A double scull is a rowing boat used in the sport of competitive rowing. It is designed for two persons who propel the boat by sculling 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 reduce drag to a minimum. They usually have a fin towards the rear, to help 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 Originally made from wood, shells are now almost always made from a composite material (usually carbon-fibre reinforced plastic) for strength and weight advantages. The riggers in sculling apply the forces symmetrically to each side of the boat.