Extension of shaft tube

[webber63]

Hello all,
I should extend the inner stern tube of my boat (a 1978 Barberis Show 34) because I would replace the stuffing box with a Volvo or Radice seal.

The shaft is 30mm.

As you can see from the pictures, the tube, whose outer diam. is 52mm while the interior is 43mm, is reduced to a stump of 20mm,
not sufficient for the installation of any type of seal.

The Volvo requires for example at least 36 mm, other types even more.

I thought to extend the tube and wonder what might be the best solution to do it.

Any opinions and / or advices?
Fair winds,
Francesco

 

[Norman_E]

The end on photo appears to show a smaller diameter tube sunk a short way inside. Is that the part with the 43mm inside diameter, or is that the inside measurement of the short outer length? If the latter it might be difficult to do the job. For a 30mm shaft the RMTA seal is available to fit a 45 or 48 mm outside diameter tube. If the inner tube is 43mm inside diameter then a piece of 45mm tube machined for part of its length to fit inside it and sealed in with epoxy adhesive would be a possibility.

 

[webber63]

Hi Norman_E, thanks for Your reply,
Yes, 43mm is the inside diameter of the short outer part.

Actually I was thinking to apply a machined tube like the one on this attached image:


The larger side would fit the outside of the short outer part.
What do you think about it?

 

[Norman_E]

It would be difficult to do it that way with only a short bit of tube to fix it to, far better in my opinion to have a fairly long piece of tube closely fitted into the existing stern tube with a 45mm diameter piece sticking out to take the RMTA or Volvo seal. The seal would fit the larger diameter of the new part with the smaller diameter secured inside the existing tube. A 1.5 mm wall thickness tube inside the existing one would still leave a 40mm hole, which should be enough clearance for the shaft unless your engine mounts are sloppy. If you can get 45mm stainless tube with a 1mm wall thickness you can even save the machining. Just cut a piece long enough to take the new seal and then cut a longer piece lengthwise, removing about 3.5mm, then close it up and force it into the 45mm piece and the existing tube. A small tack weld to hold the parts together and a good seal of epoxy will do it. You could make absolutely sure it cannot move with a screw through the existing large diameter tube into a hole drilled in the new one.

 

[webber63]

Thanks again, Norman.
I wonder if the stainless steel tube may cause galvanic corrosion to the existing bronze stern tube.
Do you think I should insulate them with Duralac, too?

 

[pvb]

Would it be possible to remove some of the fibreglass around the stern tube, to expose a longer section to clamp on to? I'd be very hesitant about relying on a glued-on extension.

 

[webber63]

Would it be possible to remove some of the fibreglass around the stern tube, to expose a longer section to clamp on to? I'd be very hesitant about relying on a glued-on extension.

Hi Pvb,
To remove the fiberglass I should remove the small portion of tube that is part of a flange screwed around the actual tube and drowned in the fiberglass.
I'm really scared to do something like this, even if I think it would be, probably, the ultimate solution.

 

[pvb]

Hi Pvb,
To remove the fiberglass I should remove the small portion of tube that is part of a flange screwed around the actual tube and drowned in the fiberglass.
I'm really scared to do something like this, even if I think it would be, probably, the ultimate solution.

Wouldn't it be possible just to remove the fibreglass, maybe using a dremel-type tool? You only need another 10-15mm to have something long enough to clamp securely on to.

 

[Keith-i]

How was your previous stern gland attached? Presumably with a rubber sleeve and a jubilee clamp. How about gluing a machined sleeve in as you suggest and also putting a rubber sleeve over the joint with a clamp on either side of the joint as a precaution. Surely to avoid corrosion just machine the new part from bronze.

 

[Norman_E]

Wouldn't it be possible just to remove the fibreglass, maybe using a dremel-type tool? You only need another 10-15mm to have something long enough to clamp securely on to.

That would not work because the diameter of it is far too big for a Volvo or RMTA seal

 

[Norman_E]

How was your previous stern gland attached? Presumably with a rubber sleeve and a jubilee clamp. How about gluing a machined sleeve in as you suggest and also putting a rubber sleeve over the joint with a clamp on either side of the joint as a precaution. Surely to avoid corrosion just machine the new part from bronze.

Bronze would be great if the OP can get a suitable piece, but unless the exact composition of the existing "bronze" tube is known the new piece might well be further from it on the galvanic scale than most stainless steels are to some bronzes. In any ene it may well be that the existing tube is not even bronze but some composition of brass. I would not in fact worry too much about galvanic corrosion but an alternative might be to source a piece of tufnol tube, which is easy stuff to machine.