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Old 21-11-2020, 17:01   #1
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Oil filled steel stern tube: a good alternative to typical shaft/stern tube setups

This is not a question, but a description of the setup on my pervious boat, a 12m “Joshua” steel double-ended ketch (same as Bernard Motiessier’s, but with an aft cockpit). This thread applies primarily to metal boats and might be useful for people building or refitting a yacht.
“Sundowner” had an 80hp 5 cylinder Mercedes engine driving a Hurth hydraulic gearbox and a 40mm stainless propshaft with a 3 bladed 22”X18” prop in an appature in the stern hung rudder. Sorry no pics: I sold the boat 20 years ago.
Starting from the prop side, the steel stern tube had a cap which bolted on to the aft face of the keel. The cap had a short section that fitted into the stern tube to center it. Then there were 3 lip seals which were an easy press fit into the stern tube. Then there was a self aligning roller bearing whose ID was a moderate press fit to the shaft and whose OD was an easy press fit to the inside of the stern tube. On the inside of the hull, the stern tube extended about 50mm, had a flange with another cap. Inside the cap were 3 lip seals.
Next was an Aquadrive thrust bearing unit bolted to a thwartships web frame. The Aquadrive was set up so the propshaft was right in the middle of the stern tube and its trust bearing took all the axial load of the prop in forward and reverse. The Aquadrive has a short shaft and CV joints to connect with the gearbox. No need to align the engine, which had soft mounts.
The stern tube had a nipple on its upper side where it extended into the hull.
This nipple was connected via a vertical copper tube to a small ss steel tank installed about 1m ABOVE the waterline level directly above the stern tube. The tank was filled with engine oil.
The positive pressure of the oil in the stern tube, due to the evevated tank, prevented water from entering the tube AND the oil lubricated the roller bearing and the lip seals. This set up is essentially what you find on big ships and worked perfectly for us for 17 years of continuous live-aboard cruising.
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Old 21-11-2020, 17:21   #2
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

You could use another oil that is not toxic just in case of a leak.
The manatees.
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Old 08-12-2020, 18:09   #3
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

I agree. This was back in “bad old days” when I wasn’t too concerned about pollution. In any case, the amount of engine oil that leaked out through the lip seals under normal conditions was negligible. In the case of total failure of all 3 of the outer lip seals, only about 2 lt of oil would be lost.
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Old 09-12-2020, 04:58   #4
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

While you could use vegetable oil etc.
I’m going to make a comment that will inflame some I’m sure, and that’s oil in small quantities isn’t really harmful, nature breaks it down. There has been oil leaking into the seas and the environment naturally for a very long time. You can even slap an “organic” label on it to make the whole food crowd feel good.
There is oil, and fuel in our exhausts, that’s why Yanmar’s are smoky.

It’s when huge amounts of almost anything is introduced that nature is overwhelmed for a time, but even then it recovers.

One of my bucket list dives that I never made was Truk Lagoon, and I believe that it had recovered before the 70’s, and lord knows how much fuel and oil was released when all those ships were sunk.
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Old 09-12-2020, 13:11   #5
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

your setup sounds uncommon for sailboats but similiar to the grease pump system on some older boats - mine for example - the stern tube is quite long and is fed by a simple automobile style hand grease pump - I think its quite a good design. I give the pump about 6 pumps when i'm putting the boat away for awhile and this seem sufficient to reduce water ingress to nothing for intervals where the boat is sitting o its mooring. Uses about one greasepack refill a year, which seems negligible from a pollution point of view. The boat was built in 1978-79 so its a pretty well tested design.
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Old 09-12-2020, 14:32   #6
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

Re grease filled stern tubes:
In fact our stern tube was originally grease filled with a hand pump. However, when the previous owner unexpectedly died, the boat sat at its berth in San Francisco for 5 months before we bought it. During that time, no one pumped grease into the tube, and we weren’t aware that it needed to be done. We took off for the S. Pacific and while in the Touamotus, water started leaking through the inner seals quite badly. We pumped grease into the tube, but every time we ran the engine, we had lots of water leaking past the seals. Luckily we had enough grease cartridges to get us through the pass at Tahiti. When we finally hauled the boat there (another drama!) and got the prop shaft out, we found that water had gotten into the tube and rusted the mild steel “cage“ in the roller bearing at the stern end of the tube; bits of the cage had then got mixed into the grease and destroyed the lip seals. The shaft was scored in the area of the lip seals and we had it metal sprayed to repair the damage.

That’s when we changed to the oil-filled set-up I described above. As long as the resevoir is kept filled, there’s no drama, and it only needed to be topped up once every couple of years. It had an external sight gauge so keeping track of it was dead easy.

The problem with grease filled tubes is that there is no constant internal pressure in the tube to counteract the external water pressure.

With regard to the kind of oil used, I’m not sure that a vegitable based oil has the necessary specs, but if you‘re a Vegan, then I guess its a big deal.

A64Pilot:
I agree that the paranoia about pollution has gotten out of hand. A bit of crap in the water is ok (fish and whales crap in the water all the time and oil seeps out of the sea floor); natural systems will take care of it in time. Its the scale that matters. A few humans on the planet is ok, but we have got a plague of them now and that’s where the problem is. Of course with all the world’s religions against birth control, this problem is not going to get addressed. My personal action was to consciously choose not to have kids...
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Old 10-12-2020, 13:49   #7
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

thanks for the explanation - makes sense and it sounds like quite a clever development to the greasegun system. what type of oil does your system use - ie 30 engine oil, 80 gear oil?
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Old 10-12-2020, 14:10   #8
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

They advertise environmentally "friendly" oil for this purpose in magazines aimed at large shipping companies.
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Old 11-12-2020, 00:06   #9
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

I sold “Sundowner” many moons ago, but as I now recall, we used 80w gear oil in the vented tank (which we made out of fibreglass).
Another good thing about this system was that, with only the one bearing at the outer end of the stern tube, it was very easy to pull the prop shaft if necessary (to replace that bearing or the lip seals). Simply disconnect the shaft from the Aquadrive (it had a keyway plus two bolts at 90˚angles going into dimples on the shaft), loosen the end cap from the stern tube and pull the shaft using the prop to pull on. The bearing‘s outer race was a “medium” slip fit in the tube and of course was always in an oil bath, so there was never a drama about it coming out. The inner race of that bearing was a fairly tight fit on the propshaft, so there was never an issue of the shaft turning in the race.

The Aquadrive is a really nice piece of kit, but costly. With a good bit of engineering skill, one could put together a similar system out of standard parts: 2-way thrust bearing, CV joints, etc.

If one was building a boat, I think this system would be appropriate for pretty much any size over say 10m. A stainless stern tube could be moulded into a fibreglass hull.

I think that if the hull design called for a “P” bracket and cutlass bearing, that would also be no problem. It just so happened that Sundowner was a “Joshua” double ended full-keel hull with the propshaft exiting out of the aft end of the full keel, so no “P” bracket.
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Old 11-12-2020, 00:38   #10
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Re: Oil filled steel stern tube: a good alternative to typical shaft/stern tube setup

Anyone who uses a 2 stroke outboard ( including me) would be a far bigger polluter than your oil filled shaft setup.
But yes too many humans is the biggest issue.
I rationalise our 2hp outboard use when I look at a superyacht parked up knowing his generator probably does more damage in a day than we do in a year.
Still want to get a 4 stroke tho.
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