The Cape Linnet is an ordinary 82,000-tonne bulk carrier with four white towers bolted to her deck. Each is 35 metres tall and spins at up to 180 revolutions a minute, using the wind to push the ship along. We joined her for a week in the South Atlantic, carrying soya beans from Santos towards Rotterdam, and filmed the crew at work.
Four towers on deck
A rotor sail is a vertical cylinder turned by an electric motor. When wind crosses a spinning cylinder, the air moves faster on one side than the other and the pressure difference produces a force at right angles to the wind. On a ship heading north with the wind from the east, that force pushes her forward. The effect has been known for a century; what changed is the price of fuel and the rules on emissions.
The Cape Linnet's owner, Brenner Bulk of Hamburg, fitted the rotors during a scheduled dry-docking. The four towers stand on the hatch-free strip between the cargo holds and can be tilted flat for loading under the port's grab cranes, which takes about twenty minutes. The motors draw power from the ship's generators, so every hour of rotor use has a small fuel cost of its own, which the bridge software subtracts before it shows the crew the saving.
Routing for the wind
On a conventional ship, the route is the shortest safe line between ports, adjusted for weather. On this one the captain draws a second line. Captain Dario Kovaฤ, a Croatian master who has commanded bulk carriers for fourteen years, checks a wind forecast every six hours and will bend the track by a hundred miles or more to find a steady beam wind.
"Before, bad weather was something to avoid," he said, standing by the chart table. "Now I go looking for a certain kind of weather. Twenty knots on the side is what I want. A headwind is useless, and the rotors come down."
The headwind in the title came on the fourth day: a front with wind straight from the north, where the ship was going. The rotors were stopped for thirty hours, and the Cape Linnet burned fuel like any other bulk carrier.
What the fuel log says
Chief engineer Joel Santiago keeps the numbers in a spreadsheet next to the official log. On the best day of our week, with a steady easterly, the main engine burned 21 per cent less fuel than the ship's baseline at the same speed. Over the full week, including the headwind, the saving was 9 per cent. Brenner Bulk says its average across the first year of operation is 7 per cent.
"People ask for one number," Santiago said. "There is no one number. There is a number for this route in this season, and the owner has to decide whether he likes it."
The sceptics
Not everyone is convinced the savings justify the cost. "Seven per cent on a trade like this is real money, but rotors only pay off on routes with reliable wind," said Karin Lindqvist, a fuel analyst at Tidewater Marine Consulting. "Put the same ship on a route with calms and headwinds and you are carrying four very expensive towers for decoration."
Charterers raise a practical point too. The towers add weight and complicate cargo operations at ports with older equipment, and some charter contracts do not yet say who keeps the fuel saving: the owner who paid for the rotors or the charterer who buys the bunkers.
What the crew thinks
The crew of twenty-one have mostly stopped noticing the towers. The steady hum of the motors is audible in the accommodation only when the ship is quiet. The cook, Arnel Bautista, says he can tell from the galley when the rotors stop, because the ship rolls differently.
The wind is one route to lower emissions. The other is changing the fuel itself, as we report in The ship that runs on its own cargo and the first ammonia bunkering.
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