Stern Tube Replacement

[emlesnick]

Hello all and thanks in advance-

In the middle of a massive refit and repower of a Shannon 51'

Here is the situation. The stern tube is a very short piece of bronze tube with an exterior bolt on fitting. The problem lies in that the end of the stern tube and stuffing box end at an IMPOSSIBLE to reach location. From the end of the stern tube there is probably a 6' run of shaft to the engine coupling. It is under the aft bunk and under the steering quadrant. Maintenance would be a nightmare and any problem or issue at sea would be a serious hazard.

The plan is to replace the existing stern tube with a longer tube moving the stuffing box forward, closer to the engine and in a place where proper maintenance can take place and any potential issues mitigated.

The plan is to align the shaft with existing setup and new engine, then remove old stern tube. Slide new stern tube in, create little "bulkheads" out of either fiberglass or starboard to fit in the bottom of the bilge and support new tube. Slide the shaft in new tube and have it run through the new cutlass bearing at the aft end and have it shimmed to center in the forward end of the stern tube. Align with engine and support with bulkheads. Glass into place, which, given its location will most likely consist mostly of chopped glass and thickened epoxy poured into the bilge to fill the area to above the level of the tube.

So first of all any thoughts?

Material for the stern tube? Fiberglass tube, SS Tube, Carbon Fiber tube?

Should I be concerned with a probably total run of 6' of stern tube for harmonics or other problems?

If I use a FG or CF tube for the stern tube is there any consideration for the set screw for the cutlass bearing? I don't want to be ripping the tube apart dealing with the set screw in future years or for that matter removal of the cutlass bearing in general.

Any thoughts or suggestions would be much appreciated!

Thanks

[chris95040]

First thing, if I were to have an important piece of metal below the waterline, dark, lonely, and forgotten, I'd much prefer to stick with bronze than change to stainless. In general its far superior to stainless with respect to corrosion. Just make sure it's a real marine bronze, phosphor or silicon bronze come to mind.

I replaced my stern tube recently since 50 years seemed like a pretty good service life, the original tube was bronze and so was it's replacement. BTW I worked with "T Norris Marine" in the UK when I did this work. I actually replaced the stern tube as well as the stern bearing housing (which threads onto the end of the stern tube where it exits the boat, also has a flange which mates with the back end of the keel, and the purpose of the stern bearing house is to hold the cutless bearing.) They made a semi-custom casting for me by tweaking one of their existing designs. Great stuff. Anyways, just in case you end up needing other parts. They've got a lot of experience with old fashioned stern gear so maybe they could advise you on your project a bit.

Regarding your plan, I'd want to be absolutely certain I wouldn't be starving my stuffing box of water somehow. I imagine since it's all well below the water line it wouldn't be a problem.. But its a long run of tube with a spinning shaft in it, not sure if some weird dynamic could screw up your water flow. Seems unlikely, and I bet there are counter-examples of such a setup working just fine.

Most folks do seem to replace the old metal stern tube + stern bearing house with just a fiberglass tube, then just have flexible hose to go from the tube to the stuffing box. Though the examples I'm familiar with had a pretty hacky approach to the cutless bearing, they just installed it inside new stern tube with a dab of 5200, seemed likely to damage the stern tube when it was time to replace. Also, at least on my boat, the stern bearing house had (and has) holes in it to scoop some water and presumably force it toward the stuffing box. Not sure if that's necessary, but replacing all that assembly with just a plastic pipe wouldn't afford that option.

So lets see, corrosion, cooling water, what else to consider. Oh! I always hear about how boats flex when in or out of the water, and you should do your alignment with the boat in the water, my run of shaft is too short for it to make a difference but I wonder about all that when I consider your plan. It all sounds nice and beefy, and this pipe + series of bulkheads would certainly stiffen things up, but if the geometry of rear end of your boat really wants to change a bit when it goes in the water, problems might arise...

[emlesnick]

Chris-
Thanks for all the info! I'll check out that company you mentioned in the UK. I'm trying to pick as many brains as possible because when it comes to something like this I'd like to make sure my grandchildren won't have to address a future issue.

As far as the flexing of the hull I don't think that will be an issue. The boat is built like a brick shithouse. My fear is having an issue and the whole works being buried in a now very solid encasement in a still very inaccessible area.

As far as water to the Stuffing box- That is a very good point and something I did not consider...It is definitely well below the water line so theoretically the whole tube should always be full of water, correct? Also, I was planning on going with a PSS Dripless Shaft Seal that has it's own water supplied to it.

The big question still remains of the whole cutlas bearing replacement.

Maybe a threaded insert glassed into the carbon fiber tube for the set screw?

I know on my old Tartan it was a fiberglass tube and can say for sure the cutlas bearing was 5200 into it. Removal was basically insert long sawzall blade and keep it flat. If you see fiberglass dust you're through the bearing! Then knock out from the inside...I remember thinking..There has to be a better way.

[Hoohaa]

Sounds like you have a long prop shaft.
I have been in the process of getting rid of a wobble in my 2.3m 8ft prop shaft for some time now, so I would suggest that you look at where the shaft is supported. My shaft is only supported at each end and it has always had some shaft wiping going on which has caused the shaft to nock on the stern tube.
I am currently preparing to install a 2nd cutlass bearing into the forward end of the stern tube which will supply an extra support near the middle of the shaft and hopefully be the final solution. My stern tube would only be around 600mm or 2ft long so do yourself a favour and think about the unsupported length of shaft.

[Oceansailor]

Originally Posted by emlesnick

Chris-

Thanks for all the info! I'll check out that company you mentioned in the UK. I'm trying to pick as many brains as possible because when it comes to something like this I'd like to make sure my grandchildren won't have to address a future issue.



As far as the flexing of the hull I don't think that will be an issue. The boat is built like a brick shithouse. My fear is having an issue and the whole works being buried in a now very solid encasement in a still very inaccessible area.



As far as water to the Stuffing box- That is a very good point and something I did not consider...It is definitely well below the water line so theoretically the whole tube should always be full of water, correct? Also, I was planning on going with a PSS Dripless Shaft Seal that has it's own water supplied to it.



The big question still remains of the whole cutlas bearing replacement.



Maybe a threaded insert glassed into the carbon fiber tube for the set screw?



I know on my old Tartan it was a fiberglass tube and can say for sure the cutlas bearing was 5200 into it. Removal was basically insert long sawzall blade and keep it flat. If you see fiberglass dust you're through the bearing! Then knock out from the inside...I remember thinking..There has to be a better way.

A better way is to let the cutlass bearing stick out one inch from the stern tube. Use no glue. Instead two set screws. That’s more than adequate to prevent the bearing from turning. When it is time to replace the cutlass bearing just loosen the two set screws. Then use a pipe wrench to turn pull the bearing out.

[Fore and Aft]

I installed a second cutlass bearing behind the stern tube and almost cooked it. The problem was no water was reaching that high when I was motoring. I just plumbed some water in there and never had a problem again.
When I am doing surveys I do not see many grub screws holding the cutlass bearing in place, mostly they are just tapped in with a bit of Sikaflex.
Cheers

[Hoohaa]

Originally Posted by Fore and Aft

I installed a second cutlass bearing behind the stern tube and almost cooked it. The problem was no water was reaching that high when I was motoring. I just plumbed some water in there and never had a problem again.
When I am doing surveys I do not see many grub screws holding the cutlass bearing in place, mostly they are just tapped in with a bit of Sikaflex.
Cheers

That is something I have been thinking about and have decided to replace the stern gland with a PSS seal and hook up the port to inject the water into the seal. Should solve the heating problem and give me a nice dry bilge in the process.