You haven't really told us even what your boat is made out of, so I will assume it is fiberglass and the plywood is a core sandwiched during construction ?
If so you need to remove the spar, open up to sandwhich from oneside or the other (I preferr top) and replace the wet crushed core with solid fiberglass, or new core, taper everthing to spread the load and increase bond area.re glass over exposed side of core. restep spar and keep new step well bedded.
That would do it.
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Brian Duff
BVI Yacht Sales, Tortola
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