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Propeller Basics

This information should give you basic knowledge on how boat propellers work
and also the terminology used when you are talking about propellers. Here is
the list of topics you will see as you scroll down the page.




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 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 phenoma 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 two times the distance from the center of the hub to the tip of the blade. 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.
Leading Edge
The edge of the propeller blade adjacent to the forward end of the hub. When viewing the propeller from astern, this edge is furthest away. The leading edge leads into the flow when providing forward thrust.
Non thru-hub Exhaust Propellers
propellers are used for inboards using shaft driven propellers, sterndrives using through hull exhaust, and on some outboards that don't route the exhaust through the lower unit torpedo.
Over-hub exhaust Propellers
have the blades attached directly to the smaller tube that fits over the propeller shaft, eliminating the larger exhaust tube. These types of propellers are often used for attaining maximum top speeds. (On some boats, the hole shot can often suffer due to the extreme exhaust flooding that occurs around the propeller blades during acceleration.)
Over/thru-hub Propellers
are a combination of thru-hub and over-hub exhaust propellers. This allows some exhaust to escape at lower RPMs, providing a controlled amount of exhaust flooding. These types of propellers will allow the propeller to be slightly easier to turn during initial acceleration, allowing for a better hole shot on some engine/boat combinations.
Thru-hub exhaust Propellers
Consist of a round barrel to which the blades are attached. The exhaust passes through the barrel and out the back, without making contact with the propeller blades. This provides a good clean water flow to the blades, usually resulting in good acceleration and hole shot.
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.
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 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.
Slip
Slip is the difference between actual and theoretical travel of the propeller blades through water. A properly matched propeller will actually move forward about 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.
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.
Propeller
Device with a central hub and radiating blades placed so that each forms part of a helical (spiral) surface, used to propel a vehicle such as a ship or boat. By its rotation in water, the propeller produces thrust on the blades, which gives forward motion to the vehicle.


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