|
|
Blade Back
Suction side. Forward side of the blade (surface facing the
bow).
Blade Face
Pressure Side, Pitch Side. Aft side of the blade (surface
facing the stern).
Blade Number
Equal to the number of blades on the propeller.
Blade Root
Fillet area. The region of transition from the blade surfaces
and edges to the hub periphery. The area where the blade attaches
to the hub.
Blade Tip
Maximum reach of the blade from the center of the hub. Separates
the leading and trailing edges. |
|
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...
Cup
Small radius of curvature located on the trailing edge of
the 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.
|
|
Diameter
Diameter is the distance from the center of the hub to the
tip of the blade x 2. It can also 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.
Hub
Solid cylinder located at the center of the propeller. Bored
to accommodate the engine propeller shaft. Hub shapes include
cylindrical, conical, radius, & barreled.
Leading Edge
The edge of the propeller blade adjacent to the forward end
of the hub. The leading edge leads into the flow when providing
forward thrust.
|
|
|
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.
Pitch is the second number listed in the propeller description.
Radius
The distance from the axis of rotation to the blade tip. The
radius multiplied by two is equal
to the diameter.
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.
Aft Rake helps to trim the bow of the boat
upward, which often results in less wetted surface area and
therefore higher top end speed.
Forward, or negative rake,
helps hold the
bow of the boat down. This is more common in workboat type
applications.
|
|
Rotation
When viewed from the stern (facing forward): Right-hand propellers
rotate clockwise to provide forward thrust. Left-hand propellers
rotate counter-clockwise to provide forward thrust.
Skew
The transverse sweeping of a blade such that when viewing
the blades from fore or aft shows an asymmetrical shape.
Aft Skew: Positive skew. Blade sweep in the
direction opposite of rotation.
Forward Skew: Negative skew. Blade sweep
in the same direction as rotation.
Slip
Slip is the difference between actual and theoretical travel
of the propeller blades through water. A properly matched
propeller will actually move forward 80 to 90 percent of the
theoretical pitch. |
|
Trailing Edge
The edge of the propeller adjacent to the aft end of the hub.
When viewing the propeller from astern, this edge is closest.
The trailing edge retreats from the flow when providing forward
thrust.
Track
The absolute difference of the actual individual blade rake
distributions to the other blade rake distributions. Always
a positive value and represents the spread between individual
blade rake distributions.
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. |
|
|
|
 |
