least stable when the oars are out of the water. The technique that often accompanies this view involves repeatedly moving the rudder back and forth over several strokes, making sure that the rudder is centered before every recovery. However, the rudder has much less steering power during the drive phase because of the very large forward propulsion force it must overcome. As a result, this technique often causes more boat drag due to longer rudder use, and the back-and-forth motion of the rudder tends to rock the boat. The cox will also need to take into account the stream and the wind as well as the river. As a general rule, still waters do not run deep: rather the stream is strongest where the river is deepest. This explains why in The Boat Race coxes tend to steer in the centre of the river. Coxless boats A boat without a cox is known as a coxless or "straight" boat. Besides single and double sculls, straight pairs and fours are the most common coxless boats. Because of their speed and lack of maneuverability, eights without a cox are very rare and dangerous. Sex Coxswains may be of either sex, regardless of the sex of the rowers, under the rules of the World Rowing Federation, USRowing British Rowing Henley Royal Regatta, Rowing Australia, and Rowing Canada. Before 2017, the World Rowing Federation (then called "FISA") rules stated that coxswains must be the same sex as the rest of the crew. In 2017, the Federation voted overwhelmingly to change the rule so that the coxswain may now be of any sex under World Rowing rules. All rule changes applied immediately. New Zealand male cox Sam Bosworth was assigned the New Zealand women's eight in March 2017, and when they won the June 2017 World Rowing Cup II in Poznań, Poland, he was the first male cox to win an international women's rowing event. Weight It is advantageous for the cox to be light – as there is less weight for the crew to move. However, weight is generally considered of minor importance compared to steering, coaching, and motivational ability. An Oxford University physics lecturer estimated that an additional of deadweight in an eight causes a 0.2% loss in speed, which would equate to 0.6 seconds for a six minute race, or , approximately one fifth of a boat length. The World Rowing Federation