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Cavitation
Bubble |
PROPELLER TERMINOLOGY
CAVITATION
Cavitation is a general term used to describe the behavior
of voids or bubbles in a liquid.
Cavitation is usually divided into two classes of behavior: inertial
(or transient) cavitation and non-inertial cavitation. Inertial cavitation
is the process where a void or bubble in a liquid rapidly collapses,
producing a shock wave. Such cavitation often occurs in pumps, propellers
and impellers.
D.A.R. Disc Area Ratio
D.A.R is the ratio of the projected blade area to the area of the
circle described by the diameter of the blades. Normally, this is
expressed as a percentage. 3-bladed propellers are normally about
50% or 0.50 D.A.R. 4-bladed around 0.7., and 5-bladed around 0.9 or
90%.
Obviously the size of individual blades influences the D.A.R.. On
propellers with overlapping blades it is not impossible to have a
D.A.R. in excess of 100%
E.A.R. Expanded Area Ratio
This is a ratio of the actual area of the propeller blades to the
area of the circle described by the diameter of the blades. This is
sometimes referred to as B.A.R. Blade Area Ratio. Normally E.A.R is
a higher percentage than D.A.R.
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WildCat
Propellers |
Screw Propeller Definitions
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Diameter(D): distance
from tip to tip
- Hub:
the connection between propeller and shaft
- Blade
Tip: the furthest point on the blade
- Blade
Root: the point where the blade meets the hub
- Pitch(P):
Theoretical distance a propeller would move in one revolution
- Pitch
Angle: Angle of the blade with respect to incoming flow.
It usually varies from root to tip.
- Fixed
Pitch:
- The
pitch is constant all the way from the blade root to the
blade tip.
- Blade is fixed
to the hub and cannot be altered.
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