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

(Rick Simeone) #1

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

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