Wind power is also being used as a supplemental power source for cargo shipping. The first commercial shipping voyage using a computer-controlled kite saved an estimated 10-15% fuel (up to $1500 USD per day).
Right. Kites are used to access higher-elevation winds which are more constant than those at surface level, though they're pretty complicated.
Another approach is modern sails integrated into masts and computer controlled, greatly reducing crew requirements and costs. The yacht Maltese Falcon is an example of this, employing the DynaRig (sometimes also "Dynaship") sail system. The rigging is said to have cost $80 million (the yacht's total cost is given as between $150 - 300 million), and the ship has a top speed of 20 kt, though it appears that that's under power from its diesel engines. I do find sources claiming that the ship can reach "nearly 20 knots" under sail power alone: http://yachtpals.com/maltese-falcon-7053
The DynaRig system was originally envisioned for cargo ships. I'm not aware of specific comparisons between DynaRig and kite technologies, though the latter should be far more amenable to retrofitting to existing vessels.
[1] http://www.nature.com/news/2008/080208/full/news.2008.564.ht...