On KiteTugs© copyright 1996, Dave Culp Speedsailing
Previous Chapter | Next Chapter | Table of Contents
Kite rigs (free-flying kites) have several inherent advantages over conventional sail, plus some distinct limitations.
A strong advantage of kite rigs is that since the rig isn't actually on the ship, minimum retrofit, and minimum deck and storage space are required4,5,6. This is particularly important while at dockside, when deck space is fully utilized. Wing masts, Flettner rotors, etc. are prone to damage or are a hindrance to cargo loading/unloading.
In addition, the kite rig is substantially manufactured away from the vessel. Downtime and retrofit costs are minimal. Further, a kite rig can be carried from one ship to another, as for varied testing, or as vessels change routes, or owners.
Kites fly at higher altitudes than conventional rigs. Wind velocities increase with altitude above the water. A large kite flying at an altitude of 1000 ft. will typically see winds of 15-30% higher velocity than a conventional rig whose center of effort is 60-80 ft. above the water7,10. As energy derived from the wind varies with the square of its velocity, this translates to 30-50% more energy available to the kite, on a per sq. ft. basis.
When on downwind courses, conventional sail becomes inefficient, due primarily to reduced apparent wind (the vessel's speed is subtracted from the actual wind speed to yield apparent wind), and also to blanketing effects from the vessel's superstructure and/or additional sailing rigs aft blanketing those forward. Not only is a free flying kite immune to such blanketing effects, the kite may also be maneuvered in the sky independent of the boat (typically in a horizontal "figure 8" pattern). This results in far higher apparent wind speeds at the kite than those experienced by the hull, and thus far more energy available than to a hull mounted rig . Calculations, and actual experience with kite rigged boats, indicate that these rigs may extract 4-10 times the energy of a conventional sail on these courses4,8,9. An analogy may be seen in a modern windmill's moving blades. The energy extracted from the wind is related to the blade's swept area over time, not to the blade's actual surface area.
Very large kites are feasible. Since deck space is not compromised, and the vessel experiences minimal heeling from the deck- or gunwale-mounted kite, far larger kites may be carried than conventional rigs. Coupled with the higher power available from winds aloft, plus the advantage of off-wind "sweeping" of the kite, far more energy is transferred to the hull. Pure sailing vessels, which do not anticipate substantial motor/sailing modes, are feasible. The kite powered ship will return a higher average fuel saving to the ship owner, without requiring a purpose-built vessel. Fuel savings and higher potential vessel speed could even crack the routing problem. A ship's master might be persuaded to sail on wind-favorable routes, rather than direct great-circle ones, at significantly greater fuel savings. The question of the vessel's net average speed, port to port, remains to be addressed. A combination of direct and wind-favorable routes is perhaps likeliest.
There are unique problems associated with using kites as sailing rigs. The greatest, perhaps, relates to keeping the kite aloft, particularly in low or no wind situations. One study concluded10 that the potential hazard of the kite falling in the water, particularly in the ship's path, outweighs any financial benefits associated with kites. Very fast retrieval methods have not been perfected, and such a grounding would destroy the kite at least, and perhaps foul the vessel's propeller and rudder, at worst.
Fast, efficient launch and retreival of heavier-that-air kites, and/or altering the vessel's heading during wind lulls may ameliorate this problem. Another solution would be to use a lighter-than-air, helium filled kite4,6, so as to maintain positive buoyancy in any wind condition. Near neutral buoyancy is desirable, so as to allow retrieval when desired. This introduces the added complications and cost of helium and its storage, as well as potential problems with reducing sail in high winds.
The general difficulty of launch and retrieval of any kite, particularly by a vessel at sea, will remain a challenge. although total retrofit costs are lower, costs of line handling winches, and reinforcement of the hull for midship towing are still substantial10. Also, the vessel might need to change course or to stop in order to launch or retrieve the kite. Though solveable4,6,11,12 specialized skills, space robbing deck mounted gear, or luck might be necessary for solid, successful launches and retrievals. This process has been compared to launch and retrieval of aircraft from aircraft carriers4. Risks and manpower requirements acceptable to a military organization are not necessarily acceptable to the merchant marine. Such a kite might only be launched and retrieved once/voyage, however.
Last, the simple "difference" of the scheme may doom it. Like many businessmen, ship owners are conservative. "Selling" a sail assist system with no historical precedent will be an uphill challange. Viability of the concept will likely have to be demonstrated to the industry before it will be accepted. This is another chicken and egg issue.
Previous Chapter | Next Chapter | Table of Contents