The combination of precedents using deck-mounted windmills to sail directly into a headwind and new developments in kite technology that drive ground level electric generators, offers the possibility of adapting kites to sail vessels directly into headwinds. 

The wind power industry has recognized that wind speed increases with elevation, including above water. Such recognition has led several developers on small wind power installations to explore possibilities involving kites and balloons. Some prototypes involve using a balloon or kite to carry airborne turbines and electric generators. Other prototypes combine airborne kite with ground level electrical generators, using reel-out-reel-in operation to drive windlass mechanisms to drive electric generators while other installations using multiple tethers to drive a ground level crank mechanisms that in turn drive electrical generators. So far, kites with ground level machinery show great promise.

In terms of power generation, wind technology power output varies to the cube of the wind velocity. A 20% increase in wind speed is often available at higher elevation and offers a potential 72% increase in power output if kite based power and low level wind turbine operate at equal conversion efficiency. When flying above certain elevation, some jurisdictions require that kites be illuminated with warning lights after sunset. Modern day LED illumination and modern lightweight battery technology is quite compatible with high flying power generation kites that could be adapted to wind driven vessel propulsion.

Kite Power

At the present day, several companies are developing technology that combines airborne kites with ground level power generation technology. One company named KitepowerTM is based in the Netherlands (Holland) and has developed kites of 25, 40 and 100-square-m capable of converting energy from high elevation winds. Some of their kites fly laterally in a figure-8 configuration. In the USA, a developer named Dave Santos used a small scale version of the Kitepower kite connected to triple tethers that drove a ground level crank handle that rotated a driveshaft and drove an electrical generator.

At present, the Kitepower technology operates in reel-out-reel-in mode to drive ground level generators. There may be scope to adapt a one-way clutch mechanism to the Kitepower technology to drive a ground level driveshaft that rotates in one direction driving a lightweight, geared fast-spinning flywheel to maintain driveshaft momentum. It may drive either an electrical generator or a water pump. Kitepower has provided access to photographs of their technology that could be used at coastal locations to quietly convert high elevation wind energy to electrical power and without causing the noise of tower-mounted technology.

Kite Powered Vessels

The company named Skysails offers frameless kites that connect to vessel bow area and fly at high elevation. When deployed, vessels can sail parallel to the direction of trade winds and use high elevation wind energy to assist in vessel propulsion. In some cases, high elevation wind energy can provide all of the propulsive requirements of smaller and lighter weight, hydrofoil equipped catamaran vessels. At the present time, windmill powered vessels have sailed directly into prevailing headwinds at speeds of up to eight knots, with potential to adapt high elevation kite power to propel vessels directly into headwinds.

Precedents initiated by Kitepower of the Netherlands and by American kite power developer Dave Santos provide a basis to adapt kite technology for sailing into headwinds. While kites gain access to faster flowing air currents that occur at higher elevation and offer the promise of greater power, kites also incur higher drag than deck mounted windmills. Further research would be required to identify suitable flight elevations for kites and suitable kite designs capable of offering higher net propulsive power than windmills on vessel decks. Kites offer the possibility of overcoming some of the limitations of deck-mounted windmills.

Windmill Limitations

Land-based wind towers have collapsed. When sailing on the ocean, wave dynamics impose additional structural stresses on vessel mounted towers, restricting turbine hub height and also turbine diameter. While vessel deck mounted towers carrying wind turbines have propelled vessels into headwinds at speeds of eight knots, there is a physical limit to the size of tower and turbine that might be adapted to sailing vessel propulsion. During powerful winds, large-diameter land-based and coastal wind turbines are often locked down and produce zero power due to threat of turbine over-speed and subsequent turbine breakage.

For operation in high wind conditions, multiple small-diameter turbines mounted along a single extended length drive shaft that is set at an angle to the wind, offers greater potential to remain operational. For maritime propulsion sailing into a headwind, the multi-turbine system is a potentially viable option for sailing in severe wind conditions, especially the ability to tilt in powerful winds to expose less cross-sectional area between wind and turbine. However, when sailing in very light wind conditions, technology capable of accessing energy from faster air currents that flow at higher elevation has the propulsive advantage.

Windmill vs Kite

Kite based vessel propulsion removes the weight of the tower required to carry a horizontal-axis wind turbine, while airborne kites gain access to more powerful higher elevation winds. The absence of large-diameter rotating machinery avoids the problem of breakage due to centrifugal force resulting from elevated wind speed. Advances in super-fiber fabric technology allows for lightweight, super high-tensile strength tether(s) between vessel deck and airborne kite. Kites made from lightweight high-strength material can withstand elevated mechanical stress forces imposed by high-altitude winds, with the option to fly kites at lower elevation during extreme wind conditions.

Adapting power kite technology to seagoing vessel propulsion offers an alternative method by which to sail directly into headwinds. There is potential for the cyclical movement of airborne power kites to generate sufficient power at vessel deck level to provide the required propulsion to sail vessels at competitive speeds into headwinds as windmill power. To assure viability, researchers will need to focus on methods by which to enable kite-powered vessels to sail into headwinds at higher speed than windmill powered vessels. Such research potentially represents the next significant challenge in wind powered vessel development. 

Mechanical vs Electrical Propulsion

There is potential to transfer all technology of a land-based power kite system designed to activate ground level electrical power generation, to a seagoing vessel. The installation might include single tether and multiple tether variants. A single tether would operate in reel-in-reel-out mode driving electrical generation equipment, with potential for a multi-tether system to drive the equivalent of a crankshaft connected to a geared flywheel and in turn drive a submerged propeller. The direct mechanical system involves less weight, lower capital cost and higher operation efficiency that an electrical power system that includes generator and motor.

In railway operation, the combination of electrical generator and traction motors can account for half the cost of a locomotive. Electrical power conversion machinery operates at peak efficiency when operating near peak output, giving an all mechanical system an advantage when operating for extended period at pert-load power. There have been new developments in gear-based variable ratio mechanical power transmissions that would likely find application in wind powered vessels that sail into headwinds. Development of an all-mechanical power kite driven propeller propulsion system to sail vessels directly into headwinds would likely require some international collaboration.

Kites vs Deck Sails

A schooner vessel is proven to be able to tack-sail at angles as tight as 20 degrees to headwind direction. During the mid-1980’s, Canadian Professor Brad Blackford entered a small windmill powered boat into a completion at Halifax and sailed directly into a headwind to beat sail-powered competitors. Spacing between multiple masts of schooner vessels is crucial to assure optimal speed when tack-sailing at an angle to a headwind. Kites offer the advantage of accessing more powerful winds that blow at higher elevation and with the possibility to allow vessels with kite-driven propellers to sail directly into headwinds.

When a sailing vessel tacks into a headwind, continuous course adjustment and continuous resetting of the sails are required. A wind-powered vessel that sails directly into the headwind requires no such continuous readjustment and could sail with a minimal crew. When surface level winds are extremely light and barely able to propel vessels driven by deck sails, kites that access more powerful winds that blow at higher elevation, gain the advantage.