Wind-assisted ship propulsion can change maritime traffic


Originally published in The beam.

By Giovanni Bordogna and Nico van der Kolk

Blue Wasp is an engineering firm specializing in wind-assisted marine propulsion, providing independent technical advice to aid stakeholder decision-making across the maritime industry. The company grew out of 10 years of research on the subject at TUDelft (in the Netherlands) and helps realize the unique economic and environmental benefits that wind support provides.

Giovanni Bordogna has a PhD in the aerodynamics of wind-assisted ships of the TUDelft. He is a goal-oriented engineer with a strong passion for green technology and entrepreneurship.

Nico van der Kolk is an experienced engineer with a broad background in experiments and numerical simulation and a keen interest in environmental policy and regulation. He did his doctorate on the sailing behavior of wind-assisted ships of the TUDelft.

The global shipping industry accounts for approximately 90% of world trade by weight and accounts for nearly 3% of man-made CO2 emissions. Together with TUDelft (in the Netherlands) the engineering company Blue Wasp wants to change the course of things.

With business unchanged, the industry’s greenhouse gas emissions are expected to increase by 50-250% from 2012 levels by 2050. In addition, the majority of the world’s fleet still burns Heavy Fuel Oil (HFO), an unrefined residual oil with a high sulfur content. For this reason, shipping is responsible for around 13% and 15% of the global sulfur and nitrogen oxide emissions from human activities. For comparison: only the 15 largest ships emit more of these harmful gases than all the cars in the world put together.

Although slowly, things have started to change. In April 2018, the International Maritime Organization (IMO) set ambition levels aimed at reducing total annual greenhouse gas emissions by at least 50% compared to 2008 by 2050 and continuing efforts to phase them out. Certain regulations to improve the efficiency of newbuild and existing ships have already been enforced, but several scientific studies have shown that these efforts are insufficient – especially when the shipping industry needs to be aligned with the goals of the Paris Agreement. Several measures are currently being discussed and it is likely that more stringent regulations will be enforced in the short term, which will have a significant impact on the business of shipping.

In addition to political environmental policy, various private initiatives have also emerged. For example, the Getting to Zero Coalition, an alliance of more than 120 major players from the fields of shipping, energy, infrastructure and finance, has committed itself to “getting commercially viable, emission-free deep-sea ships” within the next 10 years. This is a big statement that will require radical and urgent intervention to keep up with it.

To help decarbonise the shipping sector, numerous technical solutions have been proposed and several projects are ongoing to demonstrate their feasibility. Most of them include new types of fuel such as biofuels, ammonia or hydrogen, which promise to completely eliminate all pollutant and greenhouse gas emissions when burned. While this looks very promising, if these fuels are sustainably sourced, these solutions may still be a decade away as their production and distribution will require entirely new infrastructure.

The Maersk Pelican, which has been retrofitted with two Flettner rotors from the Finnish company Norsepower, is one of the largest WASP ships currently in operation. (Photo credit Wikipedia)

Wind-assisted ship propulsion (WASP) stands out among the technologies proposed for the energy transition. Wind propulsion is actually a so-called primary renewable. It does not require any new infrastructures or storage facilities, as the wind energy is converted into drive energy directly on site. This drastically reduces the cost of the application and avoids the efficiency losses associated with energy storage and conversion. Another significant benefit is that wind is a free source of energy and several WASP technologies are already available and ready to be rolled out on a large scale.

It may sound like a return to the old days when large clippers moved solely by the power of the wind (and dozens of seafarers) were used for world trade. But in fact things are very different these days. As the name suggests, wind-assisted propulsion is a hybridization for ships in which the ship’s propulsion system is split between a “sail” system and a main propulsion system. The operation of the ship is not dependent on the availability of wind, as one might think. Instead, the ship has to rely on the main engine when there is a lack of wind on a certain voyage in order to get to the next port on time.

These steel sails (some actually made of aluminum or composite) help propel the ship, saving fuel and improving transportation efficiency. Modern wind-assisting propulsion devices can be of different types. There are wing sails (like rigid, highly efficient sails), kite sails that take advantage of stronger high-altitude winds, and active devices such as Flettner rotors (high spinning cylinders) and valve foils (suction wings). The latter two are high-lift devices that require a certain amount of energy to function, but in return they can provide much greater aerodynamic thrust compared to the square sails of the past. That’s a good thing because the ships have also gotten bigger, where the electricity needs of the largest merchant ships are similar to the electricity needs of a small town.

Rendering of a large kite sail from the Airbus spin-off Airseas towing a large bulk carrier. The system is expected to be installed in mid-2021. (Photo credit:

This extra aerodynamic thrust can be applied directly to move the ship forward (as opposed to the transmission losses of the main engine), but there are some complications. As with any wing profile, the lift and drag properties of a WASP sail system generate beneficial thrust, but also a bank force. The ship’s hull has to compensate for this new transverse force, which results in a new operating state of the ship. Otherwise optimized for a very specific and symmetrical operating state, the ship’s resistance increases and this increase can actually counteract the aerodynamic thrust. Finally, the new operating status can seriously affect the controllability of these ships, especially when large systems are being considered.

Still, existing ships with a WASP retrofit can expect savings on the order of 10-20%, depending on wind conditions and the way the ship is operated. On the other hand, a newly built WASP ship that is designed to make full use of the available wind power promises savings of over 50%. This is a remarkable result that can only be achieved if the wind support devices are properly integrated into the ship from its early design phase.

Although the schedule of these hybrid ships does not depend on the availability of wind, the fuel savings during a voyage will certainly be influenced by it. Therefore, active ship navigation to find the most favorable wind conditions is an important part of the story here. To make the most of the favorable wind conditions, the ship should be operated differently than it is today. It should be able to change course and speed while keeping the arrival time unchanged in terms of business operations. This is not yet the case with the 10 WASP ships currently in operation. A rethinking is first necessary, but it is worthwhile, as active ship management has been shown to drastically increase the amount of fuel that can be saved.

Following the call to green the shipping sector, a new generation of engineers and entrepreneurs are developing the technical and practical feasibility for multiple efficient WASP devices and full integration with ship trading operating and financing models. It’s time to take on the promise of wind support. It is a mandatory short-term intervention in addition to the shutdown of fossil-fueled engines and will be an essential part of the electricity mix for the new carbon-free fleet in the future.

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