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

Pf =Propeller factor

1 .0 for boats where the propeller is

more than 2.5 times propeller

diameter forward of the rudder

(usually sailboats)

1 .2 for all other boats

Example:If we had a 28-knot, twin-
screw planing boat with two 5.28 sq. ft. (0. 49
m^2 ) rudders, the maximum force on each rud-
der at 35 degrees would be

28 knots× 1. 69 = 47 .32 ft./sec.

or

28 knots× 0. 514 = 14 .39 m/sec.

Force 0. 5

5 .28 sq.ft

=×( 1. 247 × .32,ft./sec.)^2 ×

..=8,512 lb.

Chapter 11: Rudder-Stock Size, Construction, and Bearing Specification

Slug Units of Mass

In the English system of units,weightis the

forcegenerated by the massof an object,

so

Weight (or force due to mass) =mass×

acceleration of gravity (32.2 ft./sec.^2 )

This can be rewritten as

Mass=weight (or force due to mass) ÷

32.2 ft./sec.^2

The density of seawater is 64 lb./cu. ft.,

therefore

Mass density of seawater (ρ)=

64 lb./cu. ft. ÷ 32.2 ft./sec.^2 =1.987

slugs/cu. ft.

This is so close to 2 that the mass den-

sity of seawater (ρ) is often given as 2.

(ρis the Greek letter rho.)

Slugsare the English foot-pounds-

seconds system (FPS system) units of mass,

as opposed to the force created by the

mass in Earth’s gravity, which is the weight

(confusingly though, weight in pounds is

also mass).

Lift Coefficient for

Planing Powerboat

Rudders

A coefficient of lift (CL) of 0.5 for planing

powerboats has been found by practical

experience to be not only appropriate

but conservative for the rudder calcula-

tions shown. This ties in with the many un-

knowns discussed in Chapter 10, such as

the real angle of attack and real water

speed “seen” by the rudder, not to men-

tion the complex effects of stall, ventila-

tion, and cavitation. An excellent real-

world example of this is found in Peter Du

Cane’s book High Speed Small Craft,

where he works through the rudder-stock

calculations of a Brave Class patrol boat

(88 ft. [25.8 m] LOA; 75 tons, 8,500 hp, 52

knots) using a similar method to that of

this chapter and using the same CLof 0.5.

The calculations result in a required stock

diameter of 5.94 inches (151 mm). You

would expect this to be rounded to 6.00

inches (155 mm) dia.; however, this is fol-

lowed by the note:

“In the “Brave” class F.P.B. stock dia-

meter was actually 5 in. [127 mm] and in ten

years no trouble occurred, so that even a

CL=0.5 is unnecessarily large... .”

Force = 0. 51. 214 .39 m/sec. 0 .49 m

52

××()^22 × ×

..55 3,839 kg=

NOTE: The total force on the surface

of the rudder blade is more properly indicated

by the symbol kgffor kilograms of force; how-

ever, thefis understood in this application.

2. LOCATE THECENTER OFWATERFORCE ON

THERUDDER.

Refer to Figure 11-1 to locate the geometric

center of area either by using a CAD program

or by balancing a cardboard or stiff-paper

cutout of the rudder blade.

Draw a horizontal line (a line parallel to

the DWL) across the rudder through the