Hello oscarkramer. Great suggestions. We’ll look into adding a centerboard only checkbox in advanced search.
In the meantime, the way the Draft box works in advanced search is, if you put a number in Under only, it will return boats with a minimum draft less than what you have asked for. Only boats with a variable keel have minimum draft.
Of course you all also get boats with fixed keels less than the Under you searched for but if you are searching for cruisers over, say 30 feet with a draft under, say, 2.5 feet, your results by definition will only return variable keel boats (which include lifting keel, swing keel, etc).
Hope this helps.