Kite Power for Commercial Shipping

 Can kites be economically viable for propelling commercial shipping in mainstream applications?

In my view there are two chances of this happening in the foreseeable future: Zero, none and a teensy possibility that indirect kite propulsion by way of on-board kite generated electricity driving a conventional propeller may work. 

Whoops, that's three, but you'll get the drift.   

But this is not to say it's not worth trying.  Development work in this field, providing it's eventually shared, will undoubtedly advance kite flying in general, and greenwash money seems to be available right now to support even more unlikely ideas than pulling ships around with kites.  Just don't get suckered into using your OWN money.

Disposing of the indirect possibility first; the principle here is that one of the kite energy systems currently being developed by various project teams in Europe and the USA could be mounted on a ship, and the electricity generated then used to propel the ship via an electric motor and conventional screw propeller. This is certainly technically achievable, but whether it can be economically viable is in doubt.  The advantage of an indirect system as compared to using kite pull directly for traction is that all courses become possible- even directly into the wind.  However, whether a ship driven in this way can be fast enough over a wide enough range of wind conditions to make commercial sense is far from established.  For starters, none of the kite energy systems projects have yet been able to demonstrate a fully functional system- with practical kite launching and retrieval, some degree of autonomous flying, adequate reliability and useful power generation across the available wind range.  They all seem to be still some way from meeting even these relaxed criteria, but eventual success can't yet be ruled out.  My concern is not so much that they will fail to solve the technical challenges- though developing autonomous flying (kite autopilots) that will operate reliably in marginal and turbulent conditions is probably unachievable- but that in the end they will be defeated by wind's inherent variability. When there's too much wind, above 40knots, kites cannot usually be flown safely.  Even by 30knots, experienced kitefliers generally start to pack up.  When there's too little wind (less than 6 or 7 knots), kites fall into the water.  Until there is 15knots or so they don't develop useful pull.  This leaves a usable range of approximately 15kn to 30kn; the 'steady mid range winds' that I have not often experienced in my kite flying life, not yet anyway, though I live in hope. 

That winds are so variable and unreliable also makes using kites to directly pull commercial ships unworkable in my view.  It's not believable that 'just-in-time', customers will accept 5 weeks plus or minus 3 weeks instead of the 10 days to-the-hour trans-Atlantic delivery schedule that internal combustion power allows. 

And, such delayed and irregular schedules would be the reality if kites (or sails) were used now instead of diesel engines.  In the 19th century, sail power lost out to coal fired steam power, completely and irrevocably.  By 1895, there were almost no sail powered ships plying the North Atlantic in mainstream applications (military, freight or passenger).  From the mid 20th century, steam power (even powerful and efficient steam turbines) then lost out to diesel power.   Large diesel engines have now exceeded 50% efficiency, an amazing accomplishment considering that Sadi Carnot's second law of thermodynamics sets an absolute limit for practical heat engine efficiency of not much above this.

Kites have yet to show any form even by comparison with sails, not that they might not some day. Their various inherent disadvantages (like that kites fall into the water when the wind is light), have so far prevented them being even nearly as practical as conventional sails at converting wind into propulsive force.

To suggest that, for commercial shipping, kites can not only work better than sails-on-masts, but also make up the deficit between sail power and steam power, then the deficit between steam power and diesel power is dreaming.  Sailing craft took anywhere from 75 to 200 or more days, averaging around 100, to get from England to NZ in the days of sailing ships- and there was a fair chance of them not arriving at all.  Container ships today stick to a contractual schedule of 40 days for this passage- while diverting to four or five extra ports along the way.

As well as unacceptable reliability because of wind variability, there is a technical reason why kite power cannot be useful for commercial shipping- unless container ships change to submarine form so as to reduce their above-waterline drag profile. Aerodynamic drag is the killer.  A largish container ship, with containers stacked up 15m and more above deck level can have side area above the water line of about 5000sq.m. Even if its kite is also 5000sq.m's (5 x larger than any kite that's yet been built, but probably possible), the ship's superstructure drag will reduce the efficiency of the kite/line/ship system (measured by lift to drag ratio, L/D) to less than one, and options for upwind courses disappear.  This will be so even if this kite's stand-alone L/D is 5, (a little above current state of the art for traction kites). When other inefficiencies, such as hydrodynamic drag, are also allowed for, even if the apparent wind strikes the ship's superstructure at 45degrees rather than the worst case of beam-on, upwind sailing will still not be achievable to any bankable extent.

Additionally, the usefulness of kite power for container ships is restricted because they travel FAST- and customers will not generally accept slow because the cost of holding stock while in delayed transit exceeds the extra cost of cranking up a few more knots. Fuel use per tonne km is perhaps 70 times better for a container ship than for a truck or train- they're VERY fuel efficient.

Under kite power, it can't be expected that a commercial ship will be able to achieve sustained travel at wind speed or above. Even for dedicated performance oriented recreational sailing craft, exceeding wind speed is never easy, and especially not over any sustained period.   So, the wind speed will have to be 25knots or more if kites are to propel ships at the speeds the market demands.  And unfortunately, this is true even for kite assisted motoring.  Except on reaching courses (cross wind), and barely even then, if the wind speed is less than the boat speed, the motor will effectively be towing the kite along rather than the kite assisting the motor.

Putting this limitation together with the likely upper wind speed limit of 40knots suggests that the wind range for using kites to pull such ships is say, 25kn to 40kn at the outside.

OK, so say kites are only used when the wind is appropriate by direction and strength?  Hmm, for downwind sailing, is the wind in a useful direction 35% of the time, and in a usable strength range for 10% of the time?  Let's be generous and say it's going to work for one day in every 25.  Are ships owners going to install the equipment, add extra crew, and supply specialist training for some kite assistance barely one day a month?

And then there's the associated reduced load capacity- and the downtime from running over the lines etc, and the risks to the boat itself and its crew and to other craft from out of control and escaping kites (which will happen sometimes, no matter what fail safe systems are installed) -more costs.  Oil is so effective and efficient for ship's propulsion that commercial shipping will be the last mode of transport to switch to alternatives should oil prices ever climb to unaffordable levels again. But I expect that oil will hang in at bearable prices at least long enough for our nuclear phobic generation to be but a bad memory, and kites will not be competitive against nuclear power either, so will still not be able to find a role in commercial shipping.

Come to think of it, I'll take a small bet on viable (that is long term stand-on-it's-own-feet, non-subsidised) kite power for commercial shipping not happening for now (leave me a small out for some minor, and as yet unidentified niches).

There are things that kites can do superbly- recreational kite sailing, kite boarding, buggying, sports kite flying, snow kiting, possibly kite energy systems and so on- more than enough for a kite industry many times its current size, but powering commercial shipping is not one of them.

                                   Peter Lynn, Ashburton NZ, February 2009.

                                                PS, I'm an optimist