A lot of boats have experienced this problem at some point. Indeed my boat A124 has a few similar looking screws.
You are looking at dying brass screws, which have ‘de-zincified’. Fairey Marine used these brass screws through the hull above and below the waterline to fasten the internal framing / stringers. Over the years the salt water gets to the screws and starts to corrode them. The corrosion causes the paint to crack off which is useful as a warning sign.
In many cases he screws become so weak that you are unlikely to be able to unscrew them – the slots and/or heads just break off. The corrosion can also weaken the wood in contact with the screw.
I would love to hear how other people deal with their corroded screws as I have not yet found a method I am completely happy with.
I have tried a couple of approaches:
- Drilling out the screw, priming the hole with unthickened epoxy, filling the hole with thickened epoxy, re-drill, new screw and making good
- This has worked in some cases for me.
- The challenge is drilling out the corroded screw without the drill bit diving off the screw and making a large hole beside the screw.
- Using a small hole saw around the screw to the thickness of the hull, breaking up the plug of wood and rotten screw, removing the outer layer of veneer around the hole to permit a ‘patch’ of veneer to be glued over thickened expoxy in the hole.
- Takes more time
- Involves removing more of the hull material, although the epoxy does have clean timber to bond to
Some people have, I believe, removed the corroded screw and filled the hole and then applied a new screw in a new hole near to the old screw.
Your question has made me think that we should have a ‘technical paper’ on this topic once we have collated everyone’s responses to your question 😉