Style v. substance
There is a difference. "Shallow draft" is a relative term, like "high fidelity." It can mean different things to different people. With small sailboats, I think most people accept a draft of less than three feet or so to mean "shallow" or "shoal" draft. "Swing keels" and "keel/centerboards" are different configurations of related but different stabilizers. Both give boats variable drafts (some might say "shallow"), but do so in different ways. Swing keels are fins attached to the boat on a pivot, and are raised and lowered with a winch. These keels are heavy and contain most or all of the boat's ballast. The best examples are the Catalina 22's. In Keel/centerboard models, the fins are also attached at a pivot point and are raised and lowered by a line, but these fins are usually only heavy enough to weigh themselves down. They do not provide significant ballast. The ballast is usually encapsulated in the keel, which is shallow by anyone's definition. The fin (centerboard) usually retracts into a trunk within the keel. The O'Day 23 (my boat) is like this, as is the Rhodes 22 and I think the O'Day 25.
