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

Real-World Problem with

Too Much Balance

An interesting example of a real problem cre-

ated by too much balance is presented in

Figure 10-5. This boat came into the service

yard for repairs to the running gear after

grounding, and the yard noted the sizable trim

tab bolted to the rudder, as you can see in the

photo. They asked the owner if the tab had

been bent or straight, and the owner thought

it had been straight. Repaired and relaunched,

the boat steered so strongly to port that it was

unmanageable. They hauled the boat again,

and with a bit of experimentation the trim tab

was bent 10 degrees to port. Then the steering

was fine. The original construction yard or

designer—not the service yard—had built in

the problem and then had corrected for it by

adding the tab and bending it over.

Looking at the photo, you can see the

reason: there’s too much balance (about

25 percent). A portion of the slipstream from

the prop was catching the leading edge of the

rudder in such a way that it swung the blade

to port, which caused the boat to turn to port.

Bending the tab to port counteracted the

turning effect on the rudder, forcing the blade

to starboard (back to center or “neutral”). In

theory, you could also correct this effect by

fairing away the leading edge, by grinding

down the starboard side of the rudder at the

leading edge, or by slightly bending the lead-

ing edge to starboard. In any case, you can

see both why it’s important not to have too

much balance, and how you might correct

the problem if you’re faced with it. (Inciden-

tally, there’s also too much shaft overhang

between the propeller and the stern bearing,

and the propeller nut zinc is too large, which

can cause turbulence. Also, the rudder has no

shaft-pulling hole, so the entire rudder and

probably the skeg below it would have to be

unshipped to remove the propeller shaft.)

Sizing Up the Rudder

The big question regarding rudders is how

large they should be. The smaller the rudder,

the less drag it creates and the faster you can

go. Further, smaller rudders require less

steering force and are easier to manage. If the

rudder is too small, however, you’ll end up

with insufficient steering control. Somehow,

a happy medium has to be found.

The basic rule-of-thumb rudder area

formulas are as follows:

Average Planing Boat Rudder Area= 0. 018 ×

waterline length× draft (hull only)

Displacement Boat Rudder Area= 0 .03 to

0. 04 × waterline length× draft

(including keel)

Deep Narrow Fin-Keel Sailboats= 0. 045 ×

waterline length× draft (including keel)

Moderate (Modern Long Keel) Sailboats=

0. 058 × waterline length× draft

(including keel)

Traditional Full Long Keel Sailboats= 0. 068

× waterline length× draft (including keel)

Or

The rule of thumb for planing hulls is

that the total rudder area (all rudders)

should be 2 percent of lateral plane. For

displacement motor cruisers, the rule

of thumb is between 3 and 4 percent

of lateral plane.

These rules of thumb should be used for

initial estimates and as a check. For

the most accurate results, use the

formulas below.

Displacement and

Semidisplacement

Powerboat Rudder-Area

Formula

For displacement and semidisplacement

powerboats, the following formula is useful

for determining rudder area.

PART FOUR: RUDDERS AND STEERING SYSTEMS

Figure 10-5.
Overbalanced rud-
der (Courtesy
Washburn’s
Boatyard)