A single scull (or a scull) is a rowing boat designed for a single person who propels the boat with two oars, one in each hand. Racing boats (often called "shells") are long, narrow, and broadly semi-circular in cross-section in order to minimize drag. They have riggers, which apply the forces symmetrically to each side of the boat and (usually) a fin towards the rear which helps prevent roll and yaw. Originally made from wood, shells are now almost always made from a composite material (usually carbon-fibre reinforced plastic) for strength and weight advantages.