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

CHAPTER 11

Once you’ve determined the size, shape, and

location of the rudder, you need to ensure

that the rudder stock is strong enough to

withstand any loads that may be placed on it

in service. A broken rudder stock means total

loss of steering control, which can even

result in loss of the boat.

Rudder Stock, Bearing Calculations, and Construction

To calculate a required stock diameter, pro-

ceed as follows:

1. FIND THEWATERFORCE ON THERUDDER

BLADE AT35 DEGREES, WHICHIS THEMAXI-

MUMRUDDERANGLE.

Formula 11-1. Precise Water Force on

Rudder

Where

CL=coefficient of lift for the rudder-

blade section shape at the angle being

investigated

ρ=mass density of water, 1.987 slugs/cu. ft.

(see sidebar) or 1,025 kg/m^3

Force, lb. CVAL^1

2

2

=× ×

⎛

⎝

⎜

⎞

⎠

ρ⎟

Rudder-Stock Size,

Construction, and Bearing

Specification

A =rudder area, sq. ft. or m^2

V =speed, ft./sec. or m/sec.

Since, as we’ve seen, the coefficient of

lift isn’t accurately known for most real-

world rudders, and since we’re interested in

the maximum force, which occurs at maxi-

mum helm angle (almost always 35 degrees,

or before this when the rudder stalls, in some

instances), and since the mass density of water

and acceleration of gravity are constant, this

formula can simplified as follows.

Formula 11-2. Standard Calculation of

Water Force on Rudder

or

Knots × 0 .514 = m/sec.

Where

CL=coefficient of lift

0 .5 for planing-powerboat rudders

(see sidebar)

1 .2 for all other rudders

Formula 11-1.

Formula 11-2.

Force, lb.=× ×CL(Pf Speed,ft./sec.)^2 ×Area, sq..ft.

Knots 1× .69 = ft./sec.

Force, kg C Pf Speed,m./sec.

Area, m

L

2

2

=× × ×

×

( )

552. 55