Propeller
Basics
What's
the right size propeller for my boat?
The correct propeller size (pitch) will allow your boat's
engine to operate at the engine manufacturer's maximum
recommended engine RPM at full-throttle. It is very critical
to be sure your engine is propped correctly for your boat/engine
combination and for the type of boating you will do. An
engine that does not reach the recommended RPM at full-throttle
is in an "over propped" condition, resulting
in "lugging". This severe strain can lead to
detonation, piston seizure and engine damage. An engine
that revs past the manufacturer's recommended RPM will
have higher than normal wear and can also be damaged by
fatigued parts breaking and passing through the engine.
To help with your selection process for
a replacement or upgraded propeller with a larger or smaller
pitch, use the following suggested guidelines:
Adding
1 inch of propeller pitch will reduce full-throttle RPM
by 150 to 200.
Subtracting
1 inch of propeller pitch will increase full-throttle
RPM by 150 to 200.
If you're
moving from a 3-blade to a 4-blade propeller, a 4-blade
generally turns 50 to 100 RPM less than a 3-blade propeller
with the same pitch.
Diameter
Diameter is the distance, in inches, across the imaginary
circle made when a prop rotates.
Pitch
The theoretical disatnce, in inches, that a propeller
travels forward during one revolution.
Think of pitch as speed. The higher the propeller pitch
that the engine can turn, the faster the boat
will usually go.
What
is the difference between aluminum and stainless?
Many pleasure boats are equipped from the boat manufacturer
with an aluminum propeller. Aluminum props are inexpensive
and easy to
repair, and offer good all-around performance. Stainless
steel propellers are over five times more durable than
aluminum props, which means that stainless steel propellers
are less likely to be damaged when they encounter minor
underwater debris. Stainless steel is much stronger than
aluminum so stainless steel propellers can be made in
a wider variety of shapes and sizes to increase performance
on specific boat/engine applications.
Can
I use different propellers on my boat?
If you use your boat for more than one type of activity,
such as cruising, fishing or skiing, you should consider
having more than one propeller. You may well need different
propellers for the best performance in each type of activity.
In any event, you should keep a spare propeller on board
at all times, along with a prop wrench that will fit the
propeller nut, spare prop nut and tab washer.
What's
the difference between a 3-blade and a 4-blade propeller?
Three-blade propellers are good for general purpose use
and, on many boats, are faster than 4-blade props. However,
4-blade propellers offer significantly different performance
characteristics compared to 3-blade props. A 4-blade compared
to a 3-blade propeller usually:
planes
the boat faster
keeps
the boat on plane at a slower speed
gives
improved mid-range speed at the same RPM
provides
quicker acceleration
has
better holding power in rough seas
is
less likely to ventilate in sharp turns
offers
better low-speed handling
is
not as fast on top end
If
I want to switch to a 4-blade propeller, what size should
I get?
If your current three-blade propeller allows your boat's
engine to operate within the upper end of the engine manufacturer's
recommended RPM range, choose a 4-blade propeller of the
same pitch (or next lower pitch, depending on propeller
selection) as your 3-blade propeller. Keep in mind that
the 4-blade should decrease your engine 50 to 100 RPM.
What
is propeller ventilation?
Ventilation occurs when air from the water's surface or
exhaust gases are drawn into the propeller blades. The
normal water load is reduced and the engine over-revs,
losing propeller thrust. This action occurs in turns,
when trying to plane the boat in a sharp turn or with
an excessively trimmed-out engine or sterndrive.
OK,
Let's See The Props
