Due to the increasing demand of methods and tools for the analysis of yacht behaviour in a realistic environment and in particular the development of time domain approaches able to simulate yacht motion when under sail in a seaway, in recent years a number of Dynamic Velocity Prediction Programs have been developed. Up to now, while very interesting results are available regarding dynamic effects on hydrodynamic forces acting on the yacht hull and appendages, the physics of unsteady sail aerodynamics have received far less attention. In this paper an opening simple model is presented with the aim to reproduce unsteady sail aerodynamics taking into account three dimensional effects and unsteady mainsail-jib interaction. Under the assumption that certain time and length scales for the yacht and its wave pattern are short compared with the time and length scales of the wave motion, the yacht is modelled as a single point mass constrained to move on a surface governed by the equations of wave motion: normal and tangential equations of vessel motion are derived and solutions are investigated for an arbitrary two-dimensional wave motion.

At the Institute of Naval Architecture, Marine and Ocean Engineering (ISM),Technical University of Berlin, a research project was initiated investigating prediction methods for hydrodynamic forces acting on sailing yachts. For a 33ft. sailing yacht model tests with various keel designs and RANSE calculations have been carried out. In order to verify both,a full scale sail–force–dynamometer was built which enables to record the resulting hydrodynamic forces in a seaway without scaling parameters. In addition,separate six and five component balances have been designed for measuring the forces acting on keel and rudder for the model tests as well as on the full scale boat. The design and construction of the full scale measuring device is presented. Along with some sample measurements a general calibration procedure for inclined multicomponent balances is proposed. A short review of the model tests and RANSE-calculations complement the presentation.