Well, if the ribs cracked after you got the keel on (that distributes the pressure better) there's definitely something wrong with the wood I'd say.

I usually use pine for my ribs (bends terribly, but it's all I can get without a company tax cert), and I space them about 10-12 inches apart, and I can easily jump on the boat and have never had any trouble with the ribs (except a bad bend that cracked once, but it should have been changed before skinning tbh).