Leading on from Knotty's chainplate thread and just in case this helps anyone here:
We all know electrolysis is caused by dissimilar metals in salt water, but I discovered (the hard way!) what can happen when the dissimilar metals in question just happen to be bolted to a piece of timber.
If you'll pardon the mess and the crummy camera angles, here's the 'before' pic:
It turns out that what is happening in the above pic is that the bronze bolt above the stern gland has worked itself loose over time and sea miles allowing water to slowly leak along it, down the sternpost and onto the bronze stern gland fixed with stainless-steel screws. At some stage in the distant past the original bronze nut either failed or went missing and some well-meaning PO replaced it with a stainless one hardening it down onto the original bronze washer beneath.
Not good!!... According to reputable sources, including this USDA Forest Service Research Paper
the dissimilar metals cause the sea-water to turn strongly alkaline
and basically eat away at any timber it touches.
For me, the solution to the problem was simple: replace the nut and the other stainless fastenings on the stern gland with bronze versions, wash the area with vinegar (a weak acid) to neutralise the alkaline solution, slop on a couple of coats of CPES and all is good again.
Goes to show that, even if you think you've learnt all there is to know, there is always something new to learn.