Common Propeller Terms
1. LEADING EDGE - The edge of the propeller nearest to the boat cuts through
the water first, starting at the hub it extends to the blade tip.
2. BLADE TIP - This is the farthest point that a propeller extends from the
center of the hub to the outer radius of the blade.
3. TRAILING EDGE - The edge of the propeller farthest away from the boat
where the water leaves the blade.
4. CUP - The cup on a propeller is designed to help lock the propeller in the
water to reduce slippage and prevent cavitation. The cupped area is located on
the trailing edge of the blade starting approximately 1" from the hub
extending out to the blade tip.
5. PUSHING FACE - This is the face of the propeller blade away from the boat.
More commonly called the PITCH FACE which faces the pitch block when repairing.
6. NEGATIVE FACE - This is the face of the propeller blade toward the boat.
7. BLADE ROOT - The thickest area of a propeller where the blade and the hub
are joint together.
8. HUB - The center of the propeller that fits over the propeller shaft.
9. OVER HUB EXHAUST - Exhaust gasses flow over the hub and blades.
10. THRU HUB EXHAUST - Exhaust gasses flow through a barrel of outer hub to
prevent exhaust gasses from flowing over the blades of the propeller.
11. RUBBER HUB - inner hub bushing made of hard rubber, molded to a splined
spindle to protect the drive train when shifting.
12. ACR/DIFFUSER RING - The flared ring used on through hub exhaust
propellers. The ACR/Diffuser ring prevents the exhaust gasses from backing up on
the blades which produces cavitation on take-off.
13. VENTILATION - The introduction of air from the surface or exhaust gases
being drawn into the propeller. A ventilating propeller is actually slipping and
produces very little thrust.
14. CAVITATION - A low pressure area on the blade surface that allows water
to boil at a lower than normal temperature and creating bubbles. The collapsing
action or implosion of the bubbles releases energy that chips away at the
blades, causing a ''cavitation burn”. The initial cause of the low pressure may
be nicks in the leading edge, too much cup, sharp leading edge corners, improper
polishing or , sometimes poor blade design.
15. PITCH - The theoretical travel of a propeller through
a mass per revolution. EX: a 19" pitch propeller moves approximately
19" per revolution.
16. STRAIGHT PITCH - The pitch is constant or the same from leading edge to
the trailing edge of the propeller.
17. PROGRESSIVE PITCH - The pitch increases from the leading edge to the
trailing edge. EX: Leading edge measures 17", trailing edge measures
17" pitch - this is a 3" regressive pitch.
18. VARIABLE PITCH - The pitch increases from the leading edge to the
trailing edge, and from the hub to the outer tip.
19. RAKE - The angle of the propeller blade in
correspondence with the propeller shaft.
20. FORWARD RAKE - Blades are angled toward the boat. Commonly used for
inboard propellers and small outboard propellers.
21. AFT RAKE - Blades are angled b back or away from the boat. This type of
rake is used to help lift the stern of the boat on take-off, and on top end will
help to lift the bow up - improving performance.
22. PARABOLIC RAKE - The off center development of a propeller blade used to
make the rake concave or convex.
23. DIAMETER - The overall width of a propeller.
24. RIGHT HAND ROTATION - The propeller turns clockwise on the shaft.
25. LEFT HAND ROTATION - The propeller turns counter-clockwise on the shaft.
Introduction to propeller repair