While in recent years the use of hydrofoils has experienced a substantial growth, traditional design tools such as Velocity Prediction Programs (VPP) have proven inadequate to help architects and engineers with performance trade offs which now include specific stability issues related to these foils. The quest for performance also demands a better account of the unsteadiness of the environment in which the offshore yachts evolve.

Time-domain analysis and system-based modeling allow for an improved understanding of the controllability and dynamic stability of given geometries, enabling to adapt and refine the design. This paper presents such a dynamical unsteady model, based on the superposition of several loads components, computed from either numerical, empirical or analytical models. A test case and its results are presented to show the reliability and efficiency of the developed numerical tool, by comparing response amplitude operators of a reference hull form with experimental and numerical data.

Finally, the paper outlines two 6DOF dynamic simulations of an offshore trimaran. The first case shows a simple bearaway maneuver and compares two sail tuning strategies, while the second one presents the yacht evolution in unsteady wind demonstrating how in varying conditions the boat may reach attitudes that widely differ from the steady ones.

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