VERY Common
I have a 1990 Hunter 30 with the same crack, as does every other Hunter sitting in the yard right now. It is not only common on Hunters, but on all fiberglass boats of this vintage with bolted on keels.The crack probably isn't from loose keel bolts. If your nuts are loose, you'll have seepage in the bildge.It's not a bad idea to try to tighten them if you want (I couldn't budge mine). They take a 1 1/8th inch deep socket on a half inch drive. I didn't actually check to see how much torque that size of nut takes; there is no way you could over torque. The nuts are considerably larger than auto lug-nuts, so you can just tighten them as far as you can.The crack is simply from the keel & hull flexing ever so slightly from the vast about of lateral pressure on the keel, especially when the boat is heeled.I'm hauled out right now, and after reading the many posts in the archives about this, contacted Hunter. Their recommendation was to open the crack with a Dremel tool, fill the crack with 3M5200, and glass over it. It should last at least a few years before the crack opens up again. I've got a current post about it on the Ask a Hunter Owner forum. Send me your e-mail address to D.Strong@Verizon.net and I'll forward the e-mail correspondence from the factory.It's not a tough job at all. You might check the archives to see what other creative things people have done.