When launching our 1991 Hunter 37.5 last Friday, almost as soon as we dropped her in, water began coming into our previously dry bilge. We rechecked the sea cocks and through hulls, all of which were fine. We pulled her almost immediately and discovered a crack about 8" long located approximately 6" under the joint between the keel and the hull (skeg?). For about 8" either side of the end of the crack and maybe 3" above the keel-skeg joint tapping disclosed a hollow spot under the fiber glass. Grinding confirmed what the tapping had disclosed - a hollow spot where the fiber glass and the keel had not bonded. We also found that some of the caulking between the keel and the skeg was missing in an area almost directly above the center of the crack. Our conclusions are that the hollow spot probably existed since the boat was first launched. Water in the bilge somehow found its way between the keel and the skeg (where the caulking was missing) to the hollow spot. The freeze-thaw action in northern Michigan eventaully caused the crack. The fix is to remove as much caulk between keel and skeg as is necessary and refill with epoxy, grind the fiber glass shell on keel and skeg back to solid glass, refiberglass, resurface bottom with VC tar(?) and paint with VC-17. Actually a fairly small expense as hull repairs go. Everything makes sense to us except the water path from the bilge to the keel caulking void. It does not seem that there should be any way water in the bilge could migrate to the joint between the keel and the skeg. Has anyone experienced a similar problem? One theory is that the keel bolts holes could be part of the water path. Thus, we are considering loosening the keel bolts one at a time and recaulking them. Is there any down side to this procedure?