Exploring the Future of Sail Power

Prior to the development of maritime steam power during the 19th century, wind powered ships carried the world’s trade. New developments in wind propulsion include airborne kite sails from Germany and the U.S. that fly several hundred meters above ocean surface to access winds that blow at much higher velocity, but there is more potential yet to be explored. 

Some modern sails are based on the design of aircraft wings where the curvature of the airfoil-sail causes the flow of air to generate a powerful vacuum effect on the trailing side. At the beginning of the 20th century, a seven-mast schooner built to over 400ft length appeared and was intended to carry bulk freight - except that the performance of the seven-mast configuration proved disappointing in terms of speed and power. 

During the same period, early designs of airplanes flew with double and triple wings, the classical early designs of biplane and triplane. During that era, a German physicist named Prandt suspected that the top surface of an airplane wing may contribute to flight. His suspicion was based on poured water from a full cup or mug that had been tilted over. Instead of flowing straight down, the water consistently flowed down the side of the tilted mug or cup. 

He built a scale model of a single wing airplane with a curved upper surface and flat underside, then used a fan to flow air over the scale model that while secured by a tether, lifted off the ground. A full size single wing or monoplane was built and demonstrated superior flight characteristics to that of biplanes and triplanes. 

The sail layout of the seven-mast schooner had been based on the same theory that resulted in the construction of biplanes and triplanes. 

It took many decades before maritime researchers and designers adapted an aeronautical airfoil or wing to function as a boat sail and with very positive results that included vessels being able to sail at extraordinary speeds from the energy provided by crosswinds. Some designs of wind powered vessels have actually sailed at speeds greater than the crosswinds.

An innovative discovery from the world of wind power involved two rotors of two blades being mounted at a distance from each other on the same shaft, with rotors set at 90 degrees to each other. 

During testing, the Selsam two-rotor turbine actually delivered almost double the output of a single rotor turbine, a precedent that may be duplicated in the maritime world with sails. There may be scope for a twin hull vessel to carry parallel rows of masts, a row secured above each hull. Wind direction would determine the performance of sails secured to a parallel row of masts.

If the sails were airborne, the kite sails could be flown at different elevations and different horizontal distances from the vessel to extract maximum possible propulsive power from the wind. Propulsive power varies according to the cube of wind velocity, so airborne kite sails of a ship sailing at 20-miles/hour in a wind speed of 40-miles/hour will deliver eight times the propulsive power than if the ship sailed at 10-miles/hour in a wind speed of 20-miles/hour.

The additional width of a twin-hull vessel would allow for retractable horizontal booms to extend beyond the beam of the vessel and serve as mounting points for kite sails on vessels intended to sail parallel to trade winds. 

In commercial service, transverse booms may connect between pairs of vessels that will sail side-by-side, each serving as the other’s stabilizing outrigger and with kite sails secured to the transverse booms.

While much research has gone into coupling single-hull vessels into trains, there may also be scope to design twin-hull vessels that may also be coupled lengthwise into trains, with the trailing vessel sailing in the hydraulic shadow of the vessel coupled ahead of it. If the lengthwise coupled, wind-powered vessels were sailing in crosswinds with proper aerodynamic spacing between additional deck-mounted sails with increased sail area could greatly increase propulsive power as well as vessel sailing speed. 

The use of airborne kite sails on a coupled ship should provide higher power and even greater sailing speed. While airborne kite sails can access winds that blow at elevated velocity and at higher elevations, the technology has so far been offered as a retrofit to existing vessels and to be used when sailing parallel to trade winds. There has been comparatively little research undertaken into designs and layouts of vessels that could make greater use of kite sail technology in crosswinds and when sailing parallel to trade winds. 

Extended length tows of vessels coupled lengthwise could sail on the energy of crosswinds using well-spaced kite sails that would provide greatly increased propulsive power.

Vessels that sail parallel to trade winds may be coupled parallel and at a distance to each other, with transverse booms secured between them and extending beyond the width of both vessels. The transverse booms would provide secure mounting points for multiple, well-spaced kite sails that fly at different elevations at different horizontal distances from the coupled assembly of vessels. 

There may be scope for a second pair of parallel vessels to be coupled bow-to-stern to the leading pair of vessels, with additional kite sails that will fly at higher elevations than the leading kite sails.

Conclusions

While airborne kite sails promise to reduce fuel consumption on trans-oceanic ships that sail parallel to trade winds, there is future scope to design vessels that gain even greater benefit from kite sail technology in terms of propulsive power and sailing speed. Such gains could be realized when sailing parallel to trade winds as well as when using crosswinds to provide propulsion. The ability to adjust vertical elevation and horizontal distance from the vessel or vessel assembly could even offer superior tack sailing performance or when sailing at an angle of 20 to 30 degrees into a headwind.