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

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

BM =8,512 lb.× 23. 8 in. =202,585 in.-lb.

or

TM =3,839 kg× 0 .092 m =353 kgm

BM =3,839 kg× 0 .605 m =2,322 kgm

6. FIND THECOMBINEDTWISTING ANDBENDING

MOMENT—COMBINEDMOMENT(CM).

Formula 11-3. Combined Twisting and
Bending Moment (CM)

Example: In our case

or

NOTE: Though the calculation for combined
moment (CM) should be done as shown here,
the result will consistently be close to twice
BM. Use this check on your calculation. If
the number is significantly different than 2×
BM, reexamine your work. In this case, 2×
202,585 in.-lb. =405,170; or 2×2,322 kgm =
4,644 kgm, which is nearly identical to our
calculated CM.

7. FIND THEDIAMETER OF ASOLIDRUDDER
   STOCK.

Formula 11-4. Diameter of Solid Rudder
Stock

Where

Moment =combined bending and twisting

moment, in.-lb. or Newton-meters

(Nm), for spade rudders

Dia.,in.

1 6 Mome nt

UTS

SF

=

×

π( )

3

CM 2,322 kgm 2,322 kgm 353 kgm

4,671

=+^22 +

=

(())

kgm

CM 202,585in.-lb.

202,585in.-lb.^2 30,9

=+

()+( 884 in.-lb.

407,525in.-lb.

)^2 =

CM BM=+ +BM^22 TM

TABLE 11-1. RUDDER SAFETY FACTORS

SF

Powerboat Spade Rudders 3.34

Racing-Powerboat and Patrol-Boat

Spade Rudders 4.00

Power- and Sailboat Rudders,

Bearings Above and Below Rudder

Blade 4.00

Sailboat Spade Rudders 3.34

UTS =ultimate tensile strength of the Formula 11-3.

material, psi or MPa (N/mm^2 ); refer to

Table 11-2.

SF =safety factor, refer to Table 11-1.

On metal spade rudders, there must be

no welds from 1 shaft diameter below the top

of the rudder blade up to 1 shaft diameter

above the top of the lower rudder bearing.

If welds are closer than this, then use the

“As-Welded” strength from Table 11-2.

Because in spade-rudder configuration the

bending moment will be vastly greater than the

twisting moment, we use the ultimate tensile

strength of the material from Table 11-2.

The rudder stock itself cannot be welded

from separate pieces; it must be a continuous

unwelded bar or tube.

Example:Our example has a fairly large

rudder for this speed. We want to keep stock

diameter down to reduce drag. Thus we’ll go

with a very high-strength chrome-molly stain-

less Aqualoy 22 HS.

use 3^3 / 4 - in. Aqualoy 22 HS

or

Metric structural calculations should be

done using newtons, not kilograms. To con-

vert kgm to Nm (kilogram-meters to newton-

meters), multiply kgm by 9. 8066.

so

4,761 kgm× 9. 8066 =46,699 Nm

Dia.,mm

16 46,699 Nm 1,000 mm/m

986 N/mm^2

=

××

π

33 .34 SF

3

93 .05 mm, use 100 mm Aqualo

⎛

⎝

⎜

⎞

⎠

⎟

= yy22HS

Dia.

16 407,525in.-lb.

130,000 psi

3 .34 S

=

×

π

FF

3. 76 in.,

⎛

⎝⎜

⎞

⎠⎟

=

3

Formula 11-4.