# Benchmark Sea Trials on a 6-Meter Boat Powered by Kite
## 1 Introduction  
The interest of the kite in terms of energy production is highlighted by [PERSON] in 1980 [1]. Over the past decade, kite research has taken off. However, it is essentially focused on electrical power production [2; 3; 4; 5; 6; 7; 8]. To a lesser extent, kites are investigated as an auxiliary propulsion device for merchant ship [9; 10; 11; 12; 13; 14; 15]. There is a fundamental difference between using a kite to generate electricity and using it for ship propulsion. While, in the first case, it is sought to maximise the tether tension, in the second case the aim is to maximise the component of this force in the ship advance direction. Thus, the kite flies always in dynamic mode (crosswind condition) in the case of electricity production, while flight conditions are varied in the case of ship propulsion. [PERSON] shows that the static flight is optimal under 50\({}^{\circ}\) in true wind angle [15]. For less close-hauled points of sail, dynamic mode becomes advantageous. There is a varied optimal figure-8 position and orientation for each wind angle (Figure 1). The cross wind situation is only optimal for the full downwind point of sail. Although there is a large number of scientific publications on experiments carried out onshore [3; 16; 17], there is a clear lack of experimental data for ships towed by kite. Of course, there is the invaluable feedback from the Skysails Company, which developed the most convincing industrial propulsion device based on kite [13; 18; 19; 20]. However, the data is incomplete and insufficient to allow the experiment to be replicated. Moreover, Skysails is a commercial enterprise and the authors being all staff members, it is possible that the results presented a biased view of the facts. There is also the [PERSON] et al. [16] experiment, but it takes place on the ground just like that of [PERSON] et al. [21]. Therefore, they do not allow the measurement of the interaction between the ship and the kite or the determination of the actual performance without being dependent on the limits of the modelling used (zero-mass,straight and rigid lines, roll and drift neglected etc.). [PERSON] et al. [22; 23; 24; 25] present a study with results on a kite used to propel a ship. Everything about the kite is well documented but the information on the ship is incomplete and the wind is only measured on the boat. In addition, the tests are only made for a single wind direction (\(135^{\circ}\)). [PERSON] et al. [20] report from the skysails experience that one of the major issues is the overloads caused by the vertical movements of the ship. To better understand this issue it is therefore necessary to model it. [PERSON] et al. [26; 27] proposed a numerical modelling associating the strip theory for the ship motion simulation [28; 29] and the zero-mass modelling formulated by [PERSON] et al. [15] for the kite force and motion simulation. The studies show an interesting lock-in phenomenon between the kite and the ship motions. The numerical tools are partially validated, on the one hand, for the ship motion, with the experiments from the University of Iowa [30; 31], on the other hand, for the kite dynamics, with the [PERSON] onshore experiments [21]. However, the interaction modelling between the kite and the ship requires an experimental database for validation, which is not available until now. The aim of the present study was to provide a reliable database. An earlier attempt showed the need for an instrumented vessel dedicated to this purpose [32]. A 6-m prototype was designed and built (Figure 2b). An experimental device was deployed to ensure satisfactory assessment of the wind at kite position. The phase-averaging method developed by [PERSON] et al. [21] was used in order to obtain accurate and reliable results. Section 2 presents the reference frames used. Section 3 details the measurement systems on the kiteboat and on the sailing area.