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

PART FOUR: RUDDERS AND STEERING SYSTEMS

geometric center point. This line is the effec-

tive mean chord. Measure 35 percent of the

mean-chord length aft of the rudder’s leading

edge along this line to find the center of

pressure. Use 40 percent aft on wedge-

section or parabolic-section rudders. (See the

Chapter 10 sidebar on page 167 to estimate

the location of the center of pressure at

rudder angles other than 35 degrees.)

3. FIND THETWISTINGARM(TA).

Measure the distance at right angles from the

center of water force (the center of pres-

sure) on the rudder blade to the center of the

rudder stock—the rudder-stock axis.

4. FIND THEBENDINGARM.

NOTE: If the rudder blade has bearings above

and below it, then the bending arm is zero;

see later in chapter.

Refer to Figure 11-1. The distance from

the center of water force (or center of pres-

sure) up to the middle of the lower rudder

bearing in the hull (measured along the

length of the rudder stock) is the bending

arm.

5. FIND THETWISTINGMOMENT(TM)AND

THEBENDINGMOMENT(BM).

TM =rudder force (or pressure) center ×

twisting arm

BM =rudder force (or pressure) center ×

bending arm

Example: If the twisting arm for our

example rudder is 3. 64 inches (92 mm) and

the bending arm is 23. 8 inches (605 mm),

then

Figure 11-1.
Powerboat spade
rudder