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From what I can see, the previous windlass was mounted too far forward, it was on another plywood base mounted to the shelf and sitting forward of the shelf.
My aim is to repair the shelf, then redrill a hole for the chain drop in an appropriate place - epoxying the holes for both chain drop and windlass fitments up first before redrilling.
Photos show what happened to the last windlass, and the holes drilled.
Your last photo is pretty much the same as my set up, except my windlass was not mounted on an additional piece of plywood.
The problem with the set up as shown in both yours and my photos of the windlass mounting is that the shelf on which the windlass is mounted consists of two parts. The forward part is simply an extension added on to the aft part. The aft part consists of a horizontal which extends aft from the chain locker lower aft bulkhead. Take down the lining on the forward bulkhead on the fore cabin, and you will see more easily what I’m trying to describe.
The forward part of the shelf on which the windlass was bolted to, is as I said merely an extension to the aft part, and on my boat I managed to separate the forward part of the shelf from the aft part. I repaired it initially by grinding out in way of where it had cracked, and then laid up several layers of grp before bonding a 4mm stainless steel plate across the entire area of the shelf, both the aft part and the extension leading forward. This was then stainless steel bolted through both the forward and aft parts. I think my repair is about as rock solid now as it could possibly be. Finally the windlass was bolted down with four more stainless bolts through the entire area.
The aft bulkhead of the upper part of the chain locker (above the shelf) is stepped further aft than the bulkhead below the shelf. This upper aft bulkhead limits where the windlass can be placed, as much as anything because the aft part of the windlass has a cover which must have some rearward clearance to allow the cover to be removed for maintenance (Lofrans Cayman windlass) (and I wouldn’t swap it even for a brand new Lewmar or Quick windlass).
The only part of the shelf which didn’t benefit from the stainless steel plate over it was the area in way of the forward water tank filler cover.
I understand your concern regarding the rotting plywood, and that will need to be replaced one way or another.
I would also check out the directional signal wiring while you are doing the job because on my boat that wiring consisted of plain copper wires which had corrosion on the wires throughout the entire length right back to the electric distribution board next to the chart table. I changed my wiring for tinned copper wires, and later added a £10 radio control unit which completely bypasses those signal wires with a radio receiver located directly on to, or below the solenoid control box also located next to the chart table.
Now I just carry an electronic transmitter in my pocket when I go forward to operate the windlass. I can also operate it from the cockpit, but I do prefer to be forward where I can see exactly how the chain is leading.