1 INTRODUCTION The evaluation of the hydrodynamic behavior of the flow around racing yachts is an important issue, es- pecially in the final design stage, where optimization of the hull form results in a competitive design. Kirkman (1979) discussed the evolving role of the towing tank in providing assistance to the designers and the appropriate means of using model tests in light of the contemporary understanding of scale ef- fects. Nowadays, although the role of the numerical methods in the design of sailing yachts has signifi- cantly increased, the experimental methods have al- so been considerably refined since Davidson’s memorable towing tank investigation (Davinson, 1936). Contrary to a conventional hull, sailing yachts, balanced under the combined effect of aerodynamic and hydrodynamic forces and sail in most of the cas- es in an inclined and yawed condition. Thus, the contribution of the experimental evidence to the pre- diction of their behavior is invaluable. The dynamic performance is strongly connected to the wind and wave conditions, as well as the vessels velocity. Thus, accurate and reliable predictions of the added resistance in natural waves are necessary, especially for racing yachts which participate in long races where the winner is only a few seconds faster than the other participants. On the way to investigate the dynamic performance of a ¼ scaled model of a 50-ft modern sailing yacht in waves, a series of tests have been carried out in regular and random waves. The performance of the model in waves was evaluated, for three heeling and leeway angles. The presented experimental results refer to the RAO curves for heave, pitch and three points along the hull (the bow, the midship section and the stern), as well as added resistance and added side force in regular and random head waves. 2 NOMENCLATURE A : wave amplitude BWL : waterline breadth Fn : Froude number k : wave number LWL : waterline length RCW : resistance in calm water RAW : added resistance in waves SFCW : side force in calm water SFAW : added side force in waves TCB : draft of canoe body TMAX : maximum draft (incl. appendages) t : trim V : speed β : wave heading angle (180 o for head waves) ∆ : Design displacement ∆APP : Appendage displacement λ : wave length ωP : peak frequency of the spectrum Experimental Investigation of a BOC-50 Sailing Yacht Model in Regular and Random Head Waves G. Papantonatos, D. Liarokapis, K. Sfakianaki, J. Trahanas & G. Grigoropoulos National Technical University of Athens, Athens, LSMH, Greece ABSTRACT: In this paper the dynamic performance of a BOC-50’ sailing yacht model in realistic sea states is investigated experimentally. A scaled model of the hull form with the keel-bulb configuration has been tested in the towing tank of the LSMH of NTUA. During the tests the dynamic responses, as well as the added resistance were recorded. Results referring to the resistance, the side force, the CG displacement, the pitch as well as the vertical accelerations of the model at the bow, the CG and the stern are presented. Moreover, by using a Velocity Prediction Program we calculated the polar and the stability diagram of the tested sailing yacht. Useful conclusions about the dynamic behavior of the model were obtained.