at the keel/hull joint are pretty common,whether they are serious or not depends on whether or not the crack has compromised your keel bolts and/or is letting water into the bilge or between the hull and the liner. My 1986 H28.5, for example, has a small crack at the keel/hull joint all the way around the keel. So far, no water has ingressed as best I can tell. Each spring, I grind out the joint a bit and refill it with a flexible sealer like 3M5200 or POR-15 putty. This is pretty common for these boats. But if water is getting into your boat from this crack, you do have a serious problem that would require your, or a professionals, immediate attention.