PROPELLER BASICS
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.
Diameter is the width of the circle that the propeller would make when rotating. |
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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. For
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.
Pitch is the forward movement of the propeller after one revolution assuming there is not slippage. |
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.
The curved lip on the propeller or cup helps the propeller get a better bite on the water. |
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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 as 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. |
Ventilation
Ventilation is a situation where surface air or exhaust
gasses are drawn into the propeller blades. When this
situation occurs, boat speed is lost and engine RPM
climbs rapidly. This can result from excessively tight
cornering, a motor that is mounted very high on the
transom, or by over-trimming the engine.
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 physical
damage to the propeller’s blade surface due to the
collapse of microscopic bubbles on the blade.
There may be numerous causes of cavitation such as
incorrect matching of propeller style to application,
incorrect pitch, physical damage to the blade edges, etc...
Be advised disturbances in the water flow forward of the
propeller can result in blade damage which appears to be
blade cavitation, but is actually due to non-favorable water
flow into the propeller.
Cavitation is a phenomena of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade. |
730 Main St. #346, North Myrtle Beach, SC 29582
E-mail: lensprop@boatprops4sale.com
No Refunds. Exchanges only with 25% restocking fee.
843-280-2908
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