A new validated open-source numerical tool for the evaluation of the performance of wind-assisted ship propulsion systems

Abstract

Wind propulsion is envisioned as one of the solutions for the decarbonisation of maritime transport, as it offers high efficiency in terms of primary energy comsumption. Many wind propulsion systems already exist at various development stages, but the uncertainties over their performance is a strong obstacle to their adoption by ship owners. This paper presents a general method for the assessment of steady and unsteady performances of wind- propelled ships with 6 degrees of freedom, as implemented in the open-source program xWASP_CN. Inspired by system-based modelling, the method consists in the independent modelling of the forces acting on the ship, as functions of the ship’s 6 degrees of freedom and environmental conditions. An original root-finding algorithm that leverages the specifics of the physical problem to find the steady equilibrium is presented. The method works either like a Power Prediction Program (PPP) or like a Velocity Prediction Program (VPP). As a PPP, the forward speed and course are fixed while the required propulsive power, leeway angle, heel, trim and sinkage are solved. As a VPP, only the course is fixed and the attained forward speed, leeway angle, heel, trim and sinkage are solved. This makes the method suitable for both hybrid propulsion and pure wind propulsion. The force models can be semi-empirical (usually requiring very little input data), based on preliminary experimental or numerical results (such as forward resistance curves, lift/drag coefficients and frequency-domain sea-keeping coefficients), or full-fledged flow solvers (e.g. potential theory, CFD). Thus the method is suitable for all design stages as each force can be modelled with several levels of accuracy depending on the input data available. Comparisons of an intermediate-level model with experiments on a 18-ft catamaran fitted with a Flettner rotor and a water turbine show good agreement for steady-state results.