C-Job has been actively working on the subject of wind- assisted propulsion for a number of years now. Of the numerous techniques available, the company’s attention has focused primarily on the use of deck-mounted rotating vertical cylinders called Flettner Rotors (also known as Rotor Sails).


C-Job’s research has found Flettner Rotors to be the most viable solution when compared to alternatives like kites and turbo wings. Flettner Rotors can be used to supplement a vessel’s main engines by using the Magnus effect and favourable winds to generate forward thrust. Previous Newsflash articles have highlighted the methods and the advantages.



In addition to in-house research of its own, C-Job facilitates graduate research. One notable project that is nearing completion is being undertaken by Delft University of Technology Master’s Student Robin Berendschot. Using an existing vessel as the starting point for the design process, the goal of his Marine Technology and Ship Design Master’s thesis is to develop a method to optimise a vessel design with Flettner rotors for the lowest total cost of ownership.



Multiple trade-offs

Although this initial design objective is too extensive to cover in a single Newsflash article, Robin does point to one key area of his research that is particularly noteworthy. “This is how a vessel’s fuel consumption is affected by the application of Flettner rotors. This has led to other important factors to be discussed such as using weather routing to increase performance, the effect of Flettner Rotors on stability, and the trade-off between initial investment and fuel savings.”


In designing a vessel that uses Flettner Rotors as a source of wind-assisted propulsion, he says that it is paramount to be able to have as much benefit from the rotors as possible. “There are many factors involved in the jigsaw puzzle to find the optimum design. The result of all these factors combined will determine the eventual profit of those rotors. This research is about optimising vessel design to allow larger rotors and greater fuel savings.”



Rotor sails size impact on stability

Of course, larger rotors will deliver more thrust, thereby reducing fuel costs. However, larger rotors are more expensive. This is the trade-off between initial investment and fuel costs throughout the lifetime of the vessel. “If you keep increasing the rotor size, this will affect other factors on the vessel – things like stability, structural strength, and cargo capacity.”


Flettner freighter cargo vessel Switijnk Shipping


In terms of ship design, changing dimensions such as the length/breadth ratio will impact construction costs as well as stability and resistance. “We can now include the positive effect of added breadth, by having increased stability to carry larger rotors, where we would normally only see a negative impact of additional breadth on the resistance of the vessel.”




Optimising routes

Robin’s thesis also investigates the impact of wind on the efficiency of Flettner Rotors. “Depending on the wind conditions, we are looking at double figures for fuel savings,” he says. “If you have one Flettner Rotor operating in moderate winds from a favourable direction, you can already achieve a sizeable 25% fuel reduction. Of course it is important to look at the bigger picture. Conditions are not always favourable, which leads to lower savings in average wind conditions.“


To optimise prevailing wind patterns, C-Job is using an API of NAPA Voyage Optimization, a weather routing software from NAPA, the leading maritime software, services and data analysis provider. This combines historical weather data and current weather predictions with a vessel specific ship performance model, to find the optimum route between two points. During the thesis, support for optimizing routes of vessels equipped with Flettner Rotors was co-developed.



Commercial applications Flettner Rotors

All factors considered, what type of vessel is most suitable for a Flettner Rotor retrofit? When looking at vessel types, Robin replies, the first, and most important, factor is free deck space. “While cargo that is placed on deck could interfere with the airflow around the Flettner Rotors, we can almost always find space towards the fore and aft end of the vessel. Furthermore, larger and slower vessels benefit more from rotors than smaller and faster vessels. To this end, slow-sailing Bulk Carriers, Ro-Ro vessels, Car Carriers, Livestock Carriers, Tankers and Ferries would benefit the most from Flettner Rotors.”


The trade-offs continue when analysing sailing routes. Long stretches of ocean sailing with consistent winds are more advantageous than the numerous port stops seen in shortsea shipping. Transatlantic routes, therefore, would be well suited to vessels with Flettner Rotors.



Pros and cons retrofitting Flettner Rotors

For ship-owners weighing up the pros and cons of retrofitting Flettner Rotors, the absolute crux of the matter is the effect on total cost of ownership. “Our own long-term research, supplemented by additional studies such as Robin’s Master’s Thesis and expertise from partners like NAPA, puts us in a strong position to inform ship-owners about the potential costs and savings associated with Flettner Rotor installation,” adds Thijs Muller, C-Job Lead Naval Architect. This advice includes:

  • Calculating fuel savings on an existing route
  • Calculating an optimized route to find the best wind conditions
  • Calculating the impact of ship design modifications on overall vessel performance

“This means that we are the go-to partner for ship-owners looking for an independent and dependable advice when making informed investment decisions,” Thijs concludes.

Thijs Muller, Lead Naval Architect