Propellers
Everything you need to know.

A.  Blade Tip.

The maximum reach of the blade for the center of the propeller hub.  It separates the leading edge from the trailing edge.

B.  Leading Edge.

That part of the blade nearest the boat, which first cuts through the water.  It extends from the hub to the tip.

C.  Trailing Edge.

That part of the blade farthest from the boat.  The edge from which the water leaves the blade.  It extends from the tip to the hub.

D.  Cup.

A small curve or lip on the trailing edge of the blade, permitting the propeller to hold water better and normally adding about 1/2" to 1" of pitch.

E.  Blade Face.

That side of the blade facing away from the boat, know at the positive pressure side of the blade.

F.  Blade Back.

The side of the blade facing the boat, known as the negative pressure (suction) side of the blade.

G.  Blade Root.

The point at which the blade attaches to the hub.

H.  Inner Hub.

The forward end of the inner hub is the metal surface which generally transmits the propeller thrust though the forward thrust hub to the propeller shaft and in turn, eventually to the boat.

I.  Outer Hub.

The exterior surface is in direct contact with the water.  The blades are attached to the exterior surface.  Its inner surface is in contact with the exhaust passage and with the ribs which attach the outer hub to the inner hub.

J.  Ribs.

The connections between the inner and outer hub.  There are usually three ribs.  The ribs are usually either parallel to the propeller shaft, or parallel to the blades.

K.  Flo-Torq Shock-Absorbing Rubber Hub.

Rubber molded to an inner splined hub to protect the propeller drive system from impact damage and to flex when shifting the engine, to relieve the normal shift shock that occurs between the gear and clutch mechanism. 

L. Diffuser Ring.

Aids in reducing exhaust back pressure and in preventing exhaust gas from feeding back into propeller blades.

M. Exhaust Passage.

The hollow area between the inner hub and the outer hub though which engine exhaust gases are discharged into the water.

1.  Push/Pull Concept.

This propeller has been pictured at the point where one of the blades is projecting directly out of the screen.  This is a right hand rotation propeller, spins clockwise.  As the blade that I am referring to rotates or moves downward, it pushes water down and back as your hand does when you swim.  At the same time, water rushes in behind the blade to fill the space left by the movement of the blade.  This results in a pressure differential between the two sides of the blade: a positive pressure, or pulling effect, on the top side.  This happens constantly as the propeller turns.  So the propeller is both pulling and pushing through the water.

2.  Thrust/Momentum.

The pressures cause water to be drawn into the propeller from in front and accelerated out the back, just as a fan pulls air in from behind it and blows it toward you.

The propeller draws or pulls water in from its front end through an imaginary cylinder a little larger than the propeller's diameter.  As the prop spins the water is accelerated, creating a jet stream of higher velocity water.