You know, the type of situation you describe, if I understand you correctly, sounds to me like someone whacked the keel moving along at speed. If the "soft spot" is just aft of the keel, it may well be the result of a grounding, which would push the aft end of the keel upward and delaminate and spider crack the hull aft of the keel. There would also be spider cracks forward of the keel, radiating outward toward the bow from the downward pull of the grounding. I''d check any stringers or floors immediately abaft the keel and those running directly over the keel forward for any indication of stress cracking, delamination or separation......just fixed one, and NO, I was in Puerto Rico watching a football game at the time. I still got blamed because I WASN''T on the boat. I got blamed for another boat losing a race because I WASN''T there. Ahh we.