6. 00 in. OD, or slightly

greater as convenient for the lower rudder

port tube.

or

13 mm wall× 2 =26 mm +130 mm bearing =

156 mm OD; use 160 mm OD, or slightly

greater as convenient for the lower rudder

port tube.

Rudder-Port Packing Gland

The lower rudder-port tube is fastened to the

hull and houses the rudder bearing, the packing

gland that makes the rudder port watertight,

and the upper rudder-port compression tube,

which squeezes down on the packing to

make it watertight.

Generally, the packing for the gland is best

made of standard flax packing. Even better is

Teflon-impregnated flax packing. This has lower

friction and longer life. (Never use graphite-

impregnated packing,as the graphite/carbon

will cause severe corrosion in seawater.)

The packing diameter should be approx-

imately equal to the bearing wall thickness.

The height of the uncompressed packing should

be approximately 60 percent of shaft/stock

diameter.

The rudder-port compression tube

should be machined to have the same OD as

the bearing below, and an ID with 50 percent

greater allowance than the bearing.

Example:Our bearing allowance was

0. 006 inch (0.159 mm).

0. 006 in.× 1. 5 = 0. 009 in.

so

Compression-tube ID = 3. 75 in. + 0. 009 in. =

3. 759 in. ID, + 0. 003 in., –0. 000 in.

or

0 .159 mm× 1. 5 = 0 .318 mm

so

Compression-tube ID =100 mm + 0 .318 mm

= 100 .318 mm ID, + 0 .101 mm, –0.000 mm

Compressing the Rudder-Port

Compression Tube

For off-the-shelf machined bronze and stain-

less rudder ports, the compression to

squeeze the compression tube down on the

packing is usually generated by a screw-

down nut threaded onto the top of the lower

rudder-port tube.