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

PART FOUR: RUDDERS AND STEERING SYSTEMS

Note that Nm has to be multiplied by 1,000 to

get Nmm so that all the units are the same—

mm in this case.

8. FIND THEWEIGHT PERLINEALFOOT OF

THESELECTEDSTOCK.

Formula 11-5. Weight per Lineal Foot of

Selected Stock

or

Example: Thus

0 .076 sq. ft. ×508 lb./cu. ft. =

38 .6 lb./lineal ft.

If this stock were 6 feet 2 inches (6.16 ft.)

long, it would weigh 238 pounds—minus a bit

for any tapering.

or

78 .54 cm^2 × 8 .14 g/cm^3 = 639 .3 g/lineal cm

If this stock were 1.87 m long, it would weigh

1 .87 m×100 cm/m× 639 .3 g/lineal cm ÷

1,000 g/kg = 119 .5 kg

9. OPTIONAL: FIND THESECTIONMODULUS OF

THEREQUIREDSOLIDSTOCK.

Formula 11-6. Section Modulus of

Required Solid Stock

Where

Z =section modulus, in.^3 or cm^3

Dia. =rudder-stock diameter, in. or cm

Z

Dia.

32

()^3

=

π

Section Area, cm =

100

78 .54 cm

2

2

2

π100 mm

( 2 )

=

Section Area

144

0 .076 sq.ft.

3. 75 in.

2

==

π( 2 )

Section Area, cm

100

2

2

Dia.,mm

=

π() 2

Section Area, sq.ft.

144

Dia.,in.

2

=

π( 2 )

TABLE 11-2. STRENGTHS AND DENSITIES OF STANDARD RUDDER-STOCK MATERIALS

Ultimate As-Welded

Tensile Strength Tensile Strength^1 Density

MATERIAL psi MPa* psi MPa* lb./cu. ft. g/cm^3

Aluminum 5000 series 34,000 234 16,000 110 168 2.69

Aluminum 6082^2 45,000 310 20,000 137 168 2.69

Silicon Bronze 60,000 413 50,000 344 500 8.00

Stainless Steel (316L)^3 85,000 586 70,000 482 505 8.09

Carbon Composite^4 120,000 827 NA NA 98 1.57

Aqualoy 22 HS^5 130,000 986 105,000 724 508 8.14

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

(^1) As-welded values for aluminum are the as-welded yieldstrength. Other as-welded values are
as-welded ultimate tensile strength.
(^2) Values are for 6082-T6 round bar between (^3) / 4 in. and 6 in. (20 to 150 mm).
(^3) Use only 316L (L forlow carbon) stainless to avoid pitting corrosion and weld-decay
problems.
(^4) Carbon composite requires expensive custom fabrication. Confirm mechanical properties and
engineering details with the manufacturer.
(^5) Aqualoy 22 HS is a chrome-molly alloy stainless from Western Branch Metals. Aquamet 22
from Marine Machining and Manufacturing is similar. Confirm alloy strength in the diameter
specified.
Formula 11-5.
Formula 11-6.