SELF-ALIGNING ENGINE WOES: A New Prop Shaft and Some Existential Questions Concerning Corrosion

Shaft seal

I have previously mentioned the problem I was having over the fall and winter with my engine being badly out of alignment. How I’d just run the engine anyway and eventually the prop shaft would whip it right back in line, and how this seemed to culminate in a shaft-seal leak that plagued me on the last leg of Lunacy‘s journey home from the W’Indies leaving Provincetown bound for Portland.

Given the impressive amount of water spraying all over the place as we left P-town, all of it spewing forth right from the mechanical face of the seal itself, I reckoned there was a good chance the whole unit would need replacing once I delivered the boat to the tender mercies of Maine Yacht Center. I knew I should at least have the bellows behind the seal replaced (it was about time), but I was prepared to bite the bigger bullet if necessary (see photo up top, of all new shaft seal installed).

And, of course, I asked MYC to realign the engine. And also to replace the cutless bearing, which I always assumed would be an obvious casualty of my radical self-aligning engine technique. The brains at MYC (Jeff Stack and Neil Collins) mentioned, almost as an afterthought, that we should have expert assistance and advice regarding the integrity of the prop shaft itself, as long as we had it out anyway, and so I delivered it to AccuTech Marine Propeller, a swank propeller and shaft outfit in Dover, New Hampshire, not far from where I live.

That damn shaft sure looked straight to me. One friend joked that all the folks at AccuTech would probably do was roll it across a pool table, then charge me for telling me it was straight. But no, they have very precise tools for measuring these things, and when they were done measuring my shaft they found it was in fact bent, imperceptibly perhaps, but enough to make a difference.

The new shaft, in case you’re wondering, cost almost $1,000.

Ouch! That was a bullet I was not anticipating.

New shaft in

New prop shaft and cutless bearing installed, waiting to receive the propeller

Lunacy on hard

Lunacy on the hard at MYC, reveling in a fresh coat of bottom paint


MYC staff work at wriggling the engine (which is under the sink) back in line. That’s Neil Collins down low with his head in the bilge

I can only assume it was the self-aligning engine that bent the shaft. So my current recommendation is that you not follow my example here. Once you figure out your alignment is out, you should deal with it promptly, not play on it seven months like I did.

One question I have, however, is: why didn’t I notice the alignment problem until after my insurance surveyor mentioned it to me as a possibility, based entirely on his casual eyeballing of the bellows on the shaft seal? It’s like he put the engine out of alignment just by looking at it!

While Neil Collins had his head in my bilge I also took my Man Overboard Module (MOM-8A) over to Chris Harrison at Chase Leavitt to get it serviced. This turned out to be a very smart move, as the inspection guys at Chase discovered the interior of the unit had suffered a fair bit of damage due to corrosion.

Chris told me this wasn’t a function of the MOM unit itself, as he often sees them perfectly clean and uncorroded when they are being inspected. He guessed it had something to do with the boat it was on. As in: my boat!

I said it was because my boat actually gets used a lot, and he said no, he’d seen both corroded and uncorroded units coming off very active commercial vessels. His theory is there is some property of individual boats that causes the corrosion, but he couldn’t say what exactly.

With Chris’s theory now planted in my head, I returned to Lunacy and reinstalled the revitalized MOM on its pulpit bracket. And yes, I realized, gear on Lunacy does actually seem to corrode more easily than on other boats I’ve owned and been aboard.

MOM corrosion

Evidence of corrosion on the newly serviced MOM unit

Outboard corrosion

And right after I reinstalled the MOM, I discovered the clamp threads on my brand new outboard had seized up. As you can see here, I broke off the clamp ear on one side and had to use a big crescent wrench to work the clamp bolt loose. I had the exact same problem with the old outboard engine, on the same side of the clamp! On my other boats this was never a big problem


The turnbuckles are often showing rust stains and need to be polished on a regular basis


Same with these halyard shackles

Why the hell is this? Assuming Chris’s theory and observations are correct, why should gear on one boat corrode more easily than gear on any other boat?

Should not all gear, assuming it is used and is out in the weather and is not in a direct galvanic couple of some sort, corrode equally?

Baltic ketch

Bonus photo! Saw this cool Baltic ketch at anchor when I was taking Lunacy around to her mooring after the engine realignment was completed. She’s got a yard for a raffee square-sail, plus two permanently rigged downwind poles, plus some weird jumper strut on the starboard side. That’s a lot of extra spars!

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7 Responses
  1. Charlie

    1.25 and about 4.5 feet long. And it was less than $1K, but not by much. Aquamet. Machined at the end for a Maxprop. Yes, it definitely seemed expensive. But I’m telling myself these guys do quality work.

  2. Don Joyce

    Corrosion causes are always a challenge to track down. I could find myself very paranoid, especially with an aluminum boat.

    Nonetheless, the examples you cite….shackles, turnbuckles, MOM and outboard clamp thread are probably unrelated.

    I would venture to guess that the outboard clamp was not properly grease. I also suspect the MOM container allowed sufficient sea water vapor in to cause mischief…..I would be very surprised and intrigued to learn you have an external cause of corrosion of the MOM fittings. Regarding shackles and turnbuckles, my own experience is these always require extra cleaning attention likely because of the forging process used to fabricate them. Although I’ve never been to see the manufacture of these, I’ve long suspected the dies are the root cause of surface discoloration due to non-stainless metal transfer from the dies. It would be an interesting investigative report verifying or disproving the cause.



  3. Dave

    Aquamet 22 –> Marine, Nitronic 50 –> same alloy, not labelled marine.
    Aquamet $$$$ vs Nitronic 50 $$. It’s the old tack the marine label on it trick.
    Many years ago I was chief engineer at a custom yacht builder in the midwest. Took me a while to convince them to go to Nitronic 50. You can imagine the money savings on 3″ shafts.
    PS – still looking for that MOM, but I have been distracted somewhat.
    Best – Dave

  4. Don Joyce

    Aquamet 22 –> Marine, Nitronic 50 –> same alloy, not labelled marine.
    Aquamet $$$$ vs Nitronic 50 $$. It’s the old tack the marine label on it trick.
    Many years ago I was chief engineer at a custom yacht builder in the midwest. Took me a while to convince them to go to Nitronic 50. You can imagine the money savings on 3″ shafts.
    PS – still looking for that MOM, but I have been distracted somewhat.
    Best – Dave


    Its the same alloy, but Nitronic 50 has a solution heat treatment which Aquamet doesn’t. I understand that the treatment increases the corrosion resistance of Nitronic 50 relative to Aquamet…at the cost of a slight reduction of strength.



  5. Colin A

    That ketch looks a bit like a Benford Great Pyramid Rig. Never seem one in real life and Benfords book seem a little light on the subject but it seems intriguing in pictures.

    Also seems a bit expensive on the shaft. When I used to write service estimates for a living I found a large variation in under water gear pricing from different shops with similar reps so it does pay to shop around.

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