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

Propeller Struts

You may spend hours selecting props and

engines. You will carefully consider shafts

and bearings, as you will engine mounts and

engine controls. Struts, though—until you

wipe one off by running aground—are often

afterthoughts. This hardly makes sense. After

all, the strut not only supports your propeller,

but also affects the water flowing into the

prop.

A strut that’s too weak will whip under

load, causing vibration that is often blamed

on the prop itself. (Even worse, of course,

the strut could break off entirely.) At the

same time, a strut that’s not well faired will

cause whorls and eddies in the inflow to

the prop. Not only does this turbulence

cause vibration, it also reduces propeller

efficiency.

Strut Dimensions

Figures 2-1 and 2-2 show the proportions of

proper single-leg I-struts and diverging-leg

V-struts of cast silicon bronze. These struts

are strong enough not to whip and will cause

minimal turbulence. Strut thickness for sili-

con bronze is determined as follows.

Formula 2-1a. Required Section

Modulus for Silicon Bronze I-Struts

Z, in.^3 , for I-Struts =(A ÷ B)× 6.41

(English)

Z, cm^3 , for I-Struts =(A ÷ B)× 105

(Metric)

Formula 2-1b. Required Section

Modulus for Silicon Bronze V-Struts

Z, in.^3 , for V-Struts =(A ÷ B)× 3.21

(English)

Z, cm^3 , for V-Struts =(A ÷ B)× 52.6

(Metric)

Formula 2-1c. Required Strut Thickness

for Silicon Bronze Strut with Half-Oval

Section

Where

A =hp× drop, in. (Use full engine power.)

B =shaft rpm× propeller dia. in.

Z =the required section modulus in in.^3

Or

Thickness, mm 10

Z

0.572

=×^3

Thickness, in.

Z

0.572

=^3

CHAPTER 2

Struts, Propeller Apertures,

and Shaft Angle

Formula 2-1a.

Formula 2-1b.

Formula 2-1c.