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

PART FOUR: RUDDERS AND STEERING SYSTEMS

custom fabricating and machining solid

aluminum rudder stocks of 6082. These can

be machined to any diameter desired and

tapered as well. Weight can be close to that

of carbon composite.

It is also worth considering whether such

a high-aspect rudder really pays. The differ-

ence in performance between a rudder with

an aspect ratio of 3 and one of 5 (of roughly

equal area) isn’t all that great. A lower aspect

ratio reduces the bending arm, which reduces

the bending moment, and so reduces the

required stock diameter as well as the stock

length, thereby reducing stock weight.

Checking Rudder-Stock

Strength Going Astern

Because forward speed is so much higher than

astern speed—for ordinary boats—simply

checking the rudder-stock strength in the

ahead condition is generally adequate. Some

craft, however, may have rudder configura-

tions that create greater loads when going

astern. This is because when making stern-

way, the center of water force or center of

pressure is about 20 percent of mean chord

forward of the trailing edge(which, going

backward, becomes the leading edge). This,

in turn, significantly increases the length of

the twisting arm, and so the twisting moment.

For displacement boats, use 70 percent of

maximum ahead speed as speed for sternway

calculations.

For planing hulls, use

Sternway, kts =(maximum forward speed,

kts +20) ÷ 3

If you’re in doubt about the rudder-stock

strength going astern, repeat all the previous

rudder-stock calculations, using the sternway

speed and locating the center of water force

20 percent of the mean chord forward of the

trailing edge.

Rudders Stocks with

Bearings Above and Below

the Rudder Blade

A rudder that is supported with bearings

both above and below the rudder blade, such

as in Figure 12-21, is in nearly pure torsion

with minimal bending. What bending exists

is the result of the side force of the water,

which is assumed to be distributed roughly

evenly along the height of the rudder. This

could be calculated as a “combined twisting

and bending” similar to what we did earlier,

but because the bending moment is such a

small proportion of the total load, this can

be taken care of simply by using the slightly

larger safety factor of 4, rather than 3. 34.

You could then use Formula 11-3 above for

combined twisting and bending with a zero

entered for bending arm and therefore

bending moment, but it’s more straightfor-

ward to use torque alone as simply

TM =rudder pressure center ×twisting arm

Along with a safety factor of 4 as in Table 11-1

In the combined twisting and bending of

the spade rudder, bending dominated, so we

used ultimate tensile strength (UTS). In the

current case, however, torsiondominates.

Accordingly, you need to use ultimate

shearstrength (USS), which can be taken

as 60 percent of UTS.

If we assume for a moment that our pre-

vious rudder was supported by a lower rudder

bearing on a strong skeg, then we would find

the required stock diameter as follows:

From Step 5 above:

TM =8,512 lb.× 3. 64 in. =30,984 in.-lb.

or

TM =3,839 kg× 0 .092 m =353 kgm

From Step 7 above:

Since the resulting stock will be consid-

erably smaller in this configuration, we’ll use

more standard 316L SS, with a UTS of 85,000

psi (586 MPa). Shear strength would then be

85,000 psi UTS× 0. 60 =51,000 psi USS

or

586 MPa UTS× 0. 60 =352 MPa USS

or

353 kgm× 9. 8066 =3,462 Nm

Dia.in.

16 30,984in.-lb.

51,000 psi

4SF

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2. 31 in.,use

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