Wind-assisted propulsion of a seafaring vessel using a kite sail

Abstract

Following the International Maritime Organization’s goal to reduce greenhouse gas emissions, the interest in wind-assisted ship propulsion (WASP) is on the rise. However, the adoption of WASP systems in maritime shipping has so far been limited. When compared to other WASP technologies, kite sails reach higher elevations within the atmospheric boundary layer. Kites also subject the vessel to low heeling moments and are easily retrofitted. Nonetheless, better familiarity with the technology is required. Particular consideration needs to be given to the effects of the tether weight, which will be substantial as the kite sail exploits the atmospheric wind gradient. In this study, a numerical model for a static kite sail assisting a Series 60 ship having a length of 75 m via a tether is developed in Python®. While the mass of the kite system is neglected, the weight and sagging of the tether are accounted for by assuming the tensioned tether to follow a catenary profile with a variable length. The scenario considered is the two-dimensional case in which the ship travels downwind, with the kite-tether assembly subjected to wind shear. A validation procedure for the ship resistance estimation is first conducted using MAXSURF® Resistance. The tether diameter leading to the best performance and minimum tether drag is then determined. The maximum kite area and allowable wind speed are also deduced, after which a single set of section coefficients adequate for the considered scenarios is derived for the kite sail. A proposed method of assessing the significance of the tether drag is then verified. By means of a parametric analysis, the effect of the kite area, aspect ratio and wind speed on the kite performance is investigated for a range of angles of attack and ships speeds. The results obtained indicate a positive correlation between the kite elevation and lift force in the presence of wind shear. Static operation additionally requires the length of the catenary tether to result in a weight which balances the lift, such that the kite elevation and lift force have a reciprocated effect that influences the kite thrust. In fact, the effect of the kite area and aspect ratio varies with the angle of attack. For high angles of attack, the performance of the kite sail is observed to improve with an increasing aspect ratio. The benefits of slow steaming are also highlighted as, under appropriate wind conditions, a static kite sail with an area of 320 m2 is proven to be capable of solely propelling the vessel under consideration. In general, the optimal angle of attack leading to maximum thrust depends on the wind speed and ship speed.