|Francis de Winter
|Ronald B. Swenson
P.O. Box 7080
Santa Cruz, CA 95061
We are developing the technology for the use of kites in wind powered marine propulsion. We sailed our kiteship # 2, a lightweight fiberglass proa 7.3 m long, with several sizes of kites in fresh water and in the ocean (in Monterey Bay). The kites are shaped like wings, with areas of 4 sq m, 9 sq m, and 28 sq m. The kites are flown tethered to four lines, which are fastened to the proa with a system that can run on a track mounted on the smaller (ama) hull of the proa. The two back lines serve to warp the wing for steering in the sky, and the two front lines carry most of the pull and are also used for kite control. Until now the kites have been handled with only manual control, and a lot of effort has been devoted to making this control system functional, safe, and convenient. For large kites all the kite controls will be motorized and computerized, and that is our next effort. Tillers on two coupled rudders, one at each end of the main hull, were used for steering the proa.
This is the fourth paper in our R&D program to develop the use of wing-like kites for the propulsion of large ships. Boat # 1 was a small planing catamaran built for speed, achieving up to 30 knots and up to twice the wind speed, using multiple kite arrays of up to 16 sq m in total size (Refs. 12, 13). We built boat # 2 to further develop the kite controls, and to develop the unassisted kite launching and retrieval operations. It is a lightweight fiberglass proa built with the large hull 7.3 m long, and the small hull 6.1 m long. We sailed it on a lake in the Sacramento Delta in California, and in Monterey Bay. Both sites are characterized by steady wind conditions of about 40 km/hr in large open areas.
By design, until now we have sailed in winds no stronger than about 20 km/hr. We sailed the boat with three sizes of kites: 4 sq m, 9 sq m, and 28 sq m. The smaller ones were available at the start of the program, and had been used with kite buggies on dry lakes, as well as on our previous boat. They both have an aspect ratio (of wingspan to chord) of about 3.5 to 1. The 28 sq m kite was custom made for our program, and has an aspect ratio of 3 to 1, with a chord of 3 m, and a wingspan of 9 m (see Figs. 1, 2).
The proa is symmetrical fore and aft, so it can sail in either direction. It is gibed or "shunted" by changing the direction of travel, rather than by turning the boat and continuing forward in another direction. Thus one hull always remains to windward and one to leeward, making a proa particularly suited to kite power (Refs. 1, 2, 3, 13). The steering is done with two coupled rudders, one fore and one aft, which can rotate through 360 degrees. Both of the rudders have a tiller fastened at the top, and either of these can be used for steering. The boat is very responsive to the steering. With the small kites we sailed boat # 2 with a crew of two people: a pilot to handle the kite and a helmsman to handle the steering. With the 28 sq m kite we may prefer to use three people for the steering plus all of the other control functions.
A supporting motor boat operating close at hand has been considered essential until now, not only to tow the proa as necessary, but to provide help in case of problems. Unlike future boats in our series (Refs. 1, 2, 3), this kite-pulled proa is a lightweight experimental craft with a relatively large kite area. It can be subjected to very high power and stresses by wind gusts, and it is thought advisable to have help nearby.
2. FUTURE INCENTIVES FOR SAIL POWER
In our earlier papers (Refs. 1, 2, 3) we stated that crude oil shortages (Refs. 5, 19) will provide an incentive to use wind power in the merchant marine again, to save fuel by providing some or all of the propulsion power. Shortages will develop when the reserves are depleted to the point that oil production can not be made to rise any further. In a typical oil field or oil producing region (or in the whole world), that happens when about half of the recoverable oil has been recovered, as was first described by Hubbert in 1948 (Ref. 5), and as has been shown in many thousands of oil fields of all sizes, and in many regions and nations before and after 1948. The world halfway point is expected around 2003 (Refs. 5, 19).
Recently the economic upturn in Asia caused a crude oil shortage, and oil prices tripled. Outside of the Middle East, the oil fields are already in decline almost everywhere. In the Middle East, during the years of oil gluts and low oil prices, the oil production infrastructure was not built up enough to handle the current oil demand. The result is a tight oil market, and high oil prices unlikely to go down soon, if ever (Refs. 5, 19). It will take large investments in new infrastructure and a long time to give the world any spare production capacity, and worldwide crude oil depletion will make any spare capacity disappear rapidly (Ref. 19).
It has long been known that renewable energy devices (like kiteships) would never be able to prosper so long as there was enough cheap crude oil to force them out of the "free market" (Ref. 18). When it is recognized that cheap oil is gone forever, the "free market" may offer renewable energy (including kiteships) a place at the table. That may be surprisingly soon.
3. ON KITESHIPS
When merchant marine vessels were wind-powered, only masts and sails were known and understood (Ref. 15). The use of kites would have required an understanding of the aerodynamics of wing cross sections, powered and computerized controls, and sailcloth and rigging much lighter and stronger than was available at the time. The world is now different, but currently sailing still involves primarily mast and sail combinations (Refs. 7, 8, 15), although some of it is quite imaginative (Ref. 16).
If wind power is to become cost-effective in the merchant marine again (as a fuel saving measure), it seems desirable to consider the use of large deployed wing-like kites, such as are used in some maneuverable sport parachutes. These have recently also become popular for wind surfing (also called "kite surfing"), for kite buggies on dry lakes, and for some small boats.
Kites can power a boat without the dangerous overturning moments inherent in masts and sails. Kites can fly high up in the sky, far from the effects of the waves and of the boat hull, in steadier and stronger wind conditions. Current kites can achieve a lift-to-drag ratio of about 6 to 1, which can sometimes give a boat more versatility and more power than is possible with masts and sails. One can maneuver the kite back and forth in the sky, so that the kite travels through the air faster than the boat, hence increasing the available power significantly.
The traction can be provided at one point on the boat. Since the best location for this point will change depending on the heading of the boat and the wind direction, it may be desirable to use a track mounted on the boat on which to position a small trolley to hold the kite lines, as we did on our proa, or it may be desirable to use a two legged bridle arrangement, as we may use on our larger vessels (Ref. 13). It may be quite simple and cheap to retrofit this on existing freighters and other ships.
There have been many publications regarding the potential of kiteship technology (Refs. 1-4, 6, 7, 9-14, 20-22). There were some pulling efforts long ago, with traditional kites. The ancient Chinese almost certainly used kites to help pull wagons and carts in windy areas. Benjamin Franklin did not only use kites for electrical experiments. In his youth he once flew a kite while swimming, getting pulled clear across a pond one mile wide (Ref. 20). In the early 19th century Pocock (Ref. 21) used kites to pull carriages along roads, and also to pull sizable boats on open bays. Franklin had written that kite power might be used to cross the English Channel (Ref. 20). Cody did this in 1903, and Stewart in 1977 (Refs. 12, 22). We have worked in this area since 1978 (Refs. 1-4, 6, 12-14, 22).
Our kites differ from traditional kites. A traditional kite only has to take off and to support its own weight, plus the weight of the string. As seen by the person holding the string, most kites of that type are downwind, perhaps no more than 45 degrees above the horizon. Even most of the traditional maneuverable kites are not able to cover much of the sky: they have a very small "wind window." The aerodynamics of traditional kites is quite poor. They have an inefficient airfoil, with a low ratio of the lift to drag coefficients. Benjamin Franklin could be pulled straight downwind while swimming, but even if he had been in a sleek boat with an efficient keel he would have had a limited choice in the directions he could sail. Pocock sailed his carriages and his boat upwind, but no more efficiently than other boats of his day, perhaps 60-70 degrees off the true wind.
The main thing our modern traction kites share with these traditional kites is that we also use tethered and controllable flying devices tied at the end of one or more strings. When these modern kites are inflated with air their cross section is like an airfoil - a wing. The kites are kept inflated in flight with openings in the leading edges of the wing cross section, which use the stagnation pressure to inflate the kite. The openings of the 28 sq m kite have a one-way valve, so it is easy for the air to get in but not to get out. These help to keep the kite inflated while sitting on the water and also help to keep the water out. Before the kites are launched they must be inflated. This is easy to do on land by holding the kite into the wind, but on a boat smaller than the kite (see Fig. 1) this is difficult. We use a motorized blower to inflate the 28 sq m kite quite rapidly.
The kite airfoil has a ratio of the lift to drag coefficients of about 6 to 1. It is the lifting force, shifted only slightly aft by the drag, which is supplied to the kite lines. The edge of the "wind window" is nearly 80 degrees (rather than 45 degr.), and a boat can sail in the direction against the wind more effectively and more rapidly than a modern sailing vessel using masts and sails.
4. DEVELOPMENT OF THE KITE CONTROL SYSTEM
Our kites are made of very light and strong material, and all our lines are made of Spectra, to have the minimum diameter and weight for a given strength. For the small kites the lines were led to a system of pulleys mounted on a rail on the downwind side of the proa. The pulling force or traction of the kite was transferred by the pulley assembly to the boat, and the pilot handled the control lines and forces.
With the 28 sq m kite the pull can be as high as 4,500 N (1,000 lbs). The kite traction is also transferred to the boat through the set of pulleys mounted on the rail on the proa's outrigger hull. A set of hand winches is used to launch or retrieve the kite, and to fly the kite farther out or closer in. A proprietary system allows the pilot to control the wing-warping controls of the kite. Separate controls make it possible to change the position of the attachment pulley on the rail, and to adjust the traction lines as well. Our newest control system readily lends itself to control motorization, a necessary next step.
With larger kites the control has to be performed not by hand, but with powered controls, either closely overseen by a skilled pilot, or else tied to a computer with specialized software. The 28 sq m kite is being used first with manual controls to determine the design parameters for the powered and computerized control system. We will then finalize the design, construction, and testing of powered and computerized controls for larger kites.
The speed and power obtained with these kites has been encouraging. The 4 sq m kite pulled the boat at an estimated speed of 10 km/hr in wind speeds of about 20 km/hr. Under the same conditions the 9 sq m kite produced speeds of about 16 km/hr. We have sailed with our 28 sq m kite briefly under light wind conditions in the ocean (Figs. 1 and 2). The next step is to sail with stronger winds, of up to 40 km/hr.
Results with our boats # 1 and 2 have been encouraging, and we are continuing development with boat # 2. We plan to determine the design requirements for the motorized kite control system, to begin to develop the software for computerized kite control, and to determine the needed characteristics of our boat # 3. Boat # 3 will be used to perfect the motorized and computerized kite control system, and to perform detailed kite traction tests on large kites.
6. REFERENCES AND BIBLIOGRAPHY
1. F. de Winter, R. B. Swenson, and D. Culp: "Kiteships, Sailing Vessels Pulled and Powered With a Kite," Proc. of the ASES Annual Meeting, Portland, ME, June 1999.
2. F. de Winter, R. B. Swenson, and D. Culp: "The Kiteship Project," Proc. of the ASES Annual Meeting, Madison, WI, June 2000.
3. F. de Winter, R. B. Swenson, and D. Culp: "The Kiteship (TM) Project," Proc. of the Millenium Solar Forum 2000 of ISES and ANES, Mexico City, Sept. 17-22, 2000, pp 551-553.
4. For kiteship technology and literature details, and photos and details on our work, see the website: <http://www.kiteship.com>.
5. For the future in world crude oil supplies, as well as the present situation, see the website: <http://www.hubbertpeak.com>.
6. A. M. Basque: "Have Kite, Will Sail - Researchers Hope to Prove Kite-Power Can Pull Large Boats," Santa Cruz Sentinel (the local newspaper in Santa Cruz, CA), Sat. Sept. 26, 1998, page A-4.
7. C. A. Marchaj: "Aero-Hydrodynamics of Sailing," International Marine Publishing, Camden, Maine 1988.
8. P. D. Priebe: "Modern Commercial Sailing Ship Fundamentals," Cornell Maritime Press, Inc., Centreville, MD, 1986.
9. D. Prentice, (pub.): "American Kite," Quarterly Magazine published at 13355 Grass Valley Avenue, Grass Valley, CA 95945.
10. "The Ancient Interface XI," Proc. of the Eleventh AIAA Symposium on the Aero/Hydrodynamics of Sailing, Vol. 27, Seattle, WA, Sept. 12, 1981.
11. "The Ancient Interface XII," Proc. of the Twelfth AIAA Symposium on the Aero/Hydrodynamics of Sailing, Vol. 28, San Francisco, CA, Oct. 30 and 31, 1982.
12. W. M. Roeseler and D. A. Culp: "Kitesailing Progress," Proc. Of the Eighteenth AIAA Symposium on Sailing Technology, Vol. 35, Stanford, CA, Oct. 14 - 15, 1989.
13. Journal of the Amateur Yacht Research Society (BCM AYRS, London WC1N 3XX), ISSN 0144-1396, Numbers 114 (1993), 116 (1993), 118 (1995), 122 (1996), and 124 (1997).
14. D. A. Culp: "On Kite Tugs," AYRS Journal 124, 1997, pp 29 - 45.
15. A. A. Hurst (ed.): "The Medley of Mast and Sail, II - A Camera Record," Naval Institute Press, Annapolis, MD, 1981.
16. B. Smith: "Sailloons and Fliptackers - The Limits to High-Speed Sailing," A book Published and Distributed by the AIAA, Washington, DC, 1989 (ISBN: 0-930403-65-7).
17. B. Smith: "The 40-Knot Sailboat," Grosset & Dunlap, NY, 1963.
18. F. de Winter: "Economic and Policy Aspects of Solar Energy," Proc. of the Ninth Biennial Congress of ISES, Intersol 85, Montreal, Canada, June 23-29, 1985, Pergamon Press, NY, 1986, Vol. 4, pp. 2207-2218.
19. C. J. Campbell: "Myth of Spare Capacity Setting the Stage for Another Oil Shock," the Oil and Gas Journal, March 20, 2000, pp 20-21.
20. G. L. Rogers: "Benjamin Franklin's The Art of Virtue," Acorn Pub., Eden Prairie, MN, 1990, page 192.
21. G. Pocock: "The Aeropleustic Art, or Navigation in the Air by the Use of Kites or Inflated Sails," Self-published, Bristol, England, 1827. See also http://www.kiteship.com/history/
22. W. M. Roeseler, D. A. Culp, et al: "The Case for Transport Sailcraft," World Aviation Congress, Society of Naval Architects and Marine Engineers (SNAME), Los Angeles, 1996.
Figure 1. Kite of 28 sq m Being Inflated
Figure 2. Kite of 28 sq m on Its First Flight