Bethwaite was a aircraft pilot who initially designed gliders, then designed sailing dinghies such as the Northbridge Junior and later the 4.5 metre Northbridge Senior. After founding a company Starboard Products, later Bethwaite Design, he developed the Laser 2 (two crew Laser) and the Taser. Their final boat design developed by his son Julian was the 49er FX, a new Olympic sailing class.