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
