I guess you mean keel bolts? it is the condition of the ones currently installed that would be of concern as you cant see the shank without withdrawing one (or two) Check with the seller of any yacht you are interested in if they have checked / re-newed the keel bolts and when, if they have been done. It isn't a major to pull them , or it shouldn't be. re-sealing the surounding area with whatever is the recommended bog is important as well as using suitable strength bolts if you decide to replace them. Boats are holes in the water through which you pour money, but there are many worse ways to