Getting the Shaft

By Tom Neale, 4/2/2009


Old and New Shafts at Camachee

Chez Nous was recently hauled in Camachee Yacht Yard in St. Augustine, Florida. For this haulout I had a special project that went far beyond the regular regimen of bottom painting, and I wanted a yard I could trust to do the job well. We got a new shaft, and there can be a lot more to this job than you might suspect.

Changing a shaft on an inboard powered boat is something that most boaters either never do, or do only after they’ve hit something and bent the one they have. I’ve never had to change a shaft for that reason (yet), but I change them periodically as general maintenance. It’s a very expensive item of general maintenance, but the likely consequences of having your shaft break are even more expensive.

Why should a shaft break? Well, think about it.  All the strength of your engine is transmitted via that little shaft to the propeller which spins forcefully and rapidly, its blades pushing against the water to push your boat forward. And this has a tendency to twist and whip the shaft, even though it doesn’t (hopefully) actually do so. That little tapered tip which is inserted into the prop, out there in the water at the aft end of your boat, takes tremendous stress as it spins that propeller. And the forward end of the shaft, machined with a key slot and inserted into the coupler to the transmission takes a huge amount of vibration and mechanical abuse.

The shaft is held in place by the strut (sometimes more than one) and perhaps also at the point where it goes into the hull.  At each of these points, it spins in a cutlass bearing which is usually a rubber (or similarly performing material) bushing contained within a tube.  Over the years, this rubber wears, allowing the shaft to flex more.  If the grooves in the rubber aren’t allowed to pass water through, the cutlass bearing will wear more quickly from lack of lubrication and possibly heating.  So each cutlass bearing needs to be changed periodically. If you don’t change them when needed, the shaft will have more problems quicker. When you do change the shaft, it’s wise to change the cutlass bearing anyway. Be sure that the yard you use has the appropriate tools for this job, and people who know how to use them. Camachee did.

And then there’s the issue of crevice corrosion.  This can occur in stainless steel when it is in non moving water that is oxygen deprived.  In a sense, the process is different from rusting which requires oxygen to react with the metal.  It’s also different from rusting because it’s not necessarily as obvious.  Where the shaft passes through the stuffing box (or packing gland or seal—there are different types), typically it sits, when the boat is not being used, in non moving water. This is an ideal place for crevice corrosion to develop. If the boat isn’t used much, other good places for it to develop are under the propeller, possibly in the shaft log and under cutlass bearings.  Crevice corrosion can insidiously work its way into stainless, eventually substantially weakening it.

Any stuffing box (or whatever type of device you have to keep water from coming into the hull where the shaft passes through) works in part by forming a seal against the shaft.  Some are much less abrasive than others, but most cause some degree of friction as that shaft is spinning.  Over the years, this can actually wear the shaft so that it becomes a bit thinner. This may seem hard to believe, but when you consider the number of revolutions the shaft is turning each time you use the boat, and that the water is probably a little salty or has some grit in it, this becomes easier to appreciate.

There are many grades of stainless steel. Some are almost worthless as stainless, in my opinion. And some builders may use low grade stainless because it saves money. If you have a higher grade of stainless in your shaft, you have less to worry about. But even the best grades of stainless aren’t immune to all these and other problems as the years of use go by.

This doesn’t mean that you should change your shaft every few years. Typically a shaft may serve well for many years and several owners. I change mine periodically because of the way I use my boat. I put thousands of miles on it a year in all sorts of waters and I’m often far from help. Also, I keep my boats for a long time. But it is important to carefully inspect your shaft, or have a qualified person do it. Unfortunately, it’s difficult to carefully inspect the shaft without removing it. This isn’t something that most boaters would want to do every year because on most inboard engine boats, removing the shaft is a real chore. It may even involve dropping the rudder.  There are some things that you can do that aren’t so onerous, however.

If you or the yard removes the prop at haulout, the tapered end of the shaft can be inspected for signs of corrosion, wear or other problems.  At least every few years it’s a good idea to pull the prop anyway, for inspection and often for reconditioning. Even if you’re one of those fortunate people who haven’t hit anything, eventually your blades may become a bit out of kilter with enough running, and a good prop shop can measure and “swing” it to see if there is a problem, and take care of it.  You can also inspect your shaft around the cutlass bearings. You can’t see up in the bearings, but if the shaft is at all loose in the bearing or shows any kind of wear or signs of deterioration, you should replace the cutlass bearing and carefully inspect the shaft where it passed through that bearing. Sometimes replacing the cutlass bearing requires removing the shaft.


Tom’s Tips About Shafts

 

1. Regularly, while running, look at your shaft in the area between the engine and where it exits the hull. Some amount of movement may be normal, but find out what is and what isn’t. If the shaft seems to be moving about

Click Here for More Tips

Another very important thing that you can do is to slide back your stuffing box (or whatever type of device you have) while the boat is hauled and look at the shaft underneath. You may find that a groove has been worn into the shaft over the years. This, of course, weakens a shaft. Or you may find signs of pitting which could indicate crevice corrosion. If you see something like this, rub it with fine Emory Cloth to be sure it’s not just salt deposit you’re seeing. If you find signs of pitting, unless you’re qualified to call the shot yourself, get a qualified professional to take a look.  In most cases like this, the only way you can know how serious crevice corrosion may be is to have someone X-Ray the shaft. This will probably be as expensive as replacing the shaft, which is what I would do.

You can also back the shaft out of its coupling to the engine and check that end for any indication of fracture, stress cracks, or other problems. If you have the shaft replaced, be sure the yard sends the transmission coupling half which receives the shaft to the machine shop which is preparing the shaft to be sure that the new shaft is “trued” to the coupling. (Camachee Yacht Yard knew to do this.) It often isn’t adequate to just shove the shaft in. You may also need an engine alignment, but normally you wouldn’t get this done until after the boat has been back in the water for awhile and “settled” back to its normal position.

See www.tomneale.com