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Propeller
Anatomy
Propeller
Selection
The propeller that came with your new engine isn't necessarily the
one that is best suited to your needs.
This is because many original equipment props, understandably,
are made to meet a range of average conditions for many kinds of
boats, hull styles, and loads.
This can limit their performance.
What you need is a propeller specifically selected for your
kind of boat, and your style of boating.
The
size of a propeller 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:
Diameter is two times the distance from the center of the
hub to the tip of the blade.
It also can be looked at as the distance across the circle
that the propeller would make when rotating.
It is the first number listed when describing a propeller. |
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| Pitch:
Pitch
is defined as the theoretical forward movement of a propeller
during one revolution -- assuming there is no "slippage"
between the propeller blade and the water.
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most boats, there is slippage and therefore the distance advanced
is less than the design pitch.
The amount of slippage varies from boat to boat.
Pitch is the second number listed in the propeller description. |
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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.
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.
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