Differences
I disagree with Jim about there being little difference in size. I've sailed extensively on a swing-keel Mac 21 and it is quite a different boat from my Ericson 23, which has a shoal keel and a centerboard. The Ericson probably weighs at least half again what the Mac 21 weighs, even with its iron keel. It is noticeably faster and more solid (not to take anything away from the Mac - it is a ball to sail and has served well on a one-week island hopping cruise with two adults and a child}. The 21 can be rigged and launched in about half an hour by one guy if he has the routine down. My boat takes two guys a couple of hours if they know what they are doing but take their time. Everything is bigger and heavier. But I really like the shoal keel - centerboard arrangement. There is not nearly so much strain on the pivot pin as on a swing keel, you can pull the board up and the boat still sails well, you can go into water that is about knee deep if you pull the rudder up, etc. And the boat performs well, races competitively, etc.