rowing, each rower has one oar, held with both hands. There are usually an even number of rowers – two, four or eight. Each rower's oar will extend to their port or starboard. In the United Kingdom, the port side is referred to as stroke side and the starboard side as bow side; this applies even if the stroke oarsman is rowing on the bow side and/or the bow oarsman on the stroke side. In sculling each rower has two oars (or sculls), one in each hand. Sculling is usually done without a coxswain in quads, doubles or singles. The oar in the sculler's right hand extends to port and the oar in the left hand extends to starboard. Within each discipline, there are several boat classes. A single regatta (series of races) will often feature races for many boat classes. They are classified using: Number of rowers: in all forms of modern competition the number is either 1, 2, 4, or 8. Whether there is a coxswain (also referred to as cox). Coxless sweep boats are sometimes called "straight", while sculling boats are assumed to be coxless unless stated otherwise. Although sculling and sweep boats are generally identical to each other (except having different riggers), they are referred to using different names: Sculling boat classes: Sweep boat classes: Equipment Racing shell Racing boats (often called shells) are long, narrow, and broadly semi-circular in cross-section in order to reduce drag in the water. There is some trade off between boat speed and stability in choice of hull shape. They usually have a fin towards the rear, to help prevent roll and yaw and to increase the effectiveness of the rudder. Originally made from wood, shells are now almost always made from a composite material (usually a double skin of carbon-fiber reinforced plastic with a sandwich of honeycomb material) for strength and weight advantages. World Rowing rules specify minimum weights for each class of boat so that no individual team will gain a great advantage from the use of expensive materials or technology. Smaller sculling boats are usually steered by the scullers pulling harder on one side or the other while larger boats often have a rudder, controlled by the coxswain, if present, or by one of the crew using a cable attached to one of the shoes. With the smaller boats, specialist versions of the shells for sculling can be