How does wind propulsion work on ships?
Updated: Jul 6, 2021
Decarbonization is sweeping the commercial shipping sector. Shipping has lagged behind in this area, but pressure is mounting to lower emissions, reduce fuel consumption, and get to carbon neutral by 2050. New technology like green fuels and solar are promising options to see the carbon reduction needed, but wind, one of the oldest marine propulsion methods, is also making a comeback. In the old days, wind power meant a fabric sail on a wooden mast, but modern wind power is anything but. Breakthroughs are coming in sail material, Magnus (Flettner) rotors, turbines, wind sails, and even hull designs that use wind to propel the ship. We’re going to take a closer look at some of these new innovations in wind power and how they might help to decarbonize the industry.
“VPLP’s design for a RORO vessel that will be used to transport components of the Ariane 6 uses four fully automated Oceanwings.” Image from The Engineer.
Why did wind power leave in the first place?
There are a few problems with wind power. First, wind is unpredictable. The wind isn’t always blowing and isn’t always blowing in the direction you want to go. Even if the wind is blowing in the right direction, wind speed can make voyage times highly variable. Second, wind power works very well for smaller vessels. Modern vessels are orders of magnitude larger than old wooden hulled ships, and with current technology, can’t be powered 100% by wind - in most cases wind can account for 5-20% of power needs. Last, sails have traditionally taken up huge amounts of space relative to ship size. Ships are much larger and need to pass under bridges and dock in tight ports, making tall, wide sails difficult to use effectively. Additionally, with the rise of coal, followed by oil, large amounts of energy can be packed into a relatively small space, making fuel propulsion easier. It is also much more predictable and has a widespread infrastructure built around it.
If wind can’t fully power vessels, why are we going back to it?
Current wind technology can’t fully power vessels, but it can assist. Future innovations could prove a way to produce more power from wind, enough to take up a larger percentage of the needed propulsion. The remainder could then come from green fuels, solar, or even nuclear. The Engineer has a quote from Gavin Allwright, who heads International Windship Association (IWA), “We don’t see how we can make the speed and depth of change with wind propulsion taking a significant load.” Let’s take a look at some of these technologies.
Sail technology can be divided into two areas: soft sails and hard sails. Soft sails most closely resemble traditional sails, as they are flexible and can be stowed when needed. This can also include inflatable sails which are a very new technology. VPLP designed a RORO (roll-on roll-off) vessel that uses a dual fuel LNG engine supplemented by four Oceanwings, automated wing sails on 30 meter masts.
VPLP Oceanwings designs. Image from VPLP.
Airseas is working on its autonomous Seawing system that deploys a large kite that when deployed can reduce fuel consumption up to 20%. The sail is controlled by computers that use the wing’s pull toward it’s “comfort zone” to propel the vessel. This solution is completely stowable when needed, but can greatly improve vessel efficiency.
Airseas Seawing sail. Image from Global Garland.
Hard sails don’t have the flexibility of soft sails, but they have other benefits such as the ability to cover them with photovoltaic coatings to produce solar power. Many hard sails are also telescopic so they can easily be retracted due to height restrictions or loading and unloading. A downside to large sails is that they can take up a lot of space on the deck and produce heeling, when a ship tips side to side. Technology like the suction wing is based on Jacque Cousteau’s Turbosail design. His design used a smokestack-type unit with shutters and fans that produced 3.5 to 4 times the thrust coefficient of the best sails in the world. His first ship performed well, but the second design was even better. It was able to run on purely wind power if the wind was right, supported by diesel engines when needed. Econowind has a similar concept in the Ventifoil, a vented wing with fans inside that provide additional power if the wind is right, allowing a reduction in engine usage. They expect fuel savings around 8-10%.
Econowind Ventifoil on the Frisian Sea. Image from Econowind.
Rotor designs like the Flettner rotor can be great additions to ships looking to increase efficiency. These large vertical rotors use the Magnus effect, rotating to produce lift that generates thrust for the vessel. Norsepower has the Rotor Sail, a popular Flettner type rotor that is already in use around the world. Some of these rotors are also foldable and can be reoriented almost horizontally for clearance under bridges.
Left to right wind pressure on a Rotor Sail producing forward thrust. Image from Norsepower.
Commercial vessels can use upward of 25,000-35,000 horsepower to move. Even the largest wind turbines of 14GW produce less than 19,000 hp. For a large vessel, that would mean at least two huge turbines. The Magnus (Flettner) rotors mentioned above could be super-sized, but would need their own platforms. These could be mounted on a catamaran on the stern of the vessel, requiring tow during operation. Given a large enough size and consistent wind, they could power a large commercial vessel in some areas. On the plus side, large wind power like this could make it efficient to sail vessels directly into a headwind.
New hull designs have also made headlines not only for their striking appearance, but the immense difference the design can have on propulsion needs. Sorje Lade, a Norwegian Engineer, designed the Vindskip concept vessel. The hull is turned into a wing that generates thrust simply from its design. He’s still working on it, but early tests show it could save up to 60% fuel and 80% emissions. With other renewable propulsion methods, these numbers could grow. Even though the design looks a little wild, ideas like this are getting more traction.
Vindskip concept design with pressure drag coefficients. Image from The Engineer.
What’s the way forward?
Current wind power solutions should have the ability to be retrofitted where possible. Flettner rotors, wind sails, and retractable hard sails are already in use, being installed on older ships. Newer vessels can take advantage of designing these features into the plan along with lower emissions fuels and better designed hulls. Taken together, all of the solutions can provide a significant reduction in fuel usage and emissions. Like all new technology, be it solar, wind, or other, continuing innovation will change the landscape. Ten years from now, the conversation will likely center around the most efficient hull sail design, rather than if it’s feasible. We look forward to continued maritime innovation!