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The Basics:
Propeller Diameter - is simply the distance across the circle
swept by the extreme tips of the propeller blades. Diameter is the
most critical factor in relation to power absorbed and delivered thrust.
In the majority of cases, the larger the diameter the greater the
propeller efficiency.
Revolution per minute - RPM is the number of full rotations a
propeller makes in a single minute. In most cases the
shaft RPM is different from engine RPM. In most applications, a reduction
gear is fitted between the crankshaft and the tail shaft. The primary
purpose of installing a reduction gearbox is to reduce the RPM at the
propeller so that a more efficient propeller may be used. Since larger diameter propellers are more
efficient in producing thrust, lower RPMs are generally desirable for most
installations.
Pitch - is the theoretical distance the propeller pushes the
boat forward in one complete revolution. Since the propeller is cutting
through water, the actual distance traveled by the boat is not the same as
the indicated propeller pitch. The difference in the theoretical and
actual distance traveled is called Slip.
Blade Area - is the surface area of the propeller blades. Blade
area can have direct effect on absorbed power and cavitation.
Number of Blades - the number of blades will vary from vessel to
vessel. Generally, more blades will produce smoother running.
Propeller Rotation - Marine Propellers can either be right hand
or left hand. Right handed propellers rotate clockwise as viewed from
behind the boat looking towards the stern. Left hand propellers turns
anti-clockwise.
Leading edge - is the edge of the blade that cleaves the water.
Trailing edge - is the edge of the blade where the water moves away
from the propeller blade.
Other technical terms associated with marine propellers:
Cavitation - bubbles of partial vacuum caused by excessive
propeller speed or loading. Vacuum bubbles implode against the
suction side of the propeller causing vibration and even worse is
the force of imploding bubbles is so great that it actually suck the
metal right off the surface of the propeller. Large amount of slip
and excessive tip speeds are likely to cause cavitation. The
end-result is uneven wear and vibration. You can see photos of cavitation damage by
following this link.
Ventilation - is the effect of air sucked down by the propeller
from the water's surface. Ventilation will lead to vibration and loss of
thrust. Ventilation can be corrected by just simply putting the propeller
deeper under the surface of water.
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