Boat Propulsion and Steering
Most powerboats move through the water
by the turning of a propeller, sometimes
called a screw. Propellers are multi-bladed
angled wheels that turn, drawing in water
from ahead and pushing it out astern.
Twin-screw boats have two engines, each
with a propeller.
How the boat is propelled also determines
how it is steered. Sailboats use a rudder
to steer the boat as do powerboats with
inboard engines. The most popular type of
engine today is the outboard followed by
inboard/outboard and jet drives. All of these
drives steer a boat by directing the thrust
from a propeller rather than using a rudder.
How a rudder steers a boat
A rudder is a vertically mounted flat blade.
When a rudder is turned on a moving boat,
it diverts the flow of water to one side,
causing a sideways force in the opposite
direction, which turns the boat (Figure
1-18). To be effective, the rudder needs a
flow of water; otherwise, it will do nothing.
Now add a propeller. The propeller
is mounted right in front of the rudder. In
forward gear, the propeller provides a flow
of water across the rudder. In this case, the
boat will begin to turn even if it is not moving.
However, in reverse gear, the propeller
does not direct a flow across the rudder and
nothing happens until the boat picks up
some speed so there is some flow of water
past the rudder.
Directed Thrust
Propeller thrust effects turning and trim.
The boat’s stern moves opposite to thrust:
left from starboard thrust, right from port
thrust, stern down—bow up from upward
thrust, and stern up—bow down from
downward thrust. An outboard engine or
inboard/ outboard drive steers by turning
the entire lower unit, including the propeller
(Figure 1-19). This directs the thrust to one
side, causing the stern of the boat to move
in the opposite direction. Assuming you are
operating at slow speed, the propeller thrust
(and subsequent motion of the prop, drive
unit and boat) is always opposite to the flow
of water it produces, whether in forward or
reverse. The right-hand and left hand rotations
of the propeller cause sideways motion
of the stern in the direction of rotation,
called “prop walk.” The left hand rotation
with prop walk to port assists in mooring
port side to.
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Figure 1-18. Rudder steering a boat Figure 1-19. Directed thrust