Revisiting the Case for Windmill Propulsion

Over a period of centuries, sails formed the basis of wind powered vessel propulsion until German aviation engineer Anton Flettner developed vertical-axis Magnus rotors that used the boundary layer effect to convert wind energy to vessel propulsion. During the latter 20th century, Canadian physics Professor Dr. Brad Blackford built windmill powered boats capable of sailing directly into the wind. During early 1980’s boat race at Halifax, his boat sailed into the wind at greater speed than sail-driven competitors. 

Introduction

At the present day, numerous initiatives are underway to reduce ship engine carbon emissions. In this regard, there is ongoing interest in making greater use of wind power for ship propulsion involving the combination of kite-based airborne sails to capture energy from parallel down winds and cross winds blowing at 90-degrees or more when measured from the bow. Some modern vessels such as the Maersk Pelican tanker ship carries a pair of 30-meter tall vertical-axis Magnus Rotors installed above its deck, and in service, this reduces ship emissions by up to 10 percent.

Interest in wind-assisted ship propulsion invites examination of the research and development of Blackford who by 2010 had built a hydrofoil catamaran vessel driven by a single, three-bladed horizontal-axis windmill mounted to the top of a mast and uses mechanical linkage to drive the propeller. Sailing along the North American east coast, the vessel achieved a speed of eight knots sailing into the wind and 12 knots sailing with the wind. There is scope to combine Blackford’s research with recent advances in wind energy conversion technology, to develop larger vessels or perhaps faster vessels.

Wind Turbines

While Blackford’s most recent windmill powered boat is a hydrofoil catamaran design, the question to be explored is whether there is scope to upscale the concept and to what extent? General Electric has developed three-bladed wind turbines to 12MW or 16,000 horsepower output. California based wind turbine developer Doug Selsam places a series of wind turbines spaced along a single extended-length power shaft that operates at an angle to the prevailing wind direction. There would be scope to install a parallel pair of Selsam multi-rotor Super-turbines atop a mast with a movable T-shaped member to secure the power shafts.

A single extended length power shaft would require two sets of 90-degree gears to transmit power into a vertical-axis driveshaft installed inside the hollow tube mast, with three sets of gears for parallel shafts. To adjust to more powerful winds, each power shaft would tilt with forward turbines moving downward the rear turbines moving to higher elevation. Parallel multi-turbine power shafts would involve small diameter turbines capable of spinning in powerful winds while providing vessel propulsion. Single super-size three-bladed turbines would be applicable to large vessels sailing at low speed of up to eight knots, into the wind.

Combining Innovations

Onetime Michigan University professor of business Dr. C. J. Prahalad presented the theory of new business opportunities resulting from the convergence of emerging technologies. Both hydrofoil and catamaran vessels sail through choppy water with reduced rolling and pitching. Transferring aeronautical technical innovations to maritime application would include installing a (wind powered) transverse-axis propulsion paddle wheel into the hydrofoil upper surface, to transfer the vessel to hydrofoil sailing at lower speed through choppy water. A water current driven cylindrical roller built into the hydrofoil upper surface would achieve a comparable result. 

While the objective of increasing wind propulsion for large freight ships is to reduce carbon emissions, combining various technical innovations could also produce a windmill-powered, hydrofoil catamaran tourist vessel capable of sailing smoothly through choppy water while carrying 50 to 200 guests. The proven ability of wind mill driven propellers propelling a vessel directly into the wind at higher speed than sail powered vessels of equivalent size and weight, enhances prospects of installing such technology above the bow area of both large and smaller vessels. Smaller passenger vessels used in tourist service may use multiple turbines on single power shafts.

Mechanical Power Transmission

While horizontal-axis three-bladed turbines deliver higher efficiency than vertical-axis wind turbines, the latter design reduces the complexity of transferring propulsive power to the propeller(s). Advances in constant-velocity joint technology provides smooth power transfer through angles of 45 degrees, with a pair of such couplings able to transfer power from a vertical-axis wind turbine placed high above the deck to a horizontal-axis propeller. Due to high torque at low RPM from large three-bladed turbines, expensive electric power transmission prevails. However, there are innovative methods by which to transfer power between a stern-mounted three-bladed turbine and the boat propeller. 

It appears possible to combine a stern area three-bladed wind turbine with the gear-based azimuth propeller drive system that neutralizes steering torque reaction. For applications requiring a smaller diameter turbine, the Selsam built dual rotor two-blade turbines set 90 degrees apart on an extended length power shaft with tilting capability are proven to deliver high power and operate in high winds. Applications requiring even smaller diameter wind turbines have to option to use the Selsam multi-turbine system in the stern area, keeping most of the turbines away from and behind passengers aboard to wind-powered cruise vessel.

Mobile Floating Wind Turbines

Higher wind speed occurs above ocean than over land enhances the attractiveness of offshore floating wind turbine development. Both horizontal and vertical-axis wind turbines have been adapted to boat propulsion, enhancing prospects to combine stationary and mobile floating wind turbine development for the benefit of both the power generation sector and maritime transportation sector. There are several shipping routes where air draft is unrestricted, allowing for application of the largest possible wind turbines to assist ship propulsion. Shipping routes with restricted air draft will require technical innovation to develop height adjustable wind power technology for ship propulsion.

Windmill Tug/Power Unit

The physical configuration of cruise vessels and fully laden container ships restricts onboard placement of wind turbines. A super-size multi-hull tug boat could include a pair of wind turbines connected through hinges to floating towers. While the ship would enter port areas, the size of the super-tug requires that it remain at anchor outside of ports. When a ship leaves port via tug propulsion, it would be coupled to a super-size wind turbine vessel with outer hulls and wind towers set as much as 300 meters apart, with ship superstructure providing pitch and roll stability to the wind unit. 

The arrangement could allow a pair of mega-size wind turbines to generate electric power as the ship sails directly into a trade headwind. A pair of wind turbines of up to 16,000-horsepower each (12MW) would deliver electric power to vessel electric propulsion motors and propellers. Super-size floating wind units coupled to both bow and stern could involve three turbines converting energy from diagonal headwinds for a container or cruise ship, yielding potential approaching 36MW while the lower height of bulk carrier ships could allow power from four wind turbines to generate electric power for ship propulsion.

Technical Possibilities

A telescopic Magnus rotor could operate at 30 meters height above deck while at port and extend to 50 meters height above deck when sailing across ocean. A hinge installed at the top of a wind tower mast would allow a three-blade turbine to rotate on a horizontal axis when sailing across ocean, then disengage and fold to the vertical-axis position to pass under a bridge such as along the Suez Canal. There may be future scope to develop telescopic wind tower masts that enclose telescopic splined concentric drive-shafts, to further reduce the height above water of retracted three-blade wind turbines.

A dual-purpose extended height or telescopic super-mast with a hinge could carry a tilting three-blade turbine at high elevation. At lower elevation above the deck, it would support an airfoil-sail or an airfoil-sail with counter-rotating cylinders to capture energy from wind blowing to the vessel at between 20 degrees from the bow to 90 degrees, to compliment wind turbine and airborne kite-sail propulsion. When wind speed is extreme, wind turbines need to be tilted to reduce cross sectional area relative to wind direction or have blades secured in a neutral setting to prevent high-RPM blade destruction.

Airborne Wind Power

Wind speed increases with elevation and has led to the development of various manifestations of airborne wind energy conversion. One version of the technology involves a kite or balloon carrying a wind turbine and generator, with power line built into the restraining tether. Another version combines airborne kite technology with ground level conversion of mechanical motion to electrical power. The technology is still in the early phases of development and offers promise for future propulsive power for small boats sailing into a headwind, with stern mounted tether for airborne wind power conversion technology.

Such wind power conversion with airborne technology secured via tether at the stern would be suitable for small hydrofoil cruise vessels carrying tourists. The power producing technology would be behind the vessel at high elevation, remote from passengers and with minimal sound reaching the vessel stern area. Some variations of the technology could likely involve multiple tethers connected to pulley systems that drive the power shaft and propeller(s). An all mechanical system would reduce the high cost of electric generator(s) and motor(s) with potential to offer higher efficiency converting headwind energy to propulsive power.

Combination Airborne Wind Power

A combination airborne wind power attached at vessel bow and stern has potential to provide propulsion. Retractable airborne kite-sails housed in the bow area and reeled out when needed, are proven to propel vessels using parallel downwind energy and side winds up to 90 degrees to the ship central axis. For sailing directly into a headwind or diagonally into a headwind, airborne wind power conversion technology secured via tether to the vessel stern area could provide substantial propulsive power, with the technology being towed behind the vessel when not required.

Conclusions

Courtesy of a sailing competition dating back to early 1980’s held at Halifax, Canada, Dalhousie University physics professor Dr. Brad Blackford built a windmill powered boat that sailed directly into the wind at higher speed than sail powered competing boats. His research allows windmill powered freight ships to sail directly into trade winds as reduce fuel costs and carbon emissions. There is scope to build on Blackford’s earlier innovations using modern wind turbine technology and hydrofoil innovations, to develop wind powered tourist cruise vessels capable of sailing smoothly through choppy water. 

Future maritime wind propulsion concepts would likely involve independent super-size wind turbine vessels that would operate outside of port areas, to be coupled to large container ships, bulk carriers and even cruise ships to convert wind energy to electric propulsive power as these ships depart from port to sail across ocean. Future wind powered vessels would likely use the combination of airborne kite sails and windmills that may be deck mounted, mounted on separate units that couple to a ship or even airborne wind energy conversion technology that drives vessel propellers.