A Flap Rudder is a specific type of multi-section rudder used on boats and ships.
The Flap Rudder
The flap rudder designed to improve the effective lift generated by the rudder and hence improve the manoeuvrability of the craft.
The rudder consists of two or more sections which move relative to each other as helm is applied and the angle of the main or driven section moves, thus the shape of the rudder changes dynamically as the angle of helm is changed.
References
Flap rudder Wikipedia(Text) CC BY-SA