Live and learn - I've never seen one like that before. ... If it were mine, I'd cut off the loose piece, check for holes where it was welded to the log, and see how it felt on the water. If it felt ok with the piece missing, I'd leave it. You'll just have to be sure the boat floats off the trailer rather than sliding the jagged edge of the broken keel into the trailer runners. ... If it doesn't feel right, I suppose you'll need to have a replacement piece welded back on. ... Good luck.