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

Example:So

or

93 .05 mm dia. = 9 .305 cm dia.

10. OPTIONAL: SELECT AHOLLOWPIPE OR

TUBESECTION WITH THEEQUIVALENTSECTION

MODULUS.

The pipe must be a minimum of schedule 80
or heavier wall.
Referring to the standard U.S. IPS pipe and
tube-stock table in Appendix B, a 4.5-inch OD
tube, with a^1 / 2 - inch wall has a section modulus
of 5. 67 in^3. This weighs only 22.16 lb./lineal ft.
Or
Referring to a standard heavy-wall DIN- 2448
pipe table (see Appendix B), a 114.3 mm OD
pipe, with a 11 mm wall has a section modulus
of 84.2 cm^3. This weighs only 28 kg/lineal m.
The section modulus of a hollow, round
section can be found from the following:

For the metric DIN pipe, we get
114 .3 mm OD – (2×11 mm wall) = 92 .3 mm ID

The larger diameter would create more
drag. Further, the Aqualoy 22 HS alloy isn’t
available for this size tube, so we would
have to redo the previous calculations for
316L stainless and find the pipe or tube size
for the appropriate section modulus for that
alloy.

About Welding to Rudder

Stocks

The rudder stock itself cannot be welded
from separate pieces; it must be a continuous,
unwelded bar or tube. Any welds would be

from welding external items to the stock. This

might be a sleeve to “strengthen” the stock, as is

sometimes mistakenly done. In fact—because

of the loss of strength as welded and the

increased tendency toward corrosion at

welds—welded-on sleeves are generally a

poor idea. The most common reason to have a

weld in the critical zone is when welding on

the top plate of an aluminum rudder blade to

an aluminum rudder stock. In this case, the

as-welded strength must be used.

In most instances, such an aluminum

rudder blade on an aluminum rudder stock

would be employed on a displacement boat

with bearings above and below the rudder

blade. This means that you would apply the

calculations for this type of rudder (later in

this chapter) using the shear strength based

on the as-welded strength in Table 11-2. For

aluminum, this would be the as-welded yield

shear strengthas noted on Table 11-2. For

5086 aluminum, that would be 60 percent of

16,000 psi (110 MPa) or 9,600 psi (66 MPa).

For 6082, it would be 12,000 psi (82 MPa). It’s

customary with aluminum to round 5086

down to 9,000 psi (62 MPa) as-welded yield in

shear as further insurance—and 11,000 psi

(76 MPa) for 6082 aluminum.

Saving Weight in Sailboat

Rudder Stocks

If a rudder is fitted to a strong skeg or to the

back of a traditional long keel—with bearings

above and below the rudder blade—sailboat

rudder stocks aren’t too large and heavy.

On many modern fin-keel boats, however,

balanced spade rudders can be both large

and high aspect. A solid stainless, bronze, or

even Aqualoy stock might well weigh over

500 pounds (225 kg) on a bigger boat. Such

weight in the ends of the vessel is detrimental

to performance—it increases pitching.

It makes good sense, in such instances,

to go to heavy-walled stainless or bronze pipe

or tube. Even though the rudder blade must

be thicker to accommodate the greater stock

diameter, the weight savings is well worth it.

The more expensive alternative is a custom-

fabricated, carbon-epoxy-composite stock.

Another good option is a solid aluminum

rudder stock of 6082 alloy. Some companies

(like Jefa Rudder and Steering) specialize in

Z

32

11 .43 cm OD 9 .23 cm ID^4

=×

⎛ −

⎝

⎜

⎞

⎠

⎟

π ()()^4

111 .43 cm OD

84 .2cm^3

⎛

⎝

⎜

⎞

⎠

⎟

=

Z

32

OD ID

OD

44

=

⎛

⎝

⎜

⎞

⎠

⎟×

⎛

⎝

⎜

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π – ⎟

Z

9 .305 cm

32

79 .1cm

3

==π()^3

Z

3. 76 in.

32

5. 22 in.

3

==π()^3