It definitely looks like a  hole was plugged.   I assume this is inboard of
the factory through hull for the cockpit seat drain?     I suspect it was a
plug for damage from a prior non factory motor mount that pulled out of and
damaged the transom.   Or maybe a motor was run with a bad prop and vibrated
the back of your boat to bits?     If the plug is solid I don’t know if you
should remove it or trust it.  If it was added by Stan’s shop it was
probably done right.   If it was done by some bass boat or corvette guy,
maybe not.   I bet the crack was a tiny hairline only at that time but
spread like a damaged windshield.   

You need to drill another  hole in the bottom of the crack, to stop its
spread.   I would also widen the crack to a couple mm with a jigsaw.   Cover
one side with tape and inject the defect with epoxy.   Dry it with acetone
first.   I think you also need to sand more, to get past the original blue
gellcoat and the inside to raw fiberglass as well.    Others can better
advise on the best epoxy, I made mistakes which affected painting on my 1976
first boat, from ignorance and limited local supply.   Order three times as
much of the right stuff as you think you need.   When the crack is filled
you can add new structure with woven tape and more epoxy, then fair it.  
Wear a mask and ventilate.    This should be easy (flat surface and
accessible) with enough power tools.   

Alex Cole
SV Lark



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Alex Cole
S/V Lark
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