It looks like a hard spot to me as well but I dont see how a foam filled stringer could do that. Perhaps the support of the cuddy pressing down on the hull close to that location could do it.
You need to open up the boat just above the crack for a good look and repair access. Start repair from the inside first, then you can go from the outside of the hull up to finish up the repair.
FellowShip
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Just for the Grins
Official 23 SF Antique Classic SeaCraft Owner