Wind assisted propulsion is the practice of decreasing the fuel consumption of a merchant vessel through the use of sails or some other wind capture device. Sails used to be the primary means of propelling ships, but with the advent of the steam engine and the diesel engine, sails came to be used for recreational sailing only. In recent years with increasing fuel costs and an increased focus on reducing emissions, there has been increased interest in harnessing the power of the wind to propel commercial ships. A key barrier for the implementation of any decarbonisation technology and in particular of wind-assisted ones, is frequently discussed in the academia and the industry is the availability of capital. On the one hand, shipping lenders have been reducing their commitments overall while on the other hand, low-carbon newbuilds as well as retrofit projects entail higher-than-usual capital expenditure (CapEx). Therefore, research effort is directed towards the development of shared economy and leasing business models, where benefits from reduced consumption of fossil fuels as well as gains from carbon allowances or reduced levies are shared among users, technology providers and operators. Design The mechanical means of converting the kinetic energy of the wind into thrust for a ship is the subject of much recent study. Where early ships designed primarily for sailing were designed around the sails that propelled them, commercial ships are now designed largely around the cargo that they carry, requiring a large clear deck and minimal overhead rigging in order to facilitate cargo handling. Another design consideration in designing a sail propulsion system for a commercial ship is that in order for it to be economically advantageous it cannot require a significantly larger crew to operate and it cannot compromise the stability of the ship. Taking into account these design criteria, three main concepts have emerged as the leading designs for wind-assisted propulsion: the “Wing Sail Concept,” the “Kite Sail,” and the “Flettner Rotor.” Wingsail As a result of rising oil prices in the 1980s, the US government commissioned a study on the economic feasibility of using wind assisted propulsion to reduce the fuel consumption of ships in the US Merchant Marine. This study considered several designs and concluded that a wingsail would be the most effective. The wingsail option studied consisted of an automated system of large rectangular solid sails supported by cylindrical masts. These would be symmetrical sails, which