Propellers
More about Propellers

1.  Diameter.

This is the distance across the circle made by the propeller blade tips as it rotates. 

2.  Pitch.

This is the distance that the propeller would move in one revolution, if it were moving through a soft solid.

3.  Rake

When a propeller blade is examined on a cut extending directly through the center of the hub, the face side of the cross section of that cut blade relative to a plane that is perpendicular to the propeller axis would represent blade rake.

4.  Cupping.

When the trailing edge of the blade is formed cast with an edge curl it is said to have a cup.

5.  Rotation.

The spinning direction of the propeller.

6.  Number of Blades.

A single blade propeller is the most efficient propeller if the vibration was not so bad.  To reduce the amount of vibration, more blades must be added.  This sacrifices efficiency.  4-5 blade propellers give improved acceleration and less vibration by putting more blade area into the water.  They also make for better use of the rake, generating more lift for added speed.

7.  Blade Contour.

This is the shape of the blades as viewed from directly above them.

8.  Skew.

The blade is swept back, to allow the shedding of weeds.

9.  Ventilation.

This happens when the exhaust or air enters the blade area.  This causes the engine to over rev, and lose thrust.

10.  Cavitation.

This happens when the water pressure is lowered in the surrounding water near the prop.  The water boils, causing water vapor.  This vapor then moves toward the Cup.  A higher pressure area where it turns back into liquid.  This releases energy against the blades and chips away at them.

11.  Slip.

This is the difference between actual and theoretical travel.  I have provided an example.   Lets say a prop is a 10 pitch, but the actual distance the prop advances per revolution is 8 1/2, the slip in this case is 15%.

1.  Boat speed equation. (Theoretical)

Prop Pitch (in.) x Prop Rpm

1056

Materials.

1.  Aluminum

Low in cost, fairly strong, corrosion resistant and easily repaired.  Less efficient due to flexing.

2.  Stainless Steel.

Stainless steel is 5 times stronger than aluminum, and can be made with thinner blades to improve efficiency.  Can be repaired easily although generally more expensive than Aluminum.  Good efficiency due to less flexing.

3.  Plastic.

Used primarily on small low hp motors and electric motors.  Corrosion free but none repairable.

4.  Bronze.

Used on Tournament ski boats and very large inboard powered boats.