2015 New York Safe Boating - 10

BOATING AND MOTORS

Water Jet Pump Engines

Propeller Performance

The water jet pump engine is basic- just a small propeller
(sometimes referred to as an impeller) inside of a casing or
housing, connected to an inboard engine by a shaft that passes
through a watertight opening in hull. While these engines are
traditionally used in personal watercraft, an increasing number
of boats have jet pump
engines.

There are several factors that will affect the way a prop
performs. One factor is the propeller's pitch- the distance a
propeller would travel in one revolution. Pitch is affected by
slip, which is the difference between the distance a propeller
should move in one revolution through a solid medium
and the actual distance it moves through water, a yielding
substance. Picture a bicycle wheel moving over the pavement.
Now picture it riding through a muddy field. The bicycle will
move farther over pavement than it will through the mud-
how much farther it moves is the slip.

A nozzle directs water
flow from the pump
to provide directional
control of the boat.
There is no neutral.
Boats with jet pump
engines require power
to turn and are not
responsive at idle speed. They can operate in shallow water
and are highly maneuverable and responsive; however the
pump can clog easily.

Cavitation is a condition in which the prop revolves faster
without an increased speed (thrust), which causes a loss of
power and reduced speed. This happens when a vacuum is
created around the prop. Cavitation is usually caused by one
of two problems:

Propellers
No discussion of propulsion would be complete without
a discussion of propellers, or "props." Selecting the right
propeller is critical. The right prop for your boat and its engine
will mean your boat
will handle well; the
wrong prop may make
your boat difficult to
maneuver or cause it
to perform poorly or
unpredictably.

 If the wrong size prop is on the boat the engine may

turn the prop faster then the prop should be turned.
The prop is spinning but the boat is not moving as fast
as it should. You must replace the prop.
 The prop may sit too near the surface of the water. You
must trim your boat so the stern is sitting deeper in the
water.
Be aware of the risk of grounding and damaging your prop
when you move from deep water into shallow water. A rapid
reduction in speed can cause the stern to drop and the bow
to rise, creating a bow wake. As the bow continues to rise,
the stern continues to squat (to sink lower), which may lead to
your boat grounding.

Props operate by
rotating and drawing
water from forward
and around its blades, and forcing the water into a stream away
from the stern. This creates a dynamic pressure which moves
the boat. The flow of water caused by the prop is called "screw
current."

Review Questions
1.

What are the nautical terms for the right and left side of the boat when facing the bow? ___________________
_______________________________________________________________________________________________

Why is it important to know the length of your boat? _________________________________________________
_______________________________________________________________________________________________

What is the unique propulsion system on a personal watercraft? ________________________________________
_______________________________________________________________________________________________
Answers on page 82

2015 New York Safe Boating

Table of Contents for the Digital Edition of 2015 New York Safe Boating