[The_Reader_David]

I think the idea is simply when the wind is with you, put up the kite and use less fuel, when it's against you, haul it in.

You can't tack with a 'sail' like that, sailboats and sailing ships which
tack need a keel and a sail which functions as an airfoil. A kite is only good for running before the wind.

[MeanWestTexan]

"A kite is only good for running before the wind."

That's my assumption also, but without a detailed picture, I can't tell if they have figured out some clever way of internally changing the angle of airfoils inside the kite or something.

(That said, I am wondering what they would do with the slack in the rope as the kite began to luff. I suppose if they were quick about it, they could tact before the kite hit the water.)

[USFRIENDINVICTORIA]

You're right about tacking being a problem for a tanker (it certainly wouldn't be as agile as a kite board).

However, these kites *are* actually airfoils, so they are more like a mainsail than a spinnaker. The long, deep, and nearly vertical sides of a freighter should act like a keel, to resist lateral movement. If this technology were to become popular, hull designs would likely be modified to optimize the boost from the kite, without hurting performance under power.

Also, the kite would not cause the boat to heel anywhere near as much as a sail on a tall mast. There would be little additional need of a heavy keel for ballast against heeling.