If you stop the propeller, you lose the
directed thrust and steerage becomes difficult
in the absence of a rudder. However,
the shape of the lower unit does provide
a small amount of steerage even with the
propeller in neutral gear.
Drive Mechanisms
Inboard engines are installed inside the
boat (Figure 1-20). They require a shaft (a
cylindrical rod) to transmit power from the
engine to a propeller or water jet pump.
The shaft passes through a special fitting
in the hull called a stuffing box, which
keeps out most of the water while allowing
the shaft to turn the propeller. The
small amount of water (2 to 3 drops per
minute) that does enter serves to lubricate
the fitting. However, the fitting should be
checked frequently to ensure there is no
excess leakage, for this is a major cause of
boat sinking. Since the propeller, driven
by an inboard engine, is fixed in position,
steering is accomplished using a rudder as
explained above.
Outboard motors are mounted on the
transom or on special brackets on the transom.
They move the boat with a propeller
(Figure 1-21). Outboard motors usually
weigh less than inboard engines, and do not
take up space inside the boat. They may be
tilted up in shallow water.
Inboard/outboards or I/O’s are called
stern drives. They operate with an inboard
engine that is placed inside the boat against
the transom (Figure 1-22). The drive shaft
exits through a special fitting in the transom
and, through a series of gears and
couplings, the shaft transmits power to the
propeller. The lower unit is similar to that
found on an outboard motor and may be
tilted up. Outboard and I/O drives steer by
rotating the lower unit, including the propeller.
Jet drives have no external propeller.
An inboard engine powers an impeller,
which is similar to a propeller except that
it is mounted in a hollow tube within the
boat (Figure 1-23). This creates a powerful
Section 1: Getting Started 11
stuffing box
water pump that sucks in and exhausts
large quantities of water under high pressure
through an external nozzle. The force
of the water pushes the boat through the
water. You steer the boat by moving the
nozzle from side to side.
Since you cannot reverse the flow of
water, the jet drive uses a clamshell device
over the nozzle to redirect the flow forward.
This, in turn, pushes the boat in
reverse (Figure 1-24).
Unlike a rudder, or even the mild
steerage with an outboard or I/O drive, the
jet drive has no steerage while moving with
the engine off. You must provide power to
steer.
98
99
100
101
Figure 1-20. View of an inboard
engine
Figure 1-21. Outboard motor
Figure 1-22. Cutaway view of a
stern drive
Figure 1-23. View of a jet drive
Figure 1-24. Clamshell device
over the nozzle pushes the
boat in reverse.
102 103