Stern tube in 31 ft GRP sailing yacht

[Chas1st]

On cleaning up the inboard end of stern tube for replacement of the one piece Volvo stern gland I have noticed that the tube set into the hull has corroded all the way through on its underside. It has been repaired in past with a lining of fabric and resin of some kind. As result it doesn't provide the rigidity I would like to have for creating a water tight seal. I haven't removed the repair yet so not made a thorough assessment of the extent of the corrosion. As the tube appears to be stainless steel I am surprised, even with my limited experience, that stainless can corrode to such an extent. Does anyone have experience of:
1. Corrosion of stern tubes
2. whether it might be a practical option to replace the stern tube.
Thanks

 

[Motor_Sailor]

Over the recent years, there's been an increasing awareness of problems with the corrosion of stainless steel sterntubes. It's probably an age thing as its 'x' years since they were introduced as an alternative to the far more satisfactory (and expensive) bronze alternative.

Here's a video of a pretty comprehensive solution to the problem found in this Baba 35.

 

[Keith 66]

You say that it has corroded through on the underside? Could this be down to wear from the prop shaft or partly down? I have seen this several times & it was always down to the shaft not being centred in the tube before lining up the shaft to gearbox coupling.
In all cases the engine mounts had either gone soft & collapsed or the shaft hadnt been centred in the tube before alignment was attempted.

 

[Chas1st]

Thank you Motorsailor and Keith 66 for your thoughts. The prospect of excavating the tube and realigning and resetting it is a daunting one. I will have a closer look as I cannot definitely say the damage is the result of corrosion, it was repaired some time ago and wasn't actually leaking. If is caused by mechanical damage by a misaligned shaft current thinking now is to improve the repair with reinforced resin, to get a better shape and surface for the Volvo stern gland to seal against. I didn't notice signs of wear on the shaft. C

 

[vyv_cox]

What stainless steel really dislikes is what is called 'stagnant' areas in which the oxygen content of the water reduces. Close fits that are not sealed, hoses with long term drips, wet wood, are favourites.