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

hinges entirely at its leading edge, with no

area projecting forward at all. Finally, rud-

ders may be spade rudders, with no bearing

or support below the bottom of the rudder

blade, or they may be rudders with bearings

above and below the rudder blade.

Inboard Versus Outboard

Rudders

For the same rudder shape and area, an in-

board rudder is almost always more efficient

than an outboard rudder. The hull traps the

water rushing over the top portion of the

blade, guiding the entire flow aft and forcing

this water to do useful steering work. This is

the important endplate effect, and it reduces

induced drag, which is nothing more than the

wasted energy caused by water roiling

around the top or bottom edges of such a

hydrofoil. Often, high-speed craft fitted with

outboard rudders will have endplates (some-

times incorrectly called cavitation plates) fas-

tened to them at the waterline or at the rud-

der’s top edge. The proper term is either

endplateor ventilation plate. These plates

help noticeably, but such rudders still are not

quite as efficient as a true inboard rudder.

The great advantage of outboard rudders—

at least for larger rudders in low- to medium-

speed craft—is their simplicity (Figure 10-3).

It’s comparatively inexpensive to attach the

gudgeons and pintles outside the hull on the

transom and to run the tiller to steering gear

far above the waterline, where watertightness

isn’t a big consideration. By contrast, an

inboard rudder usually requires a rudder port

and stuffing box through the bottom of the

hull—a potential source of leaks that should be

inspected at the beginning and end of each sea-

son, as well as before and after any major pas-

sage. Further, the outboard rudder allows the

propeller to be installed farther aft, which per-

mits the shaft angle to be slightly lower (closer

to parallel with the waterline) for slightly more

efficient thrust. Outboard rudders on small

planing hulls are available with premanufac-

tured housings containing the tiller arm. The

entire housing bolts to the bottom of the tran-

som, with the tiller projecting through a hole

forward into the hull or projecting over the top

of the transom into the cockpit.

Balance and the Rudder

Balanced rudders move the center of the

force of the water striking the rudder blade

closer to the rudder’s pivot axis than it would

be on an unbalanced blade. Maximum water

force occurs at maximum helm or rudder an-

gle, which is about 35 degrees to either side

of dead center—70 degrees from hard over to

hard over. (At greater angles, ordinary rud-

ders stall, lose effectiveness, and are strained

by excessive water force. Internal stops

should be fitted to keep rudders from turn-

ing farther. We’ll look at special rudders

designed for greater angles in Chapter 13.)

Because the water is striking the rudder

blade from ahead, the leading edge does more

work than the trailing edge. (This is true of

PART FOUR: RUDDERS AND STEERING SYSTEMS

Figure 10-3.
Sailboat outboard
rudders