The Oil Drum: Australia/New Zealand

It is very nice that this sail will save something south of 30% in fuel costs on what appear to be otherwise a conventional ship.

A century ago, the largest "all sail" ships ever built is the "Preussen" which had 5 masts and was 438 ft. long with a displacement of 5,081 tons, or the "France", 418 ft. long and 5,600 tons displacement.

The largest all sail ship-of-the-line is the French Valmy which displaced 5,826 tons and had a length of 64 meters.

All of these ships used sails rigged from the deck. A kite system that makes use of stronger winds higher up should be able to pull a much larger ship entirely on sail, with the on-board engine plant only used when it is entering / leaving port and the prop feathered the rest of the time. Even better, let the prop rotate and use the energy to generate electricity to carry the ship's hotel load.

An engine that is only used in such a limited fashion can be made much more powerful for a given size than a typical marine engine that is expected to operate 24/7 for the life of the ship.

Why not go for 99% sail powered?

Many trade goods like coal are bulky, low value, and would hardly matter if it spent more time in transit of the fuel savings were large enough.

That is, of course, assuming that such a sailing ship can have a small, inexpensive crew like an oil fired ship.