The awareness of the impact of air pollution on global climate change is more and more growing in society. The urgency to make ships more eco-friendly captures the interest of shipowners and naval architects. Hull designs of a ship, with regard to minimum fuel consumption have been optimized in the last decades and it has been proven difficult to improve current hull forms. Another strategy is to reduce fuel consumption by using external wind power combined with engine power, so called 'wind-assisted propulsion' or 'motor-sailing'. The potential of an extraordinary high lifting device of a mechanical sail using aspiration power, known as a 'Turbosail' introduced by Jacques Cousteau [1], has been selected as wind propulsion for an existing cargo ship. This foil can generate about 3 times higher lift compared to conventional foils, which do not use active suction of the boundary layer. Similar to the concept of a velocity prediction program for sailing yachts, a fuel prediction program (FPP) is set up to visualize the fuel savings for all wind directions relative to the vessel. For a wind force 7 Beaufort, the fuel savings for optimal wind angle can reach up to 40%, which are enormous. The working mechanism of the Turbosail is further explored and examined with CFD using ReFRESCO [38], to see if the performance of the Turbosail is as high as reported by Cousteau. Steady and unsteady RANS simulations are conducted for infinite span Turbosail using the k-ω SST turbulence model of Menter [32]. As a verification study, an expansion of two other turbulence models are included, which are the  k-ω SST-LCTM model of Langtry et al [30] and the EARSM model for a more anisotropic flow accounting for Reynolds stresses. The difference between all turbulence models appeared to be small, and a fair agreement is found with the lift and drag coefficients found by experiments of Cousteau. A steady RANS simulation has been conducted at model scale for a finite Turbosail to calculate the lift, drag and aspiration power. The distribution of air suction along the span of the foil is visualized, which gives insight for the real experiments. During this study, a Turbosail at model scale 1:2 is built with a span height of 5.5 metres and a chord length of 1 metre. The comparison with CFD is therefore very useful for validation. As a final step, the lift and drag coefficients found from the numerical result are implemented in the FPP. The fuel savings from the FPP are analysed by placing four full scale Turbosails on an existing cargo ship. In this model, there has been accounted for aspiration power needed for the boundary layer suction. Hydrodynamic forces due to leeway of the hull and rudder, propeller efficiency and specific fuel oil consumption of the engine.