A Cautionary Tale
A number of years ago as I was strolling through a boat yard, I came across a mechanic who was in the process of removing a propeller. He was using a technique that–even while writing about it well over a decade later– makes me cringe. He'd removed the propeller's nuts, and in their place had installed what I later learned was referred to as a "bang nut," an over-sized, closed-end brass fixture. The bang nut was screwed onto the shaft; however, it stopped short of making contact with the propeller hub.
To remedy this, the nut was hammered on using a substantial maul and all the user's might. As each blow connected with the bang nut, a shock wave was transmitted up the propeller shaft, through the coupling, into the transmission's bearings and gears and on to the engine block, where it was ultimately absorbed by the flexible motor mounts. With each swing the propeller reverberated with an ear-splitting twang.
After a score of mighty swings, the mechanic was visibly fatigued, yet the propeller remained fast with the shaft. The propeller ultimately yielded, sliding aft into the nut with a subdued clunk.
The physics of this approach are straightforward enough–with each blow, the shaft, transmission and engine were driven forward a fraction of an inch. The mass of the propeller, however, allowed it to be left behind in that forward advance, eventually forcing it to separate from the shaft taper. The technique worked but at what price?
It's likely that removing a propeller using blunt force of this or any kind, including striking the forward end of the propeller hub directly and driving the propeller aft, damages or shortens the life of transmission components, gears, bearings, etc. In short, this technique and others like it should never be employed for propeller removal.
It's Tempting But...
Pound for pound, transmissions are among the most costly pieces of gear aboard your boat, and repairing or replacing them is always a pricey and unpleasant experience. Some have proffered that the best way to separate a prop from its shaft is to take removal into account during assembly. Toward that end, use of anti-seize, grease or another release agent on the shaft taper is often advocated. But make no mistake about it; under no circumstances, once again, should this practice be employed.
Virtually every propeller manufacturer's installation guidelines clearly state that props should be installed "dry." The one exception to this rule is that an extremely thin application of a lightweight machine or motor oil onto the shaft taper is acceptable to help prevent binding. The primary goal is to make certain a propeller, once installed, stays put. Using anything between a propeller's bore and the shaft taper only increases the likelihood of movement
FROM THE BOATYARD
between the two, which in turn can lead to sheared keys or worse, a lost propeller. (Grease and other viscous materials are incompressible, making a hydrolock scenario very likely.) If removal is challenging, that's a good sign. Machine tapers like those used on shafts are truly amazing in their ability to reliably unify disparate parts, a desirable trait where propellers are concerned.
As desirable as it is to ensure a solid fit between a propeller and shaft, there will come a time when the two must be separated. Fortunately, there are a variety of means of easily doing this without damaging the propeller, shaft or transmission. For boat yards the Cadillac of propeller removal tools relies on the power of hydraulics. Combination tool kits are available to remove propellers, shaft couplings and strut-mounted shaft bearings. These tools enable yards to quickly and easily disassemble components using a hand-operated hydraulic pump, which actuates a ram, which in turn applies thousands of pounds of continuous rather than shock force to an assembly.
If you are in need of propeller service or shaft coupling removal, it's well worth finding a yard that relies on a tool of this sort. It's worth repeating, under no circumstances should it ever be necessary to use shock force to disassemble any of the aforementioned components, propellers, couplings and cutless bearings.
As you might imagine, the convenience of this tool comes at a price. A combination hydraulic prop, coupling and shaft bearing removal tool costs several thousand dollars and it's large and heavy. Because of this, in most cases it's housed in a rolling dolly that's about the size of a baby carriage. Alternatively, mechanical, puller or scissors-like prop removal tools are also available.
These are comparatively compact and lightweight and, consequently, much less expensive. While useful, these tools do have limitations. Primarily, they are incapable of imparting as much force as a hydraulic tool, and they must be able to "reach" between two adjacent propeller blades, a requirement that may not be possible with some four- and most five-blade propellers. Yet another option relies on a custom-made yet exceedingly simple pulling mandrel. This arrangement consists of three high-strength threaded rods and a plate that uses the propeller shaft end as its fulcrum.
The rods are screwed into matching threaded holes that have been bored and tapped in the aft end of the propeller hub. (Your propeller has these, right?) Some propellers come from their manufacturer with these holes (if you are ordering a new prop or new boat, request them). Some include just two holes, which I would argue is inadequate for this process. Some have no holes. In any event, the holes can be easily added by most propeller shops; however, this cannot be done while the propeller is installed on the shaft. With careful measurement and a simple drawing, most machine shops could make a custom tool for you at relatively low cost. (I believe that these should be provided as an option or standard by every boat builder for their propeller arrangement.)
The plate need not be made from exotic alloys; ordinary mild steel is more than adequate provided it's painted and lightly oiled. The threaded rods should be high strength and can easily be purchased off the shelf.
The final approach and the one I favor most relies on a method that bears some resemblance to the pulling mandrel; however, it uses a purpose-made tool known as a PropSmith. The PropSmith also requires a trio of threaded holes in the prop's hub; however, its fulcrum plate engages the shaft's threads, holding it rock steady and making it especially useful for in water use by a diver.
Because of this threaded shaft engagement, an added benefit of the PropSmith is its ability to aid in installation of a propeller, pushing it firmly onto the taper, as well as removal. It's not unreasonable to suggest that every vessel carry its own means of propeller removal, even if you never intend to undertake this task yourself.
Why is this necessary? If you find yourself in a boat yard for planned or unexpected propeller work, and you see a mechanic making his or her way toward your boat carrying a large hammer and a bang nut, you can intercede with your own tool. You know it will work well, it will work quickly and you can be confident that it won't cause any damage in the process.
Steve D'Antonio is a marine systems consultant offering services to boat buyers, owners and the marine industry, as well as an author and photographer. He is an ABYC-Certified Master Technician. Read more from Steve at www.stevedmarineconsulting.com