Brief guide to sail-assisted cargo ships


In March 1803 on the Forth and Clyde Canal, the steamboat Charlotte Dundas made history. She towed two seventy-ton barges nineteen miles upwind to Glasgow in just nine hours and twenty-five minutes, ushering in a new era of steam-powered boats. SS Savanna made the first steam-assisted transatlantic crossing in 1819 and the world of powered engines opened up the oceans. 

Sail and steam coexisted in peace until the 1900s, when internal combustion engines changed the face of the industry. In 1903, the French barge Petit Pierre and the Russian river tanker Vandal were fitted with diesel engines, marking the start of the motor ship era.

In 2020, we’re progressing in the opposite direction. Sail-assisted vessels are back, but they’re nothing that sailors of SS Savanna’s time would recognise. Spinning sails, kites, and vertical aerofoils work alongside alternative fuels to reduce costs and emissions.

Practical considerations

According to Lloyds Register’s Zero-Emission Vessels 2030 shipowner survey, zero-emission vessels shouldn’t increase costs by more than 10%, and technology reliability and scalability is more important than the cost. The same considerations are likely to apply to sail technologies.

Sail technology faces several practical challenges on cargo ships, including cranes, wind direction constraints, crew training, stability and maintenance.

Most tests of sail technologies are on passenger or ro-ro ships because masts, aerofoils or rotors on deck will obstruct cranes. For widespread adoption across ship types, sail technologies need to work around this.

Traditional sailing ships can’t sail directly into the wind, but we expect modern cargo ships to keep to a schedule regardless of the weather. Sail technologies that work regardless of the wind direction have an advantage over those that are restricted to certain points of sail.

In the modern world, most merchant seafarers have no experience of sail handling; most people with experience under sail have no qualifications for cargo ships. While it’s always possible to train merchant crew to sail, training adds friction to technology adoption. This is where automated solutions have an advantage over those that demand high levels of crew intervention.

As with all ships, sail-assisted cargo ships must meet the stability requirements. Depending on the sail system, some sail-assisted ships may need to consider the wind heeling requirements under the IS Code.

Mechanical systems need maintenance. Systems with simple maintenance, and standard, affordable parts have a distinct advantage over more complex systems.

Insurance and Classification

There are two categories of marine insurance: Hull and Machinery (H&M), and Protection and Indemnity (P&I). As the name suggests, H&M insurance covers the hull and machinery, while P&I insurance is a mutual insurance fund, where shipowners collectively cover each other’s losses.

While insurers can conduct their own surveys, in most cases insurance depends on compliance with relevant Conventions and Class rules. Several of the large classification societies, including DNV-GL, ABS and Class NK have either approved or issued guidelines for sail propulsion systems. This paves the way for insurance of sail-assisted vessels.

Resistance to change

Seafarers take the adage, “If it ain’t broke, don’t fix it,” seriously. When it comes to adoption of new technology on ships, shipowners and designers have to consider resistance to change.

Crew have a variety of concerns about sail systems. Some expect sail systems to create extra work that doesn’t directly benefit them, while others believe there are simpler ways to reduce carbon emissions. If the system is complicated, officers can worry about the consequences if they make a mistake or delay the ship. Seafarers are more likely to support sail propulsion technology if shipowners and designers take these concerns seriously.

Remove the barriers to adoption. Design systems that are low-maintenance, that use AI to trim the sails, or that provide some tangible benefit to the crew, and crew are more likely to adopt them willingly.

Sail propulsion systems in action

Shipping companies are experimenting with a diverse range of sail systems, from small traditional sailing ships like SV Kwai to the futuristic aerofoil hull of Vindskip. The International Windship Association lists members developing or using sail technologies to transport cargo.

Traditional sails

A small number of traditional sailing vessels carry cargo. Among the best known is SV Kwai, a topsail ketch registered in the Cook Islands. She and her crew of six carry cargo and passengers between Hawaii, the Line Islands of Kiribati, and the Cook Islands in the Pacific Ocean. Although her cargo capacity is small, she’s financially viable because she serves small islands that aren’t regularly served by conventional power-driven vessels.

Vertical aerofoils

Vertical aerofoils work on the same principle as aeroplane wings – or traditional sails. As the wind passes faster over one side of the aerofoil than the other, the Bernoulli Effect creates a low-pressure zone on the side with the fast-moving air. The normal air pressure on the other side creates a force pushing the aerofoil towards the low-pressure zone.

Vertical aerofoils impede cranes and take up deck space. In addition, to work effectively they need to be trimmed at the correct angle to the wind. Dutch startup eConowind’s ventifoil systems address these problems.

eConowind uses AI to trim the ventifoils, which fold out of the way when not in use. This keeps the deck clear for cranes and keeps the operation simple. Their containerised ventifoil system is fitted in a 40 foot container, making retrofitting straightforward. After tests on the 5,500 DWT MV Lady Christina in 2018-2019, Jan van Dam Shipping signed a contract for a ventifoil system on their general cargo vessel MV Ankie.

Rotor sails

When a cylinder rotates in a flowing fluid or gas, it generates a force at right angles to the direction of flow. This is known as the Magnus Effect, after the German physicist Heinrich Gustav Magnus. Rotor or Flettner sails work on the Magnus Effect.

Because they protrude from the deck, rotor sails face the same challenges as vertical aerofoils, save that they don’t need trimming. Their effectiveness depends on the angle of the ship’s course to the wind.

Tests of Norsepower’s rotor sails on Bore Shipping’s ro-ro ship MV Estraden confirmed fuel savings of 6.1% in 2014-2015, while the tanker Maersk Pelican saw savings of 8.2% in 2018.


While kites aren’t as popular as aerofoil or rotor sails, the German container ship MS Beluga SkySails crossed the Atlantic in 2018 assisted by SkySails’ computer-controlled kite. The system reduced fuel consumption by 2.5 tonnes per day.

The SkySails system comprises a towing kite, a control system, and a launching and recovery system. The control panel is fitted on the bridge, while the towing, launching and recovery system is housed forward. This leaves the hatches clear for cargo operations, and makes it easy to retrofit to existing ships. The real downside is that the kite only works well on a downwind course.

Aerofoil hulls

While most sail propulsion systems are fitted on deck to pull or push the hull through the water, Ladeas award-winning concept design makes the hull itself into an aerofoil. When heading into the wind, the hull shape will generate lift. The cruise control will balance the engine pitch against the wind power system, maintaining a constant speed and saving fuel.

What’s next for sail-assisted cargo ships?

Pressure is increasing to reduce emissions in the shipping industry. With growing acceptance and guidelines from flag states and classification societies, sail propulsion systems are ripe for innovation. 

Just as steam-assisted sailing ships were a common sight in the 1800s, modern sail propulsion systems will work alongside alternative fuels to create a new generation of climate-friendly cargo ships. 

As with other technologies, as shipowners, designers and seafarers gain real-world experience with sail propulsion systems, they’ll iron out the kinks. Over time, sail propulsion will become more common, leading it to become more user-friendly, easier to maintain, and more familiar to the seafarers who have to use it.

Nic Gardner

Nic Gardner is a Maritime Technology Analyst at Thetius. She is a master mariner who holds an unlimited UK CoC and has seagoing experience on capesize bulk carriers, ro-pax ferries, sail training ships, hospital ships, general cargo tramp ships, container ships and fisheries protection boats. When she is not at sea, Nic writes about a range of topics including technology and the maritime industry. Nic is also the author of “Merchant Navy Survival Guide: Survive & thrive on your first ship”, a book to give aspiring seafarers the knowledge and tools they need to make a success of their first trip to sea.