Rudders
The function of rudders is to control the direction for the boat, allowing the rider to steer it.
Although simple appendages, efficient rudders for pedal powered boats are not such a trivial thing, becouse they need to give the boat the required maneouverability and control feeling while adding minimum drag.
Meanwhile, in waterbikes usually rudders cannot be steered directly by the rider like in sailboats and canoe's ruddrs give little control and could vibrate at high speed.
Also a shallow water operational mode should sometimes be required.
Here is a collection of outstanding solutions suitable for the Open Waterbike.
Our boat is faster than mine