Kite builders have for decades installed multiple kites on a single control line, forming an airborne train of kites that are usually flown in light wind conditions. Hobbyists have adapted frame-less kites to surfboard propulsion. Large-scale frameless kites have been adapted to propel boats using cross winds and trade winds, and there is scope to adapt such kites into trains to provide vessel propulsion.
Introduction
The history of kite-based boat propulsion dates back over several hundred years when frame kites were used to tow boats over short distances, when wind direction was suitable. At the time, kite frames were made from bamboo tied together with hemp or silk thread. Woven silk was used as flight material and hemp thread used as the towing line. The idea of installing multiple kites along a single line also dates back over a period of centuries, including to tow small boats over short distances across open sea.
Single large frame-less kites are presently used to assist in vessel propulsion, usually sailing parallel to a trade wind in the case of large vessels, or perpendicular to cross-winds as would be the case for passenger cruise and tour vessels. Precedent involving trains of kites made from bamboo and silk suggests suggest adaptation of large frame-less kites for modern vessel propulsion. Problems pertaining to kite train operation would revolve around storing the kites when vessels are in port, launching the kite train and retracting the kite train upon nearing the destination port.
Future Development
One of the research challenges would involve storage, launching and retracting trains of frame-less mega-size kites, while another challenge would revolve around development of suitable lightweight high-strength tether material capable of enduring severe ocean weather conditions. If a single kite can offer 2% of vessel propulsive requirements, research needs to establish whether a train of 10 kites at progressively higher elevations between 3,000 feet and 5,000 feet would be able to provide up to 20% of vessel propulsive requirements. There would be need for night time illumination of kite trains to alert small, low flying aircraft.
Borrowing from maritime history, clipper ships of a bygone century carried international trade sailing parallel to and propelled mainly by trade winds. Modern technology allows wind-powered vessels to use more powerful wind energy that blows at over double the speed, at over 3,000 feet elevation and offering many times the pulling force. Accessing high altitude wind energy that blows at double the speed difference between wind speed and boat speed would offer the potential of eight times the power from a parallel trade wind. There would be potential to install multiple kite trains at mast spacing to convert energy from cross-winds to propulsive power.
Material Requirements
Kites assembled into airborne trains for trans-ocean propulsion will need to combine high strength with light weight, be waterproof, be resistant to solar UV radiation and endure repeated operation during severe weather conditions over many years, using synthetic textiles such as Kevlar and woven glass fibre fabric. While frame-less wind-inflated kites have been made from nylon fabric and polyester fabric, repeated long-duration exposure to solar UV radiation will eventually break the chemical bonds that keep such fabric intact.
Lightweight, ultra-high strength and weather resistant tethers would connect airborne stacks of mega-size kites to the decks of ships. Multiple tethers made from any Kevlar, glass fiber, or carbon fabric would likely be suitable for vessel towing applications.
Super-Kites
Kite enthusiasts and hobbyists have developed kites controlled by multiple tethers, allowing a ground-based operator to adjust tilt and angle relative to wind direction to move the kite sideways or vertically. While most super-kites include a frame, frame-less super kites would need to form the basis of future vessel propulsion. When sailing parallel to trade winds, the width of the vessel would determine how many parallel kite-trains would pull the vessel.
Sailing in crosswinds allows for installation of kite-trains along the length of a vessel, at spacing equivalent to mast spacing on a wind-driven sail ship. There may be scope to experiment with an extended length catamaran vessel based on canoes, involving multiple kites intended for surfboard propulsion and installed at mast-spacing in parallel kite-trains along the length of the vessel, with up to 10 kites per tether.
Launching and Retrieval
When a ship is in port, stacks of propulsion kites would need to be retracted and contained in a storage area, perhaps inside an extension built into the upper region of the bow. Tugs would move the ship from quayside and from the port terminal area, where kites might be unfurled and launched skyward. The tethers would be unrolled from drum spools that could be coordinated to adjust kite tilt angle and flight elevation. Wind speed and tether tensile strength would determine flight elevation.
On approach to destination ports, readjustment of kite tilt angle would reduce flight elevation and allow for reeling in of tethers. The combination of kite retrieval, folding and packing into storage would require a step-by-step procedure involving one kite at a time. Deck based machinery would need to repeatedly connect to each tether upstream of each kite to allow for each kite in the stack. The process would likely take 2 to 4 hours depending on the number of mega-size frameless kites in the stack, which could number as high as 50 kites.
Conclusions
Kites and trains of small kites have been used to tow small vessels over short distances in Asia, when winds were suitable. It is theoretically possible for kite-trains or stacks of kites to gain access to sufficient propulsive power to propel large vessels. The main constraint will be the tethers or towing cables that will need to combine immense tensile strength with low weight and extended usable service life in repeated adverse weather conditions, at competitive cost.
Top image courtesy John Vetterli / CC BY SA 2.0
