Sails would be good but have a few disadvantages:

You have to hang them on tall masts, which interfere with cranes when unloading, stop you going into harbours with bridges, etc.

You need a lot of crew to handle them, and they are dangerous objects (lot of energy goes into flapping them at the wrong time).

Lot of wear and tear (literally).

The wind is much slower and more turbulent near sea level than higher up.

Alternative under development is to deploy kites which go higher and just have a single atttachment point.

Check out http://news.bbc.co.uk/1/hi/world/europe/7205217.stm