Maine Sail has great information, including the use of fiberglass blocks, and I do not doubt their effectiveness. However, there are trade offs. Making or buying the fiberglass was far more expensive than the 3/4" plywood. As for rigidity and structure, the plywood is equal in useful strength to the fiberglass, meaning, they are both absolutely solid and rigid for the purpose. And finally, as for longevity and strength, I have a boat that is 23 years old with untreated plywood in various places, including the old 1/4 inch puny backing blocks, and none of it showed any signs of significant decay.
Fully encasing the new 3/4 inch backing blocks with epoxy will provide them with all the longevity I would ever need or expect, and in 20 years time I will likely want to replace them and the thru-hulls again anyway. Another reason for using 4200 and not 5200.
So I would never argue against using fiberglass, at all, it is obviously great, but I believe a solid epoxy covered plywood block will fill all the same purposes except immortality, and I don't expect that anyway.
Bainbridge Island, WA
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