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Blade Back

Suction side. Forward side of the blade (surface facing

the bow).

Blade Face

Pressure Side, Pitch Side. Aft side of the blade (surfacefacing 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 TipMaximum 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

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shapes include cylindrical, conical, radius, & barreled.

Leading EdgeThe edge of the propeller blade adjacent to the forward

end of the hub. The leading edge leads into the flowwhen 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.

RakeRake 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 thebow of the boat down. This is more common in

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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.

SlipSlip 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 EdgeThe 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 tightcornering, a motor that is mounted very high on the

transom, or by over-trimming the engine.

Propeller Basics

Most marine propellers have markings which indicated it's dimensions. There are a

number of different combinations of markings, but they should be in order of diameter,

followed by pitch. Quite often on inboard propellers the diameter and pitch will be

separated by an X or the direction of rotation. Some outboard propellers use part numbers

which have to be cross referenced to a catalogue to determine the dimensions. When

discussing or ordering propellers, always refer to the diameter first, then the pitch e.g.. 20

by 18, which indicates a 20 inch diameter propeller with 18 inches of pitch. Propellers are

specified in millimeters around the world, except for North America, where inches prevail.

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ElementsPropellers are made up of various elements.

The Hub, is the center of the propeller. It functions to provide a method ofattaching the propeller shaft to the blades.

The blade root is where the blade is attached to the hub. It is typically thethickest part of the blade.

The blade is the helical formed section of the propeller which transmits therotational torque of the propeller shaft into thrust to propel your boat. Blades are

designed in all kinds of profiles and outlines, each offering various benefits in

converting torque to thrust.

The blade pressure face is the high pressure side of the propeller bladefacing away from the bow of the boat.

The blade suction face is the low pressure side of the propeller blade facingtoward the bow of the boat.

The leading edge runs along the blade outline from the root to the tipseparating the pressure and suction face of the blade. A sharp leading edge

reduces the load on the shaft, but increases the chance of damage. Most leadingedge profiles are a trade off between strength and durability.

The trailing edge runs along the blade outline from the root to the tip. It iswhere the water exits the blade. The profile of the trailing edge is critical in

reducing noise and harmonics.

The blade tip is formed between the leading and trailing edges on the bladeoutline. The distance from the center of the hub to the blade tip times 2 describes

the propeller diameter.

MaterialsPropellers are made of various materials, including everything from plastic to titanium.

The most common materials are:

Composite. This material is very light weight and moderately strong. It isexpensive and almost corrosion proof, but is non-repairable.

Aluminum. This material is light weight and moderately strong. It isinexpensive and repairable, but it will corrode readily.

Stainless Steel. This material is heavy and very strong. It is repairable, butvery expensive. It is corrosion resistant, but will corrode rapidly without the

presence of oxygen.

Manganese Bronze. This material is moderately heavy and moderatelystrong. It is the least expensive of the bronze materials and is quite repairable. It's

main ingredients are copper and zinc. With a lack of protection, the zinc content

in the material will act as an anode to protect other underwater metals on yourboat leaving oxidized copper behind. This is indicated by a pink coloring to the

propeller. After enough corrosion takes place, the propeller will be brittle and

need to be replaced.

Nibral (nickel aluminum bronze). This material is lighter and strongerthan manganese bronze. It is more expensive than manganese bronze, and is quite

repairable. It's main ingredients are copper, aluminum, and a small amount of

nickel. Due to it's higher copper content, it's more corrosion resistant than

manganese bronze, but left unprotected the aluminum can act as an anode to

protect other underwater metals on your boat. This is indicated by dark craters on

the surface of the material ringed with a green color. Left long enough, these

craters will go right through the propeller, and it will have to be replaced.

Rotation

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Propeller rotation. For some reason, this is often a confusing subject.

Right hand. This refers to the direction of rotation of the propeller to provideforward thrust

to your boat. A right hand propeller rotates clockwise when viewed from astern.

In other words, standing behind your boat and looking towards the bow, thepropeller turns in a clockwise rotation.

Left hand. This is the opposite of right hand, and viewed from astern, rotatescounterclockwise.

Another method of determining the rotation of your propeller is to place your

thumb on the aft end of the hub, and wrap your fingers over the trailing edge of

the blade. The hand you use determines the rotation of the propeller.

Twin propellers. Normally on smaller vessels the propellers will rotateoutwards at the top, making the port side left hand, and the starboard side right

hand. On larger vessels it is often more efficient to rotate the propellers inwards

at the top, especially with single rudder installations. Either way, it is very

important to note which prop is on which side prior to removal to avoid the

embarrassment of installing them backwards.