The energy ship is a new concept for offshore wind energy capture. It consists of a wind-propelled ship that generates electricity using water turbines attached underneath its hull. Since it is not grid-connected, the generated energy is stored aboard the ship (for instance, using batteries or through conversion to hydrogen using an electrolyser). This concept has received little attention until today. In this paper, a proof-of-concept of an energy ship propelled by a Flettner rotor is presented. The experimental results are compared to numerical simulations. A very good agreement is obtained, which validates previous numerical simulations that indicate that a full-scale energy ship equipped with Flettner rotors can lead to positive net energy production.