9 times out of 10 this is a cosmetic condition. There will be some (absolutely minimal) movement at the keel hull joint due to the leverage and weight of the keel (on a otherwise sound boat). The joint is most often faired with a polyester (sometimes epoxy) filler. The lead keel, fiberglass
hull and filler all expand and contract
at different rates and hairline fractures are inevitable. The more filler, the more cracks.
In some cases the fractures may be more than cosmetic so get a competent surveyor
to look at it.
This condition is not specific to Catalina's but to any boat with keel sump integral to the hull mold
that has filler in the joint between that sump and the keel. The C&C
guys call it the C&C
smile, the CS guys call it the CS smile the .........