Any sailboat needs a vertical appendage on the lower part of its hull to avoid being blown sideways whenever the wind comes from the side. Except when the boat is sailing directly downwind, the wind always hits one side of the boat or the other and would keep the boat from sailing as nearly straight forward as possible.
Most larger sailboats have a fixed keel as a vertical appendage, which typically also is weighted to prevent the boat from heeling excessively or capsizing due to the wind in the sails (in essence, being blown over). Small sailboats, in contrast, usually have a centerboard (or daggerboard - described later) that can be lowered as a vertical appendage to prevent side-slipping. Raising the centerboard allows the boat to be put on a low trailer, carried car-top, or launched from a beach.
Most centerboards rotate up and down on a pivot pin at the top. Typically they swing up into a centerboard trunk (as in the photo) that prevents water from entering the hull. A control line is used to raise the centerboard partly or fully.
A weighted sailboat also provides ballast, like a fixed keel. A weighted board usually requires a small winch to crank up the weight.
Go to the next page for how to use a centerboard.