For thousands of years, the wind’s kinetic energy was harnessed for ship propulsion. And just as sails displaced oars as a primary means of motive power, the advent of reliable steam engines in the late 19th century meant the great age of sail was ending. Yet in the 21st century, a new era of engine-powered ships with sail-assist rigs is dawning.
Beginning in the 1960s, there were a few experiments using full-sized commercial vessels. In 1980, the Japan Ship Machinery Development Association and the Nippon Kokan Shipyard put computer-controlled rigid sails on the new build 1,500-ton tanker Shin Aitoku Maru. Oceangoing voyages between Japan and China showed promising results. The sails acted as stabilizers for the ship and provided fuel-saving motion in most sea conditions. With wind assist and a better ride, the main engine could run slower with little effect on ship speed. Under sail alone, the ship reached 15 knots. In optimum situations the fuel cost savings were as high as 50 percent.
While modest experimentation continued in several countries, these early efforts in sail-assist systems did not scale up well for commercial use. Installation costs for existing ships and new builds were significant. The new build cost of the Shin Aitoku Maru was 20 percent more than a no-sail vessel of the same size. Before the 1970s, fuel costs were so low that fuel savings was not a critical factor. Evolving ship automation also meant smaller crews, and the addition of sail assist was seen as counter to this trend. Finally, the considerable negative global environmental impact of stack emissions was not a concern in those times. There was no need to invest in this capability for commercial vessels when operating costs and regulatory compliance were acceptable.
In recent years, interest in hybrid ship technology has increased. This is because of the increasing costs of ship operations, primarily driven by the steady upward cost of marine fuels.
Recent results have been promising. Sail rigs are automated so there is no increase in crew size. Installation costs, either retrofitted or new build, can be repaid in the first two to three years of operation. Measured fuel savings are in the range of 5–20 percent. While these vessels can cruise at speeds comparable to non-hybrid ships, harmful stack gas emissions from the engines are greatly reduced.
Hybrid rigs will not work on all ships. There are practical limits, such as when added masts and rigging could interfere with normal ship operations. In the past few years there has been an increasing number of hybrid technologies proposed. The most tested are three sail-assist systems: rigid sails, Flettner Rotors, and sailing kites. Each has become operational on a few merchant vessels.
Rigid sails. This system most closely resembles a modernized version of the classical sail era. The added word “rigid” means the sails do not require elaborate rigging and large crews to attend them. They are rigged or stowed mechanically, somewhat like window shades. For optimum propulsion assistance, the sails are managed by automated computer-driven systems. The pioneering Shin Aitoku Maru had this type of rig.
Flettner Rotors. Perhaps the oldest of the sail-assist concepts, this system was developed by Anton Flettner in 1925 and uses the “Magnus Effect.” Tall, mastlike rotors are spun by motors at their base. When wind passes across the moving rotor, a lateral force is developed that is at right angle to the wind direction. Thus, if the apparent wind is at a right angle to the direction of ship travel, this will generate a propulsive force in the direction of the ship’s course. The Viking Grace, a large Baltic ferry vessel operated by Viking Ferries, is power-assisted by rotors. Also, Maersk, the world’s largest shipping company, soon will install a rotor system on one of its midsized tankers.
Sailing kites. This system resembles a jumbo version of those used by recreational wind surfers. Tethered to the bow, they are the least obtrusive of the three sail-assist systems. Automated by computerized controls, they can be adapted to almost any type of commercial ship. The kite flies between 300 and 1,200 feet above the sea surface, where higher wind velocities provide much greater power. Germany’s 10,000-ton MS Beluga SkySails, built in 2007, is a 474 TEU (20-foot-equivalent unit) container ship. Over time it has achieved at least a 5.5 percent fuel savings.
For tomorrow’s mariners, “sailing before the mast” may take on a new meaning.