Stern Tube replacement

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oilyrag

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I need to replace my (boat's) stern tube. It is currently a bronze tube glassed in. The boat is a GRP fin keel with P bracket. Access from inside is very restricted, through a cockpit locker.
I am especially looking for ideas on the best way to remove the old tube and retaining alignment when a new, larger diameter GRP tube is glassed back in.
Advice from anyone who has tackled this task would be greatly appreciated.

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cos ID of existing tube is too small for flexibly mounted Yanmar now fitted, causing noisy shaft, depending on revs. Received wisdom is that larger ID tube is only proper solution.

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Last resort, surely?

Replacing a stern tube with a larger diameter one is a big job, and not really the ideal DIY project. If the problem is that the shaft is hitting the tube, surely there are other solutions you could try first? How about less flexible engine mounts? Or maybe changing the type of stern gland?

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What state is the cutless bearing in the P bracket in?, if you can get side to side movement in this then all you need replace is the cutless bearing, and repack the stern tube.

<hr width=100% size=1>J HAMER
 
Re: Last resort, surely?

Done new mounts, flexible coupling etc. etc. It's not doing any harm and is only an irritation but getting fed up with horrified looks that say "'ere mate, your prop is about to fall off!" Yard want £2000 but my guess is that £1500 of that is to say they don't want the fiddly job.

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Have you tried....

Have you tried fitting a Volvo Penta black rubber stern gland? These have an internal moulded section, rather like a cutless bearing, which gives support to the shaft and stops it moving around too much.

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Re: Last resort, surely?

Spoken with someone with similar problem who tried an aqua drive and it didn't make any difference either. Find it hard to believe but too expensive to experiment with and the installation would be almost as difficult as the stern tube due to limted access.

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Re: Have you tried....

Pondered this one and it could well be a solution. Would be interested to hear from anyone who has changed from or to a Volvo and cured/experienced similar problem..

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Re: Have you tried....

In a moment of madness several years ago, I changed my Volvo seal to a Halyard seal. I noticed a little more vibration at certain revs, which I'm sure was the result of reduced support for the inboard end of the shaft. Now I believe that the Halyard seals are no longer produced and have a finite life; seems like another Volvo seal sometime!

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It sure a tricky job. Got the same problem coming up soon. Someone recommeded the following approach.
1. Remove chft coupling and all the gear which is in the way.
2. Use a heavy and rigid saw blade insert it to the stern tube from the outside and (patiently) make two cuts and swat the tube in half.
3. Try to collapse the tube starting from inside he hull with hammer chisels etc.
4. Extract the old tube
5. Repair any damage done to the GRP with Epoxy
6. Epoxy the new stern tube in

In your case you need to increase the size of the hole of course.

allow me to say that this seems to be a radical, time consuming and expensive exercise to cure the mentioned problem.
I would first make sure that the shaft is indeed aligned centered in the stern tube in the first place. Check whether the engine mounts are correct (most have different stiffness for aft and front). check cutless bearing for movement and use a conventional stern gland which will add support to the shaft

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It's do-able. JFkals procedure for removal is spot on, try to use an industrial hacksaw blade if possible, about 1" wide and thicker than the normal product. I also put a chisel point on to a piece of steel rod to assist in folding back the bronze tube, as a long cold chisel.

Enlarging is more interesting. I made up a pull-drill from a length of steel rod with a lathe cutter on the end ( or could be any suitably hard steel with a cutting edge), to the exact diameter of the new tube. Onto the steel rod I fitted two grp discs the exact diameter of the existing hole to form bearings for the rod and maintain the alignment of the tube hole.

The drill was inserted into the sterntube hole from inside the boat and attached to a "powerful" electric drill somewhere where the cutless bearing would normally be.

Then simply operate the drill drawing it slowly aft until you have cut a perfect, and enlarged, sterntube hole. (starting is the only tricky bit as the drill bit jumps around a little - before settling down)

It took me approx half a day to remove the old tube and about two and a half hours to drill out the hole, plus preparation time. I cannot claim any originality for this scheme, which was given to me by the very very competent leading hand at the Elephant Boatyard, who is now sadly no longer with us.

As others have said, it does seem to be a somewhat radical procedure for what could surely be cured using plummer blocks/Aquadrive etc. Good luck.

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I know what you mean when you say that you get horrified looks, i know someone with a hunter 272 with a yanmar 1gm10 and his situation is identical. The solution that was suggested to him was a shaft diameter upgrade. The reason for the noise was the shaft whipping in the tube, proven by the engineer with a dial gauge(not that it was needed as the shaft could be seen to be whippping by eye).
He is just putting up with the noise that sounds to me like a bag of coal being tipped inside.
The aquadrive option will stop most of your problem i would have thought but it does depend on the length of the shaft at the end of the day. if its long you will still get the whip.


steve

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I had this too...

I have a BetaMarine 3 cyl engine and the stern tube on my Moody is only fractionally larger that the shaft. This resulted in the knocking you describe. I also have the Volvo rubber seal you mention. I'm not sure that the seal helps with this but I can defintely recommended one if you want a dry boat.

In desperation to get rid of the noise, I moved from a 2 to a 3 blade prop and changed the original flexible mounts to some much harder ones (bit trial and error this). The engine is now held very rigidly and the knocking has gone. There is however, quite a bit of vibration transmitted through the hull from the stiffer mounts (makes the locker catches rattle at certain revs!) but to me it is a far better sound than the shaft whipping around inside the tube which sounded very disturbing.

I reckon if I had the time and money I could play around with different mounts and get it a bit quieter but I've got used to it now and have plenty of other things I should get on with instead!


<hr width=100% size=1><font color=blue>Gavin </font color=blue>
 
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