Different Directions for Flettner Rotors



There are positive indications that Flettner rotors have a great deal from now they could prove to be an untapped goldmine.  However, the criterion is getting it right.  It is a ‘make-or-break’ situation for those involved in the innovation.

Andrew Scott of the Energies Technology Institute (ETI), explains that Flettner sails are very simple.  They work by spinning a vertical tower as the wind flows around it.  The Magnus effect creates forward drag rather in the same way backspin is used by ball players to gain extra lift.

Tuomas Riski of Norsepower, the company which got such good returns on its first installation of a rotor sail on the MV Estraden that it was asked to fit a second on the same vessel, says that unfavourable crosswind conditions can be transformed into something that works.  This could give you about 2MW of difference in the best-case scenario.  Although the device results in some mass being located high above the deck, it actually works a little like a gyroscope to exert a stabilising influence.

Though the principle seems simple, its application is not so.  Different technologies are trying to find its way into the market.

The Flettner rotor installed on the MV Estraden is a fixed, lightweight structure made of carbon composite which rotates around an internal steel structure, held in place by two sets of bearings.  This makes it easy to retrofit because after adapting the deck and installing the base structure we can just take the entire external unit with all the devices inside and drop it on.

The team had to get the right materials and components to take the spin velocity.  They spent six months land testing it before constructing the first seagoing prototype.

Checks and results

The Rotor Sail was put through a year of strenuous checks.  After the vessel’s baseline profile in normal operation was established, it was activated and deactivated at random intervals in order to make sure any effects that showed up were due to the sail, and that any benefit was measurable across the vessel’s operating profile.Mr. Riski confirms that it behaved beautifully.  NAPA’s trial figures showed an average verified fuel savings of about 2.6% for a single rotor sail.

As per Norsepower and shipower Bore, the full two-rotor system on MV Estraden is achieving a touch over 6.1% fuel savings overall or 367kW.  Mr Riski says figures put the payback for an installation with two rotors at five years.  0% savings could be reached on the same route with a number of bigger rotors.

The commercial environment is a critical aspect.  Mr. Riski believes that a rotor sail has to be simple.  Currently, this is the market demand.  So while the company has recently been developing a collapsible design which folds down using a hinge just below the rotor, it still keeps to this basic premise.


CEO James Rhodesconfirms that Magnuss is aiming at something different.  There are a number of issues when it comes to installing a Flettner device on larger carriers.  An alternative is needed when it comes to crossing the rotors over into larger ships as it has to meet different demands.

The rotor sails could, theoretically, be fitted to many types of vessels.  The quest is to have it make sense.  A significant amount of a fixed Flettner rotor’s potential benefits are lost to drag in unfavourable conditions, this increases drastically if the ship is navigating a large portion of its route through open ocean waters with powerful winds.

Dry bulk carriers have big grabs that come sweeping over the deck.  Any installation on deck higher than a ship’s original profile will impede air flow.  In unsuitable conditions, it may produce unwanted drag.The clear area was very important.


Magnuss’ retractable Vertically-variable Ocean Sail System (VOSS) solution focuses on a base design for carriers of between 60,000dwt and 400,000dwt.  The company has developed both a pack-down, on-deck design for tankers while on a dry bulk carrier the VOSS will telescope down below the deck to leave a flush surface.  This gets the device out of the way for loading and unloading while in port.  At the same time, since the VOSS is retractable it simply doesn’t produce drag in adverse winds.

Magnuss VOSS is envisaged as a highly automated system that would be integral to the ship and structure.  The overall gains are well worth the work.

Mr Rhodes’ projected savings are larger than those presented by the original Norsepower sail.  He pobserves that under optimal wind conditions, fuel consumption can be cut up to 50% and under typical operating conditions, annual fuel costs can be reduced by 15% to 35%.

Magnuss has developed software for predictive analysis with firms such as AWT Worldwide to map real-life voyages as well as forecast simulations in order to gauge the savings. Norsepower provides the data but leaves the weather routing service to its end user.

Mr. Scott saya that ETI is investigating the potential of large-scale installations.  It is looking at fitting Flettner rotors to a vessel between 50,000 to 100,000dwt.  Putting several rotors on a big ship will give a high quality data on actual performance and a much better idea of what the real savings are, especially since these ships tend to spend more time at sea.

The requirements may well change over time.  While folding rotors would provide a solution for harbours that want the device stowed out of the way during loading or unloading operations, other facilities might be able to accommodate them.

ETI’s testbed project is now in the process of being firmed up along with owner, vessel, and technology supplier.

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Source and Image Credit: Norsepower


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