Installing External Chainplates

[petered]

Hi all. Novice sailer here, please forgive any gaps in knowledge/terminology. So a while ago my chainplate popped out. Columbia 38' from 1967, bought this year, cap-shroud chainplate popped out of deck while sailing, mast, fortunately, did NOT come down. Old chainplates were internal, glassed to inside of hull. Apparently chainplates were last replaced in 2011 which is not all that long ago. Oh well. Anyway, after much procrastinating it's time to get on this. After posting to this forum and consulting friends, I decided it would be best to move to external chainplates. I think I know what needs to be done, but wanted to check in here because this forum's being quite helpful in the past. And also maybe it'll help someone doing the same job down the line. My plan, illustrated below, is:

1) Grind off old (jagged) fiberglass/filler that supporting old chainplate, as well as fiberglass around bottom of the hole of deck.
2) Replace missing core material, fill with epoxy, then lay fiberglass (A) on top of deck at hole, (B) at bottom of hole in deck (C) in mats to reinforce the inside part of the hull where the external chainplate is going to be bolted through.
3) Take external chainplate (which already has holes made), carefully mark hole locations, drill through hull, supporting fiberglass that we just laid in hull, and backing plate. I am assuming hull is solid fiberglass (have yet to find information to the contrary)
4) Apply sealant (Fast Cure 4200 marine adhesive sealant) all around holes, in holes, outside of hull where chainplate will lay.
5) Immediately after applying sealant, bolt on external chainplate. (I have 4x 3/8" F593C steel bolts for each)

Questions:
A) How important is it to grind off the old fibreglass in step 1? My other thought was just to fill in the jagged bits with filler (Short strand fibreglass reinforced body filler), which seems a lot less labor intensive than grinding off the old stuff and laying several layers of new supporting fibreglass.
B) Is laying layers of fiberglass (step 2C) the best way to support the hull around where the chainplate will be bolted? If so, what's a reasonable thickness? I guess what I'm trying to avoid is a "bolts tearing through hull under upward force from cap-shroud" scenario, but I'm not sure how paranoid I should be about this.
C) How important is the backing plate? It seems the main purpose is to distribute the load of the nut. A friend suggested I just get large washers instead, is that reasonable?
D) Am I forgetting anything important?




Old chainplate (left) and new ones (right)


The scar where the chainplate used to be, on the inside of the hull (camera pointing starboard)


The exit wound

Thanks, I appreciate whatever advice any of you have for this job.

[coopec43]

I saw your earlier thread on the chain plate problem. But there are some other that perhaps you may have not seen?

I had a look at what your doing and I don't have problem with that.

I'd coarsely sand off the glass internally (about one square meter) and then start laying up using woven cloth/epoxy. The lay-up would gradually get smaller in area. I would lay up about 6 layers but that depends on the weight of glass cloth. (I would expect it to be about half inch)

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