There was some interest in a comment about chain plate restoration so I thought I would create a thread to discuss it as it relates to my 1975 Islander 30. I will post some pictures a little later of what I did.
Description of Chain Plates
The forward and mid chain plates have the stainless steel bars that penetrate the deck through a slot, and then are through bolted to the fore and aft bulkheads.
The aft chain plates are not connected to a bulkhead, because there is no bulkhead at that location. So the chainplates are of a different form. They are flat plates about 10"x5" (dimensions very approximate) with tabs (tangs) welded on to them at their approximate centre. The tangs stick up through slots in the deck and the flat plates get sucked up to the underside of the deck by the shroud tension. I deleted the screws that hold the chain plates to the underside of the deck.
Four Forward Chain Plates Problems Found and Restoration
The forward chain plates had been leaking, and of course I was concerned about rot in the deck core and bulkhead. As it turned out, I was very fortunate and there was very little. Restoration involved removing the chain plates, routing out the core material from between the two layers of fibreglass about 1/2" and then filling this with epoxy. The slot was maintained by re-installing the chain plates before pouring in the epoxy. I used a little painter's tape to stop the epoxy from sticking to the chain plates, and some sealing tape (vapour barrier tape) underneath the the deck penetration to hold the epoxy in until it hardened.
After everything was cured up, I removed the chain plates again and filed the epoxy slots a little to get a small gap to accomodate some sealing compound.
The last part of the restoration was to install some back plates similar in size and shape to the chain plates so the through bolts pull on a much larger baring surface than just the bolt heads. The forward backing plates had to be made in two pieces each, because the hanging lockers in the vberth.
The forward and mid chain plates are now really well secured to the bulkheads. Also, when (not if) new leaks occur at the chain plate penetration, the water will simply drip through, because the epoxy is waterproof. The water will not enter the core. When I see the water dripping I will know that re-bedding the chain plates is overdue. Oh - one other minor point, the top surface of the the epoxy ring-bushing that is formed when the epoxy hardens in the routed out space around the chain plate can be dished out a bit on the top to accomodate a little sealing compound underneath the stainless steel plate that fits over the chain plate on the deck. In other words, before you intall the little plate that slips over the chain plate on the deck, you can chamfer a 45 degree angle all around the chain plate to give a little place for the sealing compound to accumulate.
Two Aft Chain Plates Problems Found and Restoration
The starboard deck above the chain plate had some stress cracks in the gelcoat, and I began to see a bulge growing on the deck. It all seemed sound, and test holes drilled from below the deck showed there was dampness but no punky core yet. It was a little puzzling why the deck was deforming a little on one side, but then again, it is an old boat.
So I cut the lower layer of fibreglass out from all around the chainplate. In fact I cut it about four or five inches larger fore and aft than the existing chain plate. I removed the piece of plywood reinforcement that the chain plate seats against. (The word "I" in this case means my fibreglass man, so i can't take credit for his fine workmanship). New marine plywood of a longer length was fibreglassed into place, complete with a slot to accomodate the tang that will penetrate the deck. I was going to undercut the marine plywood reinforcement similar to what was done on the deck coring but I forgot. So I convinced myself when they leak again I will get at it from underneath and rout out the slot a 1/2" and apply the 1/2" epoxy ring from underneath. I also welded a strongback (a piece of 3/5" stainless stock on edge) along the length of the chainplate to stop it from bowing. I noticed that the plate had actually begun to bow (probably accounting for the bulging in the deck). I flattened out the chain plate in the process and it all fits nice and snug now and everything seems to pull in the right direction.
The mild deformation in the deck seemed to just press back into place and disappear. The chain plates are nice and sound now. I am not going to restore the deck stress cracks yet, but just for fun I mixed up a little batch of white pigmented epoxy (left over from the bottom restoration) and just wiped it into the cracks after cleaning them with a special wax removing solvent. It looks much better now. The rest of the gelcoat on the deck needs quite a bit of work, so I am not really worried about that part.
All in all I am very pleased with the way the chain plate restoration worked out. I think the old girl appreciated it too. Sure sails nice now along with the restored bottom - probably all in my head.
Hope you enjoyed this little explanation; I will dig up some pictures.