The plan is to start by sounding the hull to identify the region that has delaminated. As you can see in the above image, I have completed that step. I'm hoping this is the extent of the damaged area, as the taps went quickly to a dull thud in that area.
I will use my oscillating saw to to cut the interior laminate out, trying to keep things in an easily cut series of rectangles. The core will be replaced using 1/2" Divinycell foam core, laid up with West System Epoxy. I will then add a layer of 6 oz. cloth to re-establish the interior laminate.
I began by removing the hardware so I have a clean area to work in. It is clear that this boat wasn't assembled with sealant anywhere, nor were any of the hardware mounts properly potted with high density filler to avoid compression damage. The thin skins of this Lightning are great for light weight, but it really mandates proper potting for long-term durability.
The other problem I am seeing is use of plywood for backing blocks, but inadequate sealing of the wood. To use plywood you need to either seal it in epoxy (preferably) or varnish of some kind. The upper gudgeon's external block can be salvaged, but my preference is to replace it with a GRP block that's immune to degradation (once painted). The interior block is rotted, and will need to be ground out and replaced with a new GRP block. The lower gudgeon's interior mounting block is still in good shape (surprise!). I plan to sand it, then seal with epoxy and a layer of cloth for protection.
While I'm fussing with the transom, I'm also going to properly pot the transom flap openings in order to better seal them against future water damage. I would like to cover them with a 6-oz cloth layer as well, but that would require a radius on the openings that may not be feasible. I need some more time to ponder how that might work.
Stay tuned... More to come!