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

PART ONE:DRIVETRAIN INSTALLATIONS

Figure 2-3. Standard strut sections

Formula 2-3a.

Formula 2-3b.

Where

A =kW× drop, mm ÷ 18.95

B =shaft rpm× propeller dia. mm ÷ 25.4

Z =the required section modulus in in.^3

Formula 2-2a. Strut Length (Any

Section) at Hull Bottom

Length, in., at hull bottom =6.5× thick-

ness, but not less than 4× shaft

diameter (English)

Length, mm, at hull bottom (minimum) =

6.5× thickness, but not less than

4 × shaft diameter (Metric)

Formula 2-2b. Strut Length (Any

Section) at Boss

Length, in., at propeller-shaft boss (at

bottom) = 4 × shaft diameter

(English)

Length, mm, at propeller-shaft boss (at

bottom) = 4 × shaft diameter

(Metric)

Example: Say our boat has twin diesels

delivering 300 hp (224 kW) each at 2,800

rpm. Its reduction gear is 1.51:1, so the

shaft rpm is 1,854. Propellers are 20-inch

(508 mm) diameter 4-blade on 1^1 / 2 -inch

(38 mm) bronze shafts, and the strut drop

is 15.5 inches (394 mm). We want silicon

bronze struts with a half-oval section. We

would then find:

A =300 hp× 15.5 in. drop =4,650

B =1,854 shaft rpm× 20 in. prop. dia. =

37,080

Z for I-Struts =(4,650 ÷ 37,080)×

6.41 =0.80 in.^3

Length at hull bottom =6.5× 11 / 8 in. =

7.31; use 7^3 / 8 in.

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

shaft diameter =6 in.

Or

A =224 kW× 394 mm drop ÷ 18.95 =4,657

B =1,854 rpm× 508 mm prop. dia. ÷ 25.4 =

37,080

Thickness, in.

0.80 in.

0.572

(^3) 1.118 in.,
3
==uuse 1 / in.^18
Formula 2-2a.
Formula 2-2b.
Z for I-Struts =(4,657 ÷ 37,080)× 105
(section modulus, cm^3 ) =13.19 cm^3
Length at hull bottom =6.5× 30 mm =
195 mm
Length at propeller-shaft boss = 4 × 38 mm
shaft diameter =152 mm
Note that the thickness obtained from
Formula 2-1c is for the half-oval section
shown in Figures 2-1 and 2-3. Three standard
strut sections include, in addition to the half-
oval section with rounded ends we’ve been
discussing, a rectangular section with
rounded ends and an NACA 16-series foil
section.
Formula 2-3a. For a Rectangular
Section with Rounded Ends
Z =0.159× L× T^2
Formula 2-3b. For an NACA 16-Series
Foil Section
Z =0.0891× L× T^2
Thickness, mm 10
13.19 cm
0.572
(^3) 28.46 mm;
3
=× =
use29or30mm