Understanding Boat Design (Revised)

For new boating enthusiasts-even if they've been at it awhile-there are scores of burning questions. If one boat has a round bottom and another's is veed, what difference does it make in the way they perform? What are the advantages of a cutter rig over a sloop? Why does one sailor swear by a full keel, while others won't have anything but a fin keel? Why does one powerboat have more flare in its topsides than another? And while we're at it, what's flare and what are topsides? Why do some hull shapes look "right?" How big an engine and propeller will it take to move that powerboat? What are the elements of a safe boat, or a comfortable cruising boat?