Propellers 101
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- LEARNING THE LINGO: YOUR GUIDE TO BOAT TERMINOLOGY
- SELECTING THE RIGHT PROPELLER FOR YOU BOAT AND ENGINE.
Learning the Lingo:
Your Guide to Propeller Terminology
Propeller Size
The size of a prop is described using two sets of numbers. These correspond to the
diameter and pitch. The pitch always follows the diameter when describing a propeller.
Diameter
The first number listed in a propeller size. Diameter is defined as two times the distance
from the center of the hub to the tip of the blade. It can also be looked at as the
distance across the circle that the prop would make when working.
Pitch
The last number listed in a propeller size. Technically speaking, pitch is the theoretical
distance a prop moves forward in one revolution - assuming there is no
"slippage" between the prop blade and the water. In the real world, there is
some slippage and therefore the distance advanced is less than the design pitch. Pitch can
be visualized as the tightness or looseness of the blades as they swirl around the prop
hub. If the blades appear tightly wound and angled sharply inward toward the hub, the
pitch is high. If the blades appear loosely wound and angled outward from the hub (that
is, they appear flatter), the pitch is low.
Cupping
Many of today's propellers incorporate a cup at the trailing edge of the propeller blade.
This curved lip on the propeller allows it to get a better bite on the water. This results
in reduced ventilation, slipping, and allows for a better hole shot in many cases. A
cupped propeller also works very well where the motor can be trimmed so that the propeller
is near the surface of the water. The cup will typically result in higher top end speed on
one of these applications.
Ventilation
Air from the water surface or exhaust gases from the exhaust outlet being drawn into the
prop blades causes ventilation. When this situation occurs, boat speed is lost and engine
RPM climbs rapidly. This occurs most often with high transom mounting, over-trimming the
engine, and sharp turns.
Cavitation
Cavitation (which is often confused with ventilation), is a phenomena of water vaporizing
or "boiling" due to the extreme reduction of pressure on the back of the
propeller blade. Many propellers partially cavitate during normal operation, but excessive
cavitation can result in metal erosion or "cavitation burn" to the prop's blade
surface. There are numerous causes of cavitation such as incorrect matching of propeller
style to application, incorrect pitch, physical damage to the blade edges, etc.
Rake
Rake is the degree that the blades slant forward or backwards in relation to the hub. Rake
can affect the flow of water through the propeller, and has implications with respect to
boat performance. Aft rake helps to trim the bow of the boat upwards, which often results
in less wetted surface area and therefore higher top end speed. Aft rake propellers also
typically "bite" better on ventilating type applications. Forward, or negative
rake, helps hold the bow of the boat down. This is more common in workboat type
applications.
Selecting the Right Propeller For Your Boat and
Engine
The best propeller size for your boat and engine combination is based on the recommended
operating range at wide open throttle (w.o.t.) for your engine, which you will find in
your operator's manual. This will be expressed in terms of a certain horsepower at a
certain RPM (revolutions per minute).
The goal in prop selection is to determine what propeller style and size will maximize
performance for your boat, while allowing your engine to operate in the recommended RPM
range. The correct propeller will prevent the engine from over-revving, yet allow it to
reach the minimum RPM where maximum horsepower is produced.
Run the boat/motor at w.o.t. under normal operating load to determine the maximum RPM you
are able to obtain. A tachometer is necessary for this test. Adjust the motor trim angle
for the optimum performance. If during this test, you begin to exceed the maximum rated
RPM of the engine, reduce throttle setting to a position where maximum RPM is not
exceeded.
If your test results in your being able to over-rev the engine, you need to increase the
pitch of the propeller. Increasing the pitch increment by 1" will result in
approximately a 200 RPM drop. If your testing shows, however, that you are only able to
obtain a RPM somewhat lower than the maximum rating given by your engine manufacturer, you
would need to decrease pitch. Decreasing pitch would increase your RPM.
Example:
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Switching from an uncupped to a cupped propeller will also reduce your RPM. The cupped
propeller of the same pitch and diameter will typically reduce your RPM by approximately
200.
Once your wide open throttle RPM falls within the recommended range of the engine
manufacturer, you have a propeller that is suited correctly for your boat with respect to
RPM. If you use your boat for fishing, cruising and skiing, one prop probably won't do all
three things equally well. It is best in circumstances like this to have two propellers.
One to accommodate one set of circumstances and the other to perform best under the
different load. It could, in fact, be that more than one propeller would be suitable for
your boat and motor combination depending on your usage. It is imperative, however, that
the wide open throttle RPM fall within the range specified by your engine manufacturer