Q&A - Securing the prop shaft - Soundings Online

Q&A - Securing the prop shaft

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Q: How can you keep your propeller shaft from backing out?

Q&A - Securing the prop shaft

Q: How can you keep your propeller shaft from backing out?

A: If you’ve been out and about in boats long enough, you’ve probably seen those moments when someone throws it into reverse to stop before crashing into a dock, and the shaft comes loose from the coupling, resulting in the shaft and prop spinning off astern. This also occasionally happens when sailboats are sailing along and the crew hears a thump astern as the shaft slides out, fouling the prop with the rudder, and water begins pouring in through the shaft hole. If the shaft has come all the way out, the water might come in at a rate greater than your bilge pump capacity. (Always use extra capacity bilge pumps.)

Propeller shafts usually are secured to the coupling at the transmission by set screws tightly screwed into dimples in the shaft, and sometimes the coupling clamps the shaft and its key. There’s a simple trick that can help stop the shaft from backing out if it loosens from the coupling, but first a thought about preventative steps.

The set screws should be secured from turning with stainless wire, but even this isn’t a guarantee they won’t vibrate loose. Always check your engine space before a run. (Be careful to avoid moving machinery — all should be off — and electric shock risks.) Make sure the set screws are tight, with no indication of movement. I prefer to occasionally torque them down with a wrench that I keep nearby for that purpose. Also, check the shaft to be sure it hasn’t moved away from the coupling. Usually you can use the end of the key or keyway as a reference point. If that doesn’t work on your engine, mark the shaft where it enters the coupling with an indelible fine-point pen or a piece of electrician’s tape.

At an appropriate time, verify that the shaft has dimples in its engine end for the set screws. These usually are there, though occasionally they are omitted. You likely can check by removing a set screw and looking into the hole with a small flashlight. (Make sure you reinstall the screw securely.) If you can’t tell, the shaft might have to be removed from the coupling. This may be difficult to do without professional assistance, but you should consider having it done, perhaps when you’re hauled or getting other work done in the engine room. The pro also can check the end of the shaft for signs of impairment. Not only do you want to be sure that the shaft has dimples, but also that the set screws are bottomed out in the dimples, rather than just off to the side.

When replacing set screws, it’s important to look with a small light into their holes to be sure the dimples are aligned properly with the hole. Some feel that dimples weaken the shaft at its end. If the dimples are too deep this could be possible. But a slight dimple, done by a qualified professional, shouldn’t significantly weaken the shaft end, and the risk of this is much less than the risk of throwing a shaft when there is no dimple.

Now for the trick of the trade. Consider tightly attaching a donut zinc to your shaft just forward of where it passes through. Depending upon your boat, this likely will be just forward of the stuffing box. Don’t clamp it too close to the stuffing box. (Ours is about a foot forward of it.) You don’t want it to get in the way of servicing that component, and there should be plenty of clearance for the zinc collar to rotate without fouling anything, including when the engine is in reverse.

If your shaft pulls loose, this should stop it from going very far. There is no guarantee that this will work. The tactic might be inadequate for the circumstances, but it should at least help. It might damage the stuffing box or shaft log upon impact, but this is preferable to having the propeller jammed in your rudder with water pouring in, or the shaft on the bottom of the sea and a huge stream flooding the boat.