On KiteTugs© copyright 1996, Dave Culp SpeedSailing
Previous Chapter | Next Chapter | Table of Contents
Here we'll consider two scenarios and make a number of assumptions. Scenario one considers a KiteTug of 30,000 sq. ft. It's dimensions might be: 350' span x 100' chord, by 22' thickness of canopy. The manned nacelle might be suspended 100-150 ft. below the kite itself. Scenario 2 considers a KiteTug of 15,000 sq. ft. This kite might span 240' x 70' x 15'.
The 30,000 sq. ft. kite is expected to cost $3 million in prototype4, which is in line, on a per-pound basis, with other large high tech prototypical structures (experimental aircraft, wingsails, etc.). It has been estimated that offshore (Far East) sub-contracting of such a structure may save 35% of this cost, and that mass production (on the order of ten units/run) will save approx. 20%. These savings are cumulative, i.e. foreign production units might be 52% (65% of 80%) of a domestic prototype's costs1.
This kite is well sized for crewed commercial flight. Taking the nacelle, its instrumentation, crew, supplies, and auxiliary engines and fuel into account, plus the weight of the canopy itself, a structure this size will contain a sufficient volume of helium to achieve positive buoyancy. Some of the volume of the kite will be filled with air, in order to maintain near-neutral buoyancy. The kite's altitude will vary from zero to perhaps 1500 ft, so some provision must be made for expansion of the helium.
We'll assume that the KiteTug spends 250 days/year at sea. This is a fairly widely accepted average for commercial ships. At first blush, this seems too few for the KiteTug, as commercial vessels typically spend 4 days of every 14 at the dock, loading and unloading. Since the KiteTug doesn't need to stand by while the vessel loads, she ought to be able to spend more days at sea, and probably will. With ships, however, general maintenance takes place at these docked times also, with one 2-4 week overhaul/year. The KiteTug will need maintenance as well. For purposes of this study, we'll stick with industry averages.
Of the 250 days at sea, we'll assume that half the total mileage will be dead-headed, and half will be under paying tows. This is probably conservative, given an efficient dispatch service and a bias for routes with favorable wind conditions. Ref 1 finds that approximately 1/3 of all winds at sea are <14 kts. In addition, by definition, 1/3 of all courses are <60 degrees from the eye of the wind. Assuming a KiteTug will consider light winds and courses close to the wind unprofittable, approximately 1/2 of all ship voyage/hours will have favorable wind directions and true wind velocities >14 kts. This figure is without regard for actual course sailed, and is a composite of all possible vessel courses. We will further assume that the KiteTug's speed while deadheading will be 50% faster than while under tow, so we'll budget 100 days/year for deadheading, and 150 days under paid tow.
Of the days under paid tow, we will assume 65% of the time we are able to act as "pure" sail, providing all the ship's motive power, and 35% of the time we will be "sail assisting," while the towed ship runs her engines concurrently. This will be during times of reduced wind, and thus reduced ship speed. We will assume that a kite of this size and power will be able to maintain 80-110% of the ship's normal cruising speed for 65% of the time chosen for KiteTug assist. We'll use 90% of full cruise speed as an average for calculations. It is assumed that at some threshold speed (80% of cruise speed?), the ship's master will decide to re-start his engines. Since marine engines do not generally do well at low power settings3, we'll assume that, under all "sail assist" scenarios, the ship run her engines at half power settings, and thus will burn 1/2 of the fuel she normally burns under her sole power; thus towing rates will need to be reduced by 1/2 for these times, in order to remain economically viable. These numbers are completely arbitrary and may appear optimistic. They will, in fact, vary greatly with the size of vessel under tow. The KiteTug, however, has the ability to "pick" its tows, and to abandon uneconomic tows for better ones elsewhere.
Market forces, in the persona of the ship's and KiteTugs' captains, will determine at what point a tow becomes "uneconomical." Physical distance to a more lucrative tow will be a factor as well. A ship's captain might be willing to accept a slower boat speed in order to entice a KiteTug to remain on station. Similarly, the Tug's captain might accept the reduced per diem income stream to avoid a long deadhead to another tow. A computerized matrix, constantly updated by the Tug's dispatch, will assist in making these decisions.
A KiteTug this size is capable of economically towing ships from about 25,000 tons to about 50,000 tons. It is estimated that a 30,000 sq. ft. kite, pulling a vessel at 14 kts in a 20 kt crosswind, might generate 200-400,000 pounds of towing force. This equates to 10-20,000 thrust horsepower (Shaft horsepower is multiplied by gear and propeller efficiency, typically 75-80%, to yield thrust horsepower)4. This implies that, at 30,000 tons, the KiteTug/ship combination might split "pure" sailing and "sail assist" in the 2/3:1/3 ratio envisioned above. At 50,000 tons, the ratios will perhaps be reversed, only 1/3 of the time will the vessel sail "pure." However, the larger vessel's much higher fuel consumption, and thus potential fuel savings, will result in higher average tow rates chargeable, and the KiteTug will favor large vessels over small. Below, we will assume a vessel of 30,000 tons, burning approximately 36 long tons of diesel oil, at $320/lt. per day. A 50,000 ton ship might burn closer to 50 tons. Even larger vessels may also be towed, but likely only in "sail assist" mode. Expected net fuel savings, and thus maximum tow rates chargeable, will be the only deciding factor in choosing vessel size and type.
Finally, we will assume that a ship owner will pay 80% of the cost of his actual fuel saved, as a towing rate. As the industry matures and KiteTugs become accepted, this number will likely rise (current conventional sail assist schemes offer to provide as little as 10% average fuel savings, with the ship owner absorbing the cost of the retrofit, to boot).
Thus, we have 100 days/year in which the KiteTug replaces 90% of 36lt of diesel fuel burned per day. At 80% of 90% of $320/lt, this would lead to average fees charged of $8,550/day, or $855,000 on an annual basis. In addition, the tug will have 50 days in which it can only charge an average of 1/2 normal fuel costs, so will add another $237,500 annual income. This gives a total annual income stream of $1,092,500.
Previous Chapter | Next Chapter | Table of Contents