Propeller Repair FAQs

Can my propeller be repaired?
In most cases, aluminum, stainless steel, and bronze or nibril propellers can be repaired.  Curled blade edges, missing pieces of the blade edge, and bent blades can all be straightened and filled.  The central hub in aluminum and stainless steel propellers can be replaced.  Usually, if more than 1/3 of a blade is missing or the metal of center hub of the propeller is damaged, the propeller cannot be repaired.  In addition, aluminium propellers that have been repaired multiple times may not be able to be repaired due to metal fatigue.
How are propellers repaired or reconditioned?
There are really several phases to reconditioning a boat propeller.  The first step is to verify and correct any pitch problems.  This is accomplished by placing the propeller in a special jig  and aligning the blades with a pitch block using heat and hammers.  Once the pitch is set, the propeller is sandblasted to remove any corrosion, old paint, or any other residue.  After sandblasting, any broken areas are cleaned up by grinding away areas that won't hold new metal.  The missing pieces of the blade are welded back with the appropriate metal.   In the next step the blades are ground back to the original equipment manufacturers shape.  Following the grinding the propeller is tested for balance and adjusted by removing small amounts of metal.  The propeller is then cupped, if necessary, then finished by polishing or painting.  Once completed, the propeller is checked one last time for balance and corrected if necessary.
What is pitch?
The pitch of a propellers determines how far a propeller will travel in water in one rotation.  A 19" pitch would travel 19 inches in one full rotation.  Of course the pitch value of a propeller is theoretical in nature and does not account for slippage and other environmental issues within the operating arena.  
What is cupping?
Cupping consist of changing the shape of the blades at the trailing edge.  Slightly curling the edge toward the direction of rotation.  Cupping a propeller effectively increases the pitch of a propeller.   The cup gives the propeller more grip in the water and will bring the boat to plane quicker  This puts more strain on the engine, thus the effective increase in pitch.
What is a hub?  or why can't I go anywhere?
The hub part of the propeller is the part that makes contact with the propeller shaft.  Most outboard and inboard/outboard props these days are splined.  That is, they have gear teeth that meshes with the propeller shaft.  There are different numbers of gear teeth depending on the make and model of the engine.  The two most common have 13 or 15 tooth splines.  There are two different kinds of these hub systems, the rubber hub and the exchangeable hub.  The rubber hub is composed of a splined shaft surrounded by vulcanized rubber.  The hub is pressed into the propeller hub cavity using a hydraulic press, funnel, and hub driver.  This is not a field serviceable hub system.  If you "spin" a hub you must have it professionally replaced.  The second kind of hub system is the exchangeable hub system.  Originally invented by Mercury and known as the flo-torq hub system, this hub system has become more common.  It is a field serviceable hub system consisting of a specialized thrust washer, a urethane drive sleeve, and a shaft adapter.  The urethane drive sleeve replaces the rubber in the pressed in rubber hub system and works on geometry rather than friction.  In both cases, the rubber hub or exchangeable hub system the rubber or the drive sleeve are failure points.  Should the propeller strike something in the rotational plane (e.g. tree stump, rocks, gravel, rebar, etc.), the rubber or the drive sleeve are designed to release to protect the lower unit propeller shaft, carrier bearing, drive shaft, etc.  The release or failure of the rubber hub or drive sleeve is know as a spun hub.

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