4 Reasons You Should Never Purchase Used Props

Captains frequently seek to economize when they need to replace a wheel, and to this end often search for used props on eBay, Craigslist, or with a prop shop that inventories used running gear.  They often don’t realize the downside of buying someone else’s discards.  To understand why, we need to first look at some principles of hydrokinetics.   

The transference of energy from a rotating engine to forward motion of the boat relies on hydrodynamics, the science of the energy of moving water.   There is an optimal curve of each blade that results in the maximum production of both thrust of water aft, and negative pressure forward of the wheel, literally sucking your boat forward as it turns.  Any deviation from that perfect blade shape results in inefficiency – the failure of the prop to transfer all its energy from the drive shaft to the water. ISO484 Class 1 is a standard of how closely the blade shape conforms to that optimal curve. When your prop-shop “tunes” your propellers, they are reworking the blades to conform to that perfect geometric shape.

Propeller blades must endure enormous amounts of stress.  Think about how thick and rigid the oar of a row-boat needs to be to.  Have you ever rowed with a cheap aluminum and plastic oar, only to have it break off in your hands?  When it happens you wish you had one made of solid hickory.  That oar has to survive a lot of force to move your little jon-boat through the water.  If you have a modest sized diesel engine in your cruiser, the force on your prop blades is 500 times greater than what an oar must withstand!  So the blades have to be intensely strong. Anything that diminishes the strength of the blades will result in flexing and distortion of the blade profile, leading to inefficiency.  Vibration often follows due to the uneven nature of one blade’s distortion versus the others, which is not just uncomfortable for passengers, it will destroy your cutlass bearings and may even damage your transmission.    

When you buy a used prop, it has likely been subjected to numerous forces that cause the blades to weaken.  So while it may be have perfect geometry on the bench, it loses its shape in the water under load.  Following are the most common causes:
#1.  Galvanic Corrosion.  How often do you see a boat hauled out that has a zinc missing from the drive shaft?  It happens all the time.  Captains wait too long to change their zincs.   Any time two dissimilar metals are near each other in salt water, they lose material and structure.  Once the zinc is gone from the drive shaft, the weakest metal remaining is the prop, which starts to lose material.  How many months has that used prop you’re considering buying been sitting in salt water without a zinc to save it?  The wheel could be 30 years old and has lost 15% of its mass.  You just don’t know.  
#2.   Re-working.  Bronze and NIBRAL are not infinitely malleable, meaning you can’t bend them repeatedly and expect them to maintain their strength. Every time they get re-built, re-tuned, or re-pitched, someone takes a hammer to them.  The bending and flexing causes small amounts of degradation of strength.  After 3 or 4 sessions on the anvil, a blade will no longer reliably maintain its shape under load.  You have no idea how many times a used prop has been reworked.  Whatever the seller tells you – don’t believe it. 
#3.  Cavitation.  No matter how well maintained a boat is, cavitation takes its toll over the life of a prop.  Low pressure areas on the leading side of the blade result in boiling of the water, which is like a thousand microscopic explosions next to the surface of the bade.   Because of customer demand for speed, many boat manufactures power their designs near the threshold of allowable blade loading, which results in cavitation.  Even a modest amount of cavitation will thin out the blade over time.  Thin blades flex under load.  
The forth reason to forgo used props is the principle of “Total Cost of Ownership.”  When you buy used wheels, there are any number of hidden costs that make the deal just not worth it, particularly when Class 1 wheels are available at factory-direct prices from ZZ-Prop.   With used props you always have to compromise.  The perfect size is never available, so you’ll have to have the wheel re-pitched, or a sleeve added to the bore.  Secondly, you won’t get the performance or longevity from a used wheel that you will a new one.   When you consider the cost of haul-out, mounting, and re-launch, the difference just isn’t that great.   The added cost of new wheels can be easily made up in economy over the first two tanks of gas.

Your props are where “the rubber meets the road.”  The perfection of the geometry of each blade is essential to the proper transfer of energy from your engines into forward motion of the vessel. With the price of Class 1 NIBRAL props from ZZ-Prop so economical, there’s no reason to gamble on someone else’s cast-offs. So let some other captain have those used props you found on eBay. Let him find out they just don’t perform like what he had before.  Two or three haul-outs later he’ll finally realize that the problem isn’t the pitch, or the cup, or the balance.  What he needs is the rigidity and strength of some new NIBRAL in the water to push him around.
John T. Cox VP Operations
​Timbolier Industries, Inc.