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Kite-Powered Ship Launched
The Grand Poobah writes "The big-kite technology we discussed last month has officially launched in Hamburg, Germany. Reuters has a writeup of the new technology, which aims to cut fossil fuel use on sea voyages by an estimated 20% by means of a huge computer-controlled kite. The link includes a video."
This is a great start but not the whole answer. The position of the kite allows it to only work with the wind coming from abaft of beam. (from behind)Also creating a "lee-helm", driving her nose down wind. To get the 20% fuel savings (I am guessing closer to 15%, from experience) 50% of the time, a second kite would be needed amidships. The wind would in fact be much stronger at altitude, But with little to block it 100 feet above sea level, I believe a schooner rig would be as productive, more often. "Down-wind" is not the best point of sail. "Close-hauled" is. Where the wind comes from either side of the bow. Creates an airplane wing effect that sucks ship forward. (Positive to negative) With kite alone, the ship would lose a lot of energy trying to stay on course due to rudder angle. Under perfect conditions, this will work great. Just not often. I am retired from the U.S. Merchant Marines and have worked and sailed on many tankers and schooners. I do believe we tossed sail aside to quickly. Amazing, free and renewable energy.
I took up kitesurfing in the summer. I can attest to the enormous power a kite can yeild - far more than a sail of the same size. Most people kitesurfing use a kite around 10-12 metres long depending on their size and the wind conditions. When I was learning the ropes (sorry), I used a 3 metre kite on land. I am not a small chap. But that 3 metre kite picked me up as if I was hardly there and threw me around.
Power kites are quite hard to learn how to control properly. I think the leap in the technology here isn't the wind which, as you point out, has been done before, but the control systems to keep the kite in the air, stable and effective.