Wind assisted ship propulsion has returned in ship design

Wind assisted ship propulsion (WASP) enjoys growing shipping interest, driven by the need to curb climate change. One of the most proven technologies in this field are Flettner-rotor vessels.

“It has in fact become a sport to consume as little fuel as possible at each voyage”, says Antje-Friederike Herbst, the master of this century’s first commercial Flettner rotor vessel, the E-Ship 1.

Merchant sailing ships reached their technical peak during the 1840’s. Improved steam engines, and later diesel engines combined with fast growth of sea trade -requiring more and larger vessels – made sailing merchant ships obsolete. The development of Flettner rotors or rotor sails in the 1920’s marked the return of ‘modern’ wind in ship propulsion.

The initial ideas of German aerospace-engineer Anton Flettner were scientifically based at utilising the Magnus effect. Flettner technology, despite reasonable performance during major voyages, lost its significance in the early 1930’s due to rising popularity of marine-diesel engines burning inexpensive heavy fuel oil. Shipping interest in Flettner technology revived in the 1980’s for reducing fuel use, with commercial application start in 2010.

E-Ship 1 is a RoRo vessel with 16-person crew that can either transport 853 TEU, rolling cargo and complete turbines and main components and the first and only Flettner-rotor vessel developed in-house from scratch by a wind turbine supplier, German direct drive wind turbine pioneer Enercon. The company is also known for innovative variable-speed hydropower and wind powered water desalination plants, and an advanced wind-diesel hybrid unit with multiple storage solutions. All were built around core wind power know-how.

First ideas originate from 2000 and focused at maximising sustainability performance. Research effort commenced the same year, followed two years later by testing the Flettner-rotor technology at an in-house test rig. Concept development and full-scale Flettner-rotor testing were key next steps towards E-Ship 1 construction start in 2007. Despite E-Ship 1 complexity and delays caused by the initial shipyard’s bankruptcy, the unique 21-st century vessel made a maiden voyage with cargo in August 2010.

Antje-Friederike Herbst is E-Ship 1 master from 2014, and according her estimate one out of around ten female captains within the German merchant fleet. She gained earlier experience in the heavy-lift sector, and at historic tall-ships.

Herbst: “My first voyage was bringing turbines from Emden in Germany to Ireland, and this was a special experience. Above all, E-Ship 1 is technically a smart idea, but the technology principle and operation of the four rotors is remarkably simple. After starting-up the rotors in open water, E-Ship 1 like a tall-ship lifts the hull at a constant 3 – 5-degree tilt. The absence of engine noise and vibrations thanks to the diesel-electric propulsion system felt immediately pleasant and one can even hear the bow-wave under clear weather conditions.”

Design features of the 10,020 dwt vessel, 130 metres by 22.5 metres, are a streamlined double-skin hull and a wheelhouse with minimised flow resistance below and above the waterline. The 7 MW diesel-electric propulsion system comprises 7 diesel-generators burning ‘environment-friendly’ Marine Gasoline Oil (MGO).

Two variable-speed electric motors, modified marine-adapted generators used in Enercon E-82 turbines, drive the propeller. The in-house power management system integrates operation of wind-diesel and Flettner systems. Rotor aerodynamics and flow dynamics know-how resulted in an optimized propeller-rudder design. One of several other system optimizing measures is feeding engine exhaust gasses waste heat through a steam turbine for additional shaft power.

Since commissioning, E-Ship 1 travelled over 535,000 nautical miles with main routes the Mediterranean, South and North America. The rotors reduced annual fuel consumption by 20 per cent (annual mean) compared to similar-size conventionally powered vessels, or 920 tonne MGO savings at 13 knots cruising speed.

Long-term annual mean cargo capacity utilization is according Enercon statistics around 76 per cent. Hamburg-based shipping company Auerbach operates E-Ship 1 since 2014 for owner Enercon, which did not build additional Flettner vessels ‘because this is no more part of the turbine core business.’ Original plans for third-party licensing of “E-Ship 1” technology was not pursued either.

Herbst adds that Enercon’s initial concept envisaged fully automated hybrid system operation, including stepwise engaging and disengaging engines together with continuous adaptation of rotor revolutions for maximizing wind power utilization. However, this advanced power management system did in practice not work completely as expected and the hybrid system operation is now performed manually but again aimed maximizing wind power utilization and fuel saving.

Herbst: “It has in fact become a sport to consume as little fuel as possible at each voyage, but operational flexibility is required too because there are specific conditions whereby fast delivery is key. The Flettner rotors can be utilized up to severe gale and storm conditions corresponding to Beaufort 9 – 10, whereby similar sails reefing effect is achieved through reducing rotor revolutions. Under such best sailing conditions with maximized wind power utilization, we can disengage up to five engines and another one must always be engaged for powering the rotors. Engine seven provides ship supply or hotel power.”

Maximum forward propelling force is achieved when wind flow passes the rotors from either port [left] or starboard [right] sides perpendicular the ship’s hull centre line. Forward thrust remains substantial with wind flow directional deviations up to 35 degrees from the optimum angle. Mapping optimal sailing routes requires a pragmatic approach and common sense said Herbst: “This could for instance mean shifting routes by a few degrees if this shows beneficial, but never in travelling many extra miles with a single focus at maximizing wind utilization.”

Herbst added that the rotors in practice do not impose restrictions to loading and offloading while in port. She further observed that spinning rotors add substantially to overall E-Ship 1 dynamic stability performance [aerodynamic damping and gyroscopic absorption] and thus considers it essential to keep them running while at sea. “This includes for instance letting the rotors counter-rotate in pairs under unfavourable wind conditions insufficient to provide forward propelling force”, she concluded.

This article was previously published in Offshore Energy Magazine by Eize de Vries.