The stainless-steel chainplates through the deck with a slotted coverplate was the virtually standard meathod most builders used back in the 1960-80's. It worked fine and was structuraly strong as built. The problem was that over time even the best sealants will break down and leaks occur. Cutting the slot through a cored deck without sealing the core with a penetrating resin would have helped a lot (as would making the area of the deck around the penetration solid glass, no core as is often done now). But remember, most of our O'DAYs were built while technology was just being learned, these were production line boats, and yet were built to pretty good quality considering the price. Were 1980's O'DAYs built the same quality standard as a Benneteau? No, but they were never as high priced as a Benneteau either. Actually, O'DAY did use the idea of rods to connect deck-mounted chainplates to the interior liner on just about all the boats 27 and larger during the 1980's. Smaller models often used stainless-steel "U" bolts through the deck backed up with connections to the bulkheads below or at least a backing plate under the deck.Most of us did not buy our O'DAYs brand new, and thus are dealing with the lack of previous owner upkeep. Fittings may need resealing, bulkheads may need repair because a leak let water in and it was never repaired before rot developed. We used to own a 1970 CAL 21, my Parents bought that boat brand new and we owned her for 38 years. The chainplates were stainless-steel bar-stock, through a slot in the deck and bolted to a small bulkhead below deck. They had plastic slotted covers at the deck and sealant (silicone?) around the deck penetration and under those plates. We kept after the sealant and rebedded them a few times over the 38 years, at one point we had to replace the plastic covers due to UV degradation, (metal plates would have been much better from the factory, but plastic was stock) I made new ones out of a better plastic. One secret to getting a good seal. is to bevel the top edges of the slot and squeeze sealant down into the slot forming a gasket, then slide the plate down over it. I think one reason those stainless plates were not welded on at O'DAY is that if they were, each chainplate would need to be made specifically for that model and for the location on that model (extra $$ to fab and would mean needing to keep a supply of specific parts in stock, costing more $$) . By using separate cover plates, O'DAY could use the same chainplate and cover for many models as location and angle of the plate was adaptable to the use. One other factor, when it is time to rebed the chainplate, or at least renew the seal at the deck, with the plate as a separate piece, you only need to remove the plate, not the whole chainplate/cover plate assembly. Anyway, attached below is a good technique for sealing chainplates.
BTW: The chainplates on my DS II are easy to keep sealed, they are just bolted through the external hull-deck flange, no penetration through the cored part of the deck.
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