chain plate backing plate

This is very easy and you do not need to take your rig down.

The plate is held in by the four nuts on the underside. In the top of the plate there is a recess to the top of the hole. The S/S rod has a disk shaped head on it that sits down into that recess. (like a round head of a bolt)

Let all the tension off the rig and support the rig temporarily with halyards. Unscrew the four nuts under each plate. Drive the holding screws/bolts from the bottom up through the deck and chain plates. Then the rod can be bent a little inboard and unscrewed from its lower anchor. If you look closely at the rod, you will find two flat spots ground into it to allow you to put a wrench on it. It simply unscrews from its bottom anchor place. With the rod removed, the plate slips off the bottom. Either clean it up an put it back or replace with another Aluminum plate made out of T-6 grade. Sometimes this is referred to as the 6000 series. Basically, the 5 series is crap for signboards, etc, T-6 is structural and marine use, and 7 is higher grade structural for aircraft, etc. When done, put it all back together and seal with 5200. Easy job.

The plates are identical. I just made a pair for a friend.

In the system, the deck is only a point that locates the chainplates. It does prevent the point of anchor for the shrouds from moving in and out, but the deck does not support the rig (shrouds) at all. There is nothing wrong with overdrilling the existing holes, filling them with high density epoxy filler, and then redrilling the holes. On boats that we have worked on, we have made up thin plates of carbon fiber (not more that 2 mm thick, and placed them on the deck under the chainplate and epoxied them down. This was mostly done just to add some comfort to an area of the boat under the plates that looked a bit tired.

Regarding the tension of the rod, you may have once noticed that the area of the deck under the chainplates looked like it was trying to be pulled up. That will be because these boats are 25 plus years old and they flex and yield under the constant tension of the rig. Also, the rig being under compression, the boat hull tries to pull in......in essence it is being squeezed narrower, so the rod ends up being higher than when first set. My suggestion is that when the rod is re installed, that it be tensioned until quite snug.....as though it was trying to pull the chain plates down into the deck. Don't over-due this, but I suggest this to compensate for the tension that is going to be put on the rod when you tension the rig. This will try to pull everything up. In a perfect world, the boat shape would be stable and nothing would move........alas our old boats don't dwell in a perfect world. The hull will yield under tension. You are trying to achieve neutral tension on the deck. Nothing up and nothing down. Achieving it is a guess. However if you see upward deflection on the deck, then your rod is not tight enough. If you over tension the rod, you may see downward stress under the deck or dimpling under the chainplates on top. If this doesn't disappear when the rig is tensioned, then the rod is too tight.

No doubt Bill has done this dozens of times more than I and may be able to contribute here, but I would say that a little prestress applied when the rod is installed will be what you are looking for so that when tension is added you may achieve something close to neutral at deck level.

Hope this helps.