On KiteTugs©  copyright 1996, Dave Culp Speedsailing

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Why KiteTugs©?

First, a definition: "KiteTug" refers to a crewed and independently powered and maneuverable, lighter-than-air dirigible kite. It will closely resemble a "powered parachute," or paraglider, though far larger. It will be helium inflated, yet retain ram air over-pressurization to retain rigidity. Its crewed portion will be a "nacelle" suspended within the canopy's bridle. This nacelle will contain all living quarters, instrumentation and auxiliary power. The KiteTug will be flown in three modes;

  1. As a pure kite, attached to a vessel on the water's surface. In this mode, it is the sailing rig for the vessel, and "tows" it on its voyage4,6,14.
  2. As a powered airship or dirigible, free of attachment. In this mode, it can fly inland to hangar or docking facilities, or fly through windless areas, or over land enroute to a paying tow. Its motive power will likely be petroleum fueled internal combustion engines, but solar/electric, photo voltaic/hydrogen/Stirling engine or other combinations of alternate energy systems are certainly feasible.
  3. It will deploy a small hull carrying a keel, paravane, or "water kite," (or deploy a hull-less paravane or "hapa"4,6,13. In this last mode, it operates as a sailing vehicle in and of itself, and is capable of long voyages, at high speed, without using power other than for auxiliaries and control.

The KiteTug's control system will make extensive use of computerized fly-by-wire technology4,15,16,17. Its autopilot will monitor not only altitude, direction and speed, but also very accurately its rate of turn, rate of climb, plus local air flow and pressure characteristics throughout the canopy's structure. In addition, all sailing, navigation, and course-keeping control will be from the Tug. The ship becomes a "dead" tow and may even lock her rudder. The KiteTug will monitor the vessel's speed, course, surface winds, and, if applicable, her power and fuel usage functions. Her computers will then optimize course, canopy attitude and shape to maximize power transmitted to the ship's hull. Physical control of the kite may be through actuated control surfaces (rigid flaps, rudders, etc.)4,15,18, but more likely through sophisticated wing warping, accomplished by varying bridle line lengths between the nacelle and canopy. This is an efficient and elegant method of controlling a flying wing, not normally available to conventional aircraft.

The KiteTug would likely not be owned by an individual ship or ship owner, nor would it be assigned to a single vessel. Rather, it would roam the world's oceans in search of paying tows, typically on routes or during the portions of voyages where wind patterns favor sail power. This solves a number of general sail assist and kite rig related problems.

Advantages of KiteTugs©

First, such a scheme requires no retrofit to vessels of any kind. Indeed, the concept requires no long term commitment from ship owners at all. They would be met at sea (or, more likely, through a KiteTug dispatch service via radio), and offered a tow. Through the KiteTug owner's foreknowledge of the specific ship's fuel requirements for the existing conditions, a tow rate is offered which would be substantially lower than fuel costs for the vessel. Tow lines are passed over and the job begins. The ship takes lines from the KiteTug to her fore and aft mooring bits. The KiteTug varies the length of these lines (likely from a second, smaller nacelle near the ship where the bow and stern lines join to become the main flying towline), in order to vary the vessel's course in relation to the KiteTug's position. The ship's rudder will only be used in emergency maneuvers, and perhaps for tacking ship. The tow continues until either the voyage ends, or wind conditions drop to the point where tow rates become uneconomic, or until the KiteTug finds another tow available, steaming in more favorable wind, and within economical sailing distance. The KiteTug then disengages, sails (at speed) to the new tow, and re-attaches to the new vessel. The KiteTug can thus "cherry pick" only the most lucrative jobs, and tow vessels only during the best wind conditions. Revenue streams will remain high, as the gear remains fully utilized and seldom becalmed.

A dispatch service will need to be created, recording and predicting weather and ship movements worldwide. Embryonic versions of such services exist today, and are used to route long distance balloon and experimental aircraft voyages. It will need to maintain an extensive database, preferably specifying every sea-going vessel's capabilities, current load factor, and likely fuel usage at all times. Ship owners would be expected to comply, and to provide historical performance and fuel usage records, in order to "qualify" for KiteTug assistance. Tow rates offered by the KiteTug will vary widely, based on how much it can save a particular vessel in a particular set of conditions. Rates will be for towed miles, in order to account for variable speeds of the vessel due to wind variations. In addition, differential rates will need to be calculated and charged for when a ship's captain decides that the towed speed is unacceptably slow, and re-starts his engines, effectively converting the "pure" sailed tow to a "sail assist" tow. For purposes of calculations in this study, average days under tow are used rather than towed miles.

There will be minimal or no light wind or "contrary" wind conditions, as with conventional sail. In these conditions, a conventionally rigged ship either derives a much reduced utility from her rig, or suffers a penalty as she carries her drag-producing rig upwind. In these conditions, the KiteTug simply disengages, and goes in search of more lucrative tows. Further, the KiteTug can generally avoid both gales and doldrums, through careful planning via her dispatch service. If caught, she can disengage and fly through either condition on her own power, in free-flying mode.

In addition, the KiteTug may free-fly overland enroute to lucrative tows. Panama and Suez come immediately to mind, but trans-Florida, trans-Mexico via the Yucatan, and perhaps even trans-Iberian Peninsula flights are feasible. In and out of the Great Lakes, the Black and Caspian seas, and across the Straits of Tiera del Fuego may be profitable. Similar shortcuts present themselves throughout the island nations of the Western Pacific and Indian Oceans.

The KiteTug, which may become very large, will not normally inflate and deflate her canopy between voyages. Indeed, she will have few "betweens" at all. KiteTugs will be either towing vessels, or deadheading to new tows, nearly all the time. The Tug can come to land-based hangar facilities, or more likely mooring masts, for maintenance or overhaul. She'll be deflated only occasionally, for major maintenance or canopy replacement.

Limitations of KiteTugs©

The fact that a KiteTug is crewed means manpower is needed. A well planned and computerized conventional sail assist system might add no additional crew load to a ship at all1,2, although the crew will have to be specially trained, and their workload will increase. A KiteTug requires a minimum of two crew, and more likely three to four. Although costs are effectively split over the many tows the Tug is involved with, there will be a net increase in "crewed miles" for towed vessels, and thus inherent cost increases. However, these are offset by increased revenue streams.

A KiteTug is essentially a tethered aircraft. Such devices are potentially very dangerous as they are susceptible to fairly fast-onset oscillations and crashes. The best controllable kites today still occasionally smash into the ground or sea. A crewed event would be disastrous. There are two attributes of KiteTugs which are expected to ameliorate this. First, the kite is very large. Sizes to 15-30,000 square feet will be commonplace4. Such large structures tend to be more stable than small ones. They do not react to relatively small gust cells in the wind, and events like stalls happen relatively slowly. Second, the KiteTug will be heavily instrument and largely computer controlled (fly-by-wire)4,15,16,17. It is quite possible to fit the entire canopy with pressure sensors and to model pressures and flows throughout the structure via computer. Unstable events will be discovered and corrected before any human becomes aware of them. In addition, an emergency cut-away system may be rigged. When a situation arises, such as a high velocity dive below a specified altitude, an emergency system could cut away the tow, which would instantly convert the KiteTug into a low flying and stable glider15. After recovery and correction of the problem, the KiteTug will start her auxiliaries, maneuver back alongside the vessel, and continue the tow.

Another issue concerns KiteTug handling and safety in high winds. The KiteTug cannot be effectively reefed. Historically, however, other large sailing and inflated flying structures of this size have shown the ability to continue operations in these conditions. The largest sailing ships 100 years ago were on the order of 400' long and spread upwards of 50,000 sq. ft. of working sail. These ships rarely reefed, and gave their best performances in the Southern Ocean, where winds average 30-40 kts19. The Graf Zeppelin class of dirigibles, to 700' and flown in the 1920's and 30's, powered through all normal storms in order to maintain their schedules. KiteTugs' control systems will need to be capable of reducing the Tug's coefficient of lift to low levels, while maintaining stability and control. While this is a challenge for human controlled kites, it will be within the computer controlled and sensored KiteTug's ability.

Last, we need to consider damage or catastrophic deflation to the canopy and emergency landing and/or self rescue at sea. First, the crewed nacelle will be on the order of 40-60 feet long. It will be a watertight, boat-shaped unit, capable of operating on its own at sea. It will have decent handling characteristics, and include effective sea anchors, or other position maintaining devices. These will be needed to re-launch the kite, if if repair is practical. Large inflated kites, when tethered by their trailing edges, are relatively docile and will lay on the surface unattended for long periods. Thus, an emergency procedure would entail reducing altitude (by any of several means) until the nacelle is waterborne, then cutting away (or more practically, quickly lengthening) all forward lines of the bridle, leaving the kite tethered by its rear lines. It may then be deflated and retrieved, or abandoned. If repairable, and for initial launch, the kite will be re-inflated and re-launched by reversing the procedure. If the canopy is not repairable, a second, much smaller "jury-rig" kite is deployed, and the nacelle may be sailed back to harbor as a kite-rigged boat herself.

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