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

the force being off to that side will tend to
push the bow to starboard going ahead
and the stern to starboard going astern.
Thus, installing a right-hand wheel off cen-
terline to port helps cancel out the to-port
tendency caused by the propeller. If
instead you put the shaft off to the star-
board side, it would increase the to-port
tendency caused by the right-hand pro-
peller, which would not be good. A left-
hand wheel behaves exactly the opposite.
NOTE: On twin-screw boats, the star-
board propeller is usually right-hand and the
port propeller is left-hand (Figure 2-11).
Offset parallel propeller shafts are not
uncommon on sailboats, where they help
make more room on the side away from the
engine and also avoid running the shaft
through the structure of the keel on centerline
(Figure 2-12). The offset distance shouldn’t be
more than about 8 percent of beam. Note that
this offset puts the rudder out of the direct
propeller slipstream, reducing steering re-
sponse under power.

It is also not uncommon to angle the prop

shaft so the propeller is still on the centerline,

with the engine angled off to the port side on

a right-hand wheel boat (Figure 2-12). This

also counters the to-port tendency of a right-

hand wheel. The maximum installed shaft

angle in plan should be about 3 degrees. With

this configuration, the rudder is still in the

slipstream.

Occasionally, larger single-screw power-

boats will use a similar configuration, with

the prop on the centerline and the shaft an-

gled up to 3 degrees to port on a right-hand

wheel (opposite for left-hand). The primary

advantage here is when the engine is at mid-

ships or forward (Figure 2-13). The shaft

angle creates some added room on the star-

board side of the engine. This might allow a

passageway to starboard or some large item

of machinery to be installed that otherwise

wouldn’t fit.

Twin-screw boats can also use shafts an-

gled in plan. V-bottom hulls can angle the

shafts out at the stern and in toward the

Chapter 2:Struts, Propeller Apertures, and Shaft Angle

Figure 2-11.

Standard

direction of

rotation for twin

screws (viewed

from astern)