Ecoliner Case Study: 2 vs 4 Flettner Rotors of 18m

In our previous blog posts, we explained why wind propulsion matters, how WASP devices work, and why performance prediction must go beyond simplified assumptions. In this article, we move from theory to a practical case study taken directly from our publication «High Fidelity Modelling and State-of-the-Art Evaluation of WASP Systems’ Fuel Savings on Major Shipping Routes», published in the RINA Wind Propulsion 2024 Proceedings.

The question is straightforward: for the Ecoliner test ship, is it possible to double the savings if doubling the number of wind propulsion systems? To answer to that question let’s analyze the vessel equipped with two and four Flettner rotors.

The Configurations

The technical publication compares the same Ecoliner vessel in two layouts: x2FR and x4FR. Each rotor is 18 m high, 4 m in diameter, and fitted with a 6 m end plate, with a maximum allowed rotational speed of 180 RPM. The analysis uses a high-fidelity approach that includes the hull of the ship, the superstructure and the rotor layout for the aerodynamic modelling.

That matters because WASP devices do not operate in isolation. The aerodynamic field around the hull and superstructure changes the performance of each rotor, and the rotors also interact with one another. So, the real design question is never just how many devices can be installed, but how much useful thrust each added device still delivers once these interactions are included.

Behind this 4 rotors configuration, the idea was to fit the maximum number of systems onboard and compare against the reference 2 rotors layout. Let’s see if it was the right decision from a performance and economic point of view.

In terms of trim optimization, the best common RPM is found for each sailing condition in both cases. So, all rotors will be working at the same speed. Individual trim optimization could offer further improvement, but it would require deeper analysis and advance onboard control systems.

What the Polars Show

The first practical insight appears before routing begins: doubling the number of rotors does not double the driving force. Interaction losses appear immediately, especially in close upwind conditions, where drag can worsen if not tilting mechanism is available. Another critical area is apparent wind reaching conditions due to the proximity of the pairs of front and aft rotors in the x4FR configuration.

The high-fidelity polars show that x4FR does not outperform x2FR across all conditions. At 10 kn true wind speed, the crossover appears around 75 degrees TWA. At 20 kn, it moves to about 55 degrees. At 30 kn, it is close to 50 degrees. Below those angles, the two-rotor solution can still be the better performer.

This is a critical message: more devices do not automatically mean better overall performance. The marginal value of each additional rotor must be checked on the actual ship layout. The systems’ distribution must be taken carefully into account in order to maximize each system’s contribution.

Results on the North Atlantic Route

For routing computations, a tilting mechanism is assumed to be present and have zero added drag. It is activated as soon as power savings are negative (in close upwind conditions). The selected route corresponds to a North Atlantic route, from Plymouth to New York.

For the route results shown here only GC studies at a constant boat speed of 12kn were performed. The goal is to use the most common routing strategy with and without FRs on board and evaluate the benefits of the installation without the need of applying any operational changes.

The high-fidelity routing results make the practical message very clear: x4 performs better than x2, but the difference depends strongly on route direction and operating assumptions.

On the westbound leg, adding two more rotors raises savings by roughly 2 percentage points. On the eastbound leg, the gain is closer to 4.5 to 5 points, because the vessel spends more time in reaching conditions where the rotors can generate more useful thrust.

What This Means in Practice

The Ecoliner case does not say that four rotors are a bad idea. It says that the business case must be judged on marginal benefit, not on installed count alone. For this ship and rotor size, four rotors provide a measurable improvement, but the gain depends strongly on route direction and operating conditions.

To analyze this from a business case perspective, let’s calculate the ROI for these 2 configurations under the same benchmark conditions used in the technical publication:

• Round trip Plymouth – New York
• Route over Great Circle at constant 12kn including waves
• Fuel price of 700$/ton
• 300 operational days
• Flettner Rotor acquisition cost of 500,000$, plus 100,000$ of installation and 100,000$ for the tilting mechanism considered per device (these numbers are a rough estimate of the actual system cost and must be reviewed by a manufacturer).

These ROI numbers must be considered as a rough estimate as we did the exercise as an example. Avoiding large waves, controlling the speed of the vessel, or optimizing the route according to weather conditions will considerably reduce these values.

Conclusion

For the Ecoliner test ship, four 18 m Flettner rotors outperform two, but not by a factor of two and not under all sailing conditions. The practical lesson is simple: extra devices create extra opportunity, but also extra interaction losses.

That is why robust performance prediction matters so much. The right configuration is the one that delivers the best net benefit on the real ship, on specific route, under realistic operating assumptions. For wind propulsion projects, that difference is what separates a persuasive concept from an investment-grade decision.