Updated replacement for old Stuart Turner stern gear

[nickcrowhurst]

In 1977 I launched our Eventide 26 after three years spare time (hard) work. I installed a used P55 Stuart engine, which I later replaced with a Yanmar 1GM10. The stern gear is the original from Stuart Turner, with the grooved rubber outer bearing, and the inner traditional greased rope inner stuffing box. This inner bearing seems to be lined with a nylon type of material, but that's a guess. I had thought it was white metal, but 35 years since I installed the tube is a long time. I installed a white plastic flexible coupling to the 1GM10.

Last month I hit a submerged object in the upper Tamar, and when I put her ashore, found that the stern tube had snapped off where it emerged from the deadwood, as had the two bronze coachscrews securing the stern bearing. I had evidently hit something very solid.
I've now completely removed and dismantled the stern gear, and I now have the bare engine and a hole through the deadwood. The prop is okay, but the 1" bronze shaft needs to be replaced. I wish to assemble the new system with three main aims:

1. To update the stern gear with any developments in stern gear technology since 1975.
2. To have a safe seamanlike system that will last many years.
3. To install a system with an inner bearing that does not drip, even occasionally, as the Eventide has a very shallow bilge, and a cupful of greasy water goes all over the area under the bunks.

The quick and dirty solution would be to carve an inch off the back of the dead wood (I've already done that while diagnosing the problem), put a thread on the broken end of the stern tube, fit a new prop shaft, and chug happily over the horizon.

However, I like to improve systems during replacement, so I have some questions to help install an improved stern gear:

1. I have 110mm clearance for the inner bearing, between the 1GM10 female flange coupling and the shaft log, so I could fit a Volvo seal, at 36 + 66 mm. Is this a good plan? What about a bearing? I would need a new stern tube to replace the 1.5" O.D tube, presumably non-threaded at the inner end. Are replacement stern tubes available? The original tube has some slight de-zincification. This type of seal is water lubricated. The inner bearing is about 6" under the water line. What bearing is used at the inner end, as I assume the Volvo seal is just a seal, and not a bearing? Would I have enough space for a bearing and a Volvo seal? The new stern tube would need to be installed so that water could not leak along the outside of the stern tube. Traditionally it might have been white lead. How could I secure a watertight installation, while being able to remove the tube in the future, if necessary for maintenance? The current inner bearing was sealed against the shaft log, and the outer bearing was sealed against the deadwood, so the tube did not need to be installed with a completely watertight sealant.

2. The outer bearing with its attached rope stripper is a bronze housing with a fluted rubber-type bearing. It's time to replace this rubber. Are there better modern alternatives? Are the replacement rubbers available?

3. Would a stainless steel shaft be better than bronze? Would stainless introduce electrolysis problems? The engine does very few hours each year (perhaps only 20), but the water is silt-laden.

4. Has there been any development of systems to enable propellor and/or shaft coupling removal without the traditional trauma, heat, plus-gas and, eventually, the angle grinder?

Basically, I can start from scratch, as if I were building the boat again, so what would would be the ideal modern system, bearing in mind the limited gap between the shaft log and the 1GM10 coupling? I'd be very grateful for any advice.

Nick.

 

[sarabande]

what's the condition of the wood surrounding the stern tube ? If there has been dezincification or other corrosion causing a weakness which might have contributed to the failure, there may be some waterlogging or other effects.



Welcome to the forum BTW - even though you have registered a long time ! I think you hold the record !


If I were starting truly from scratch, I'd use an engine as a (constant speed) DC generator to drive an electric motor for the prop via a belt drive, but I suspect that system losses on a small engine would make that impractical.

 

[cliffordpope]

I had similar considerations a few years ago when I converted from an abortive P55 ST to the small Yanmar.

Several points need to be borne in mind:

1) A traditional shaft/log with outer whitemetal bearing and inner stuffing box is solid - ie it doesn't waggle about at the inboard end. The Yanmar engine is famous for rocking about rather a lot, so you can't rely on simply very accurate alignment. (The ST was solid bolted to the bearers with no movement) Presumably your flexible coupling was able to cope with this movement, which could be up to half an inch sideways?

2) Did the old tube have an actual bearing at the inboard end, or just the fibre packing? It does really need a bearing in view of the considerable force of the engine kicking about from side to side.

3) As I understand them, modern set ups have a loose outer bearing (cutlass) and no bearing at all inboard. The shaft is free to flap around inside the tube, centralised by approximately aligning the engine, so that even the most extreme movements don't cause the shaft to bang on the tube. You don't use a flexible coupling, but do obviously need a very secure flexible rubber boot to stop water gushing in.

My personal feeling is to a phobia of rubber seals, prefering an old-fashioned tight wad of greased fibre held by an honest bronze cup, but I realise millions of boats aren't constantly sinking because their rubber seals have split, so that must be just my prejudice.

Plan A was to just have the traditional stuffing box and a short flexible propellor shaft with hardy-spicer joints, but Plan B added a simple bronze bearing block to give more support to the fibre stuffing.

Regarding sealing the void between the tube and the log, a method I read about and employed was to drill a hole sideways into the wood until I just touched the tube, cement in a grease nipple, and then pump in silicone sealant with a grease gun.

 

[Tranona]

I have exactly the same set up in my Eventide 26, except that I have just replaced the Yanmar with a Nanni 14.

Had 3 goes to get this right, but from gearbox output back. Bullflex coupling - expensive but far superior to the Nylon type; Volvo seal fitted onto original ST inner bearing housing machined down. White metal (at least mine was) in inner housing replaced with a length of cutless bearing. You might get away with no bearing in this housing as the Volvo "seal" is also a water lubricated bearing very similar to the outer ST bearing. The outer bearing is still available, but I had mine machined to take a cutless - cut down to 3 5/8" from the standard 4". I also believe that Montrose make a custom cutless to fit straight into the housing. an alternative is a composite bearing that Neil Young of H4 Marine would make for you.

Not sure what you should do with the damaged stern tube, but similar ones are still made - Lake Engineering in Poole is one source, Teignbridge another. I am surprised you managed to get it out - the reason I modified mine is that I could not get it out to replace it. Don't think I would cut back the deadwood as better to get a new tube made to use the existing end fittings. You definitely need a stainless steel shaft - the bronze one in theory is not strong enough. You can get new screws (2"*20g) from Classic Marine - just bought 2.

Key thing with the installation is to support the shaft at both ends and let the engine bounce independently as there is not enough clearance in the tube if you leave it floating. Do not be persuaded by other dripless seals. The only viable alternative is Tides and that is physically too big for your space (as well as twice the price).

Don't know where you are based, but my boat is in Poole and is currently in pieces, so you are welcome to come and look at it.

 

[Keith 66]

T Norris of Isleworth good quality gear, be aware for a full set of stern gear inc new tube, cutless housing & inner flexibly mounted stufing box plus new stainless shaft it will set you back at least £500.

 

[nickcrowhurst]

Those excellent replies are much appreciated. In response to some of the points raised:

1. The mahogany deadwood was as sound and dry as when I fitted it 35 years ago, and it was reclaimed timber then.

2. The system has coped with the movement of the 1GM10 for 25 years without any problems.

3. The old stern tube had an inboard bearing behind the stuffing box. It appears to be a white nylon type of material, lubricated by the grease from the remote greaser. Remarkably, it appears to be unworn. Would this material be satisfactorily water lubricated in conjunction with a Volvo seal? Probably safer to go with the Cutless ruber type?

4. Pumping silicone sealant round the stern tube is a good plan. However, I would like to be able to remove the stern tube for maintenance. When I built the boat I made the tube a sliding fit, and relied on sealing the ends. That's why yesterday I was able to slide the tube out by hand. Perhaps I could use silicone for an inch at each end, so that this could be cut when required.

5. Superb details of the Eventide retro-fit, thank you. Machining the inner bearing housing to take the Volvo seal is a grand idea. I will certainly follow up the suppliers mentioned, and come back to you all when I hit snags.

6. The Norris system is of considerable interest, as the package would presumably be well developed and proven, and the price would not be a deterrent.

Thanks again for all your help and interest.
Nick.

 

[Tranona]

The inboard bearing could well be composite - my 1963 one was white metal. However, as you say it is grease lubricated so if you are going water lubricated then change it to a water lubricated composite or a cutless. Not sure I would use silicone sealant - Polysulphide is more usual underwater.

You should be able to get a new tube made to the right length and threaded then modify the end bearings - much more economic than trying to replace the whole lot!

Good luck.

 

[nickcrowhurst]

You should be able to get a new tube made to the right length and threaded then modify the end bearings - much more economic than trying to replace the whole lot!

Today I went to C & O Engineering, at Abbotskerswell near Newton Abbot, Devon, tel. 01626 367782, and spoke to the proprietor, Graham, who knows exactly what he is talking about. His company's sole business is stern gear refurbishment. By next tuesday I should have a new stern tube and 316 stainless prop shaft, new propellor (12 x 8), new grooved rubber sleeve in the outboard bearing, and a new 1" cutless bearing in the inboard bearing, with the inner Stuart Turner bearing stripped of its packing assembly and machined down to take a Volvo seal #3819724.
Brilliant, thank you.
Nick.

 

[Tranona]

Good. Hope it all goes well. Prop seems a bit on the small side, but depends on the reduction ratio. I had a 13" fixed 3 blade prop on my 1GM with 3.1:1 box, then a 15" feathering prop, but you can't swing them with a lower ratio - worth checking though as the larger diameter is potentially better behind the long keel.

 

[nickcrowhurst]

The engineer checked prop size and pitch using some software on his PC, using data from my Yanmar handbook and my estimate of cruising speed. He wouldn't just replace my old prop with the same size without checking. He said 12 x 8 was slightly over the ideal, but 12 x 7 was under. I was impressed.
Nick.

 

[rjp]

You can get a phenolic shell 'cutless' bearing as a direct replacement for the Stuart Turner rubber one from Countrose Bearings in Birmingham (about £34 two years ago and it wasn't in their catalogue so I had to call them!) Just presses into the original stern fitting, with some effort in my case.

 

[nickcrowhurst]

Good information, thank you. The engineering company has now decided on 12 x 10 for the prop, after looking at blade size on the original. With a stainless steel shaft for the first time I'm now having to plan more for electrolytic action than in the past 35 years with a bronze shaft. Previously I've only had a zinc on the prop rope cutter. I presume I have to have a large hull-mounted zinc anode, wired to the prop via the shaft (somehow), and wired to the engine and the stern tube. I'll do some searches on the forum.
I've always had my user cp option set to "immediate notification by email" when a new post is added to my thread or post, but I never receive the emails. Is this a cookie issue?
Nick.

 

[Tranona]

You could, of course replace your rope cutter with another make that does not need an anode!

When I put in my 1GM and stainless shaft, I fitted a hull anode, wired to the gearbox and a braided copper bridge over the R&D coupling. This worked fine - very slowly eroding and no problems with my fixed 3 blade prop. When I fitted the Bullflex coupling I could not bridge as there is no metal contact with the shaft. However the anode still continues to erode slowly. I also now have a JF feathering prop with its own anode and this also only erodes slowly. My Ambassador Marine cutter is made of 316 so is compatible with the shaft, so does not need an anode.