When looking at fuel consumption alone over the measurement period, the savings were not immediately evident. However, when NAPA and ABB reviewed the data, they isolated a change in the propulsion power breakdown of Viking Grace, caused by the Rotor Sail.

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The same result was confirmed with a strain gauge analysis, where forward thrust of the Rotor Sail was measured and converted into propulsion power. Based on the different analysis, the expected long-term change in Viking Grace's annual fuel consumption because of the Rotor Sail has been verified to be between 231 and 315 tonnes on annual basis.

According to the analysis results, the Rotor Sail provides more forward thrust on the open sea legs of the route for the Viking Grace, but due to the route being located mostly in the archipelago, the annual fuel savings potential is on the same level within both route areas.

For this reason, Viking Line and Norsepower agreed to continue collaboratively using and enhancing the Rotor Sail on the M/S Viking Grace with the technology now fully operational.

Operating in the archipelago between Turku (Finland) and Stockholm (Sweden), the 57,565 GT M/S Viking Grace was equipped with one medium-sized Norsepower Rotor Sail Unit in April 2018, making it the first-ever global LNG / wind electric propulsion ship.

The Norsepower Rotor Sail Solution, which can be installed on new vessels or retrofitted on current ships without off-hire costs, is a modernised version of the Flettner rotor, which is a spinning cylinder that uses the Magnus effect to harness wind to propel a ship. The solution is automated and senses whenever the wind is strong enough to deliver fuel savings, at which point the rotors start automatically.

In addition, Norsepower’s Rotor Sail Solution is also fitted onboard the Bore’s M/S Estraden, a 9,700 DWT Ro-Ro carrier and Maersk Tanker’s 110 000 DWT Maersk Pelican.

Tuomas Riski, CEO, Norsepower, said that when the test period started, they faced had some challenges with the product, but were able to fix them quickly, and since the end of September 2018, the technical availability of the Rotor Sail has been around 97%. He added that the project has confirmed that this technology works also with high-speed cruise ferries and that favourable results can be achieved with a service speed of 21 knots. Moreover, based on harsh weather experiences so far, the Rotor Sail can be operated around the year without any weather-related issues.

For its part, NAPA mentioned that:

In the actual navigational conditions there is always variation in the environment, which has important effect on the fuel consumption. This is why a statistical regression model was applied, to overcome the variations in the comparison conditions and to disclose the effect of the rotor sail

Before the test results, LR had approved the structure and the risk-assessment related to the installation of a rotor sail on the Viking Grace, in line with its Guidance Notes for Flettner Rotor Approval. The approvals aim to ensure that the rotor would not have a negative impact on the safe operation of the ship or the safety of the crew.