NavList:

I wonder if they had the boat surveyed before they bought it. It seems like a sailboat could not be going fast enough to knock the keel off, unless there were structural issues.

You don’t need to be going very fast in a fin keeled vessel to get quite a jolt if you hit something heavy enough and solid enough.  It’s certainly enough of a jolt to almost throw someone off the vessel and make you fear grave damage either below waterline, around the bulkheads, or at the chain plate attachments.  It's not so much the speed as the deceleration.  Even if you experience a soft grounding, if there’s any tide under you, the vessel will within a few seconds be on her side swivelling about her fin shedding vortices bow and stern causing a wrenching effect on the fin. 

There’s no suggestion in the article of hidden weakness in the hull.  With the number of Columbia 28s built and the time they’ve been sailing, there can’t have been much wrong with the basic design or construction.  A previous owner’s assessment of possibly the very same yacht appears here http://www.columbia-yachts.com/c-28info.html .  As with any 49 year old vessel, one can’t entirely rule out a previous knock causing weakness, which a sharp eyed surveyor might or might not have spotted.  The Columbia owner’s site also mentions some 28s having suffered keel bolt corrosion, but how serious and how easy to check each of the seven bolts without time consuming and expensive X-rays or withdrawal isn’t stated.  DaveP