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

Formula 2-3c. For the Flat Half-Oval
Section with Rounded Ends

Z =0.080× L× T^2

Where

Z =the required section modulus from

Formula 2-1a or 2-1b in in.^3 or cm^3

L =length from Formula 2-2a and 2-2b,

in. or cm

T =thickness to achieve the specified

section modulus Z, in. or cm

NOTE: in metrics, for the preceding equa-
tions, enter dimensions here in centimeters,
notmillimeters.
You can enter different values of thick-
ness and length in different combinations to
get the required Z; however, proportions
should follow Formulas 2-2a and 2-2b.
Using standard engineering references,
you can use Z to find the required thickness
and length for any other strut-section shape.
You can also refer to ABYC standard P-6 for
additional details.

Adjusting Strut Dimensions

for Other Materials

The preceding calculations are for silicon
bronze, which has a yield strength of 55,000 psi
(379,000 kPa). If you use another alloy, you
have to determine its yield strength and ad-
just Z proportionately. Say you use 316L
stainless, which has a yield strength of
42,000 psi (289,500 kPa). Divide the yield
strength of silicon bronze by the yield
strength of stainless to get 1.309. Accord-
ingly, you would increase Z by 1.309 times
and then find the required dimensions. If
you used 316L stainless for the half-oval-
section struts in the earlier example, they
would work out as

55,000 psi yield silicon bronze ÷ 42,000 psi

yield 316L stainless =1.309

Z for silicon bronze I-struts, you found,

was 0.80 cu. in.

Z for 316L stainless I-struts =0.80 cu. in.×

1.309 =1.047 in.^3

Length at hull bottom =6.5× 11 / 4 in. =

8.125; use 8^1 / 4 in.

Length at propeller-shaft boss = 4 × 11 / 2 in.

shaft diameter =6 in.

Or

379,000 kPa yield silicon bronze ÷

289,500 kPa yield 316L stainless =1.309

Z for silicon bronze I-struts, you found,

was 13.19 cm^3

Z for 316L stainless I-struts =13.19^3 ×

1.309 =17.26 cm^3

Length at hull bottom =6.5× 32 mm =

208 mm

Length at propeller-shaft boss = 4 × 38 mm

shaft diameter =152 mm

TABLE 2-1.YIELD STRENGTHS OF

COMMON STRUT MATERIALS

Alloy Yield, psi Yield, kPa

Commercial bronze 37,000 255,000

Manganese bronze,

SAE No. 43 65,000 448,000

Manganese bronze,

SAE No.430 (GrA) 90,000 620,500

Manganese bronze,

SAE No.430 (GrB) 110,000 758,400

Silicon bronze 55,000 379,000

Phosphor bronze 55,000 379,000

Aluminum bronze 55,000 379,000

Stainless steel 304 35,000 241,000

Stainless steel 316 42,000 289,500

Stainless steel 316L 42,000 289,500

Aluminum 5000 series 34,000 234,400

Aluminum 5000 series

as welded 14,000 96,500

Discussion of Strut Alloys

The longest-lasting alloys by far are silicon

bronze or one of the phosphor or aluminum

bronzes. All the other alloys are second best,

even though the manganese bronzes have

very high yield strengths. Of the steels, I

Thickness, in.

1.047 in.

0.572

(^3) 1.22 in; u
3
==sse 1^1 / 4 in.
Chapter 2:Struts, Propeller Apertures, and Shaft Angle
Thickness, mm 10
17.26 cm
0.572
(^3) 31.13; us
3
=× = ee32mm
Formula 2-3c.