There are more effective ways of making a ship more efficient, like slowing down.
The way the shipping
industry works is that those with the faster ships and those that can keep a schedule get paid more money
. A sailing ship could do neither very well and still compete.
There are also economies of scale and ships are as big as many ports
can accommodate their deep drafts and bridge clearances.
Also, the longer a ship takes to cross, the more the company pays out in wages and depreciation costs for a given amount of cargo carried. A quicker run means more cargo can be carried in a given amount of time which means fixed costs are lower relative to tonnage carried. Time is money
Kites have been flown from the decks of ships but on a percentage basis compared to overall costs, the cost savings is very minimal.
The faster a ship travels, the more aft the apparent wind
shifts, which means for a fast container ship the time when the wind is aft of the beam is very minimal compared to a relatively slow tanker. The faster the ship needs to be, the less the idea works.
I'm all for the idea and sailing ships would be great if time and meeting a schedule did not mean money.