Wind Assisted Ship Propulsion

Wind-assisted ship propulsion (WASP) provides the principal means to significantly reduce fuel consumption and emissions from waterborne transport without resorting to scarce and / or expensive alternatives like carbon-neutral fuels (DNV, 2025). Additionally, wind propulsion is one of the very few alternative propulsion options that potentially offers real zero-emission ship propulsion. In recent years significant effort has been put into the research and development of the wind-assisted propulsion systems (WAPS) themselves, but research on the interaction with the vessel itself and optimization of operation to reduce emissions using WAPS has been limited (IMO, 2024), limiting the possible gains in return.

To maximize the gains available from us ing WAPS it is essential to take a holistic view on ship design and operation. The EU should facilitate research not just into the WAPS themselves but into operation of hull and systems under the influence of sails as well as the interaction with logistics and the requirements for as well as the gains available from taking the pe culiarities of sails assisted ship propulsion into account.

Recent studies involving today’s fuel pric es and installation/ running/ maintenance costs of WASP systems show return of investment times that are still longer than any planning horizon of ship owners (Hansen et al, 2025). This shows the need for further incentives to motivate ship own ers to invest in WASP systems. However, many ship owners willing to invest in WASP systems are still unable to estimate the real savings and costs of WASP for their ships.

While an accurate performance prediction of WASP ships is generally possible, using sophisticated tools to predict the aero- and hydrodynamics of sails and hull, the real-life fuel savings are much more complicated to predict due to the high dependency of the WASP perfor mance on the environmental condition. The propulsive contribution of sails, and their negative effects such as heeling mo ments and side forces, highly depend on the wind speed and wind direction. While a WASP system can provide up to 100% of the ship’s propulsion power in some conditions, it can lead to increased power de mand in other conditions. While not a sin gle study (respecting real-life weather) showed negative effects of WASP on the fuel consumption, the expected savings differ largely from some mere percent to a full 50% reduction of the fuel consumption along a route. These differences occur due to different sail systems and sail sizes and different routes and highlight one of the main problems concerning WASP: an objective benchmarking of different sail systems and sizes considering the opera tional areas of a ship and an objective pre diction of long-term savings with variable routes. To achieve a trustworthy long-term prediction and a benchmark, multiple sails must be thoroughly analysed in a large variety of conditions and reason able weather scenarios must be defined. Further, prediction tools must be able to consider both, real weather (forecast or hindcast) along different routes, and sta tistical weather approaches to predict the long-term savings. Ideally a benchmark factor involving multiple criteria should be defined to compare different WASP systems, involving costs, fuel savings, size/ space requirements and operational diffi culties.

Further it must be noted that most of todays WASP ships are convention ally designed ships, retrofitted with WASP systems. Studies have shown that (Renzsch et al, 2023) the hydrodynamical performance of hulls sailing at a drift an gle (both, the course keeping ability and the added resistance from drift) is highly dependent on the hull design. Effort must thus be put into developing tools and guidelines to rapidly analyse and predict the hydrodynamics of hull designs for the operation with WASP.