3 minutes ago, J.C.MacSwell said:

They spin an upright cylinder such that the boundary layer effectively creates a very inefficient airfoil shape. You don't need waste energy to do that...just put up a (hopefully more effectively shaped) sail.

Yes, this ship seems more of an advertising gimmick than a real commercial investment. It's used to carry wind farm turbines and you have to say it's an eye-catcher, so it will generate plenty of attention. But as a practical commercial venture, they haven't taken off. You need a side-wind for best thrust, so the ship will need to tack, from side to side to sail into the wind, which will add to time lost and diesel used. Also, when you do tack, you have to stop the rotors, and drive them in the reverse direction, otherwise they will give a backwards thrust. 

Interestingly, the original marine diesel generators rated at 3.5 MW had to be changed for 6.6 MW ones, due to "technical difficulties" after 3 years. 

https://en.wikipedia.org/wiki/E-Ship_1    

11 hours ago, J.C.MacSwell said:

4 times the energy per volume of air, but the flow is double, so as you suggest it's a cube relation.

 

Yes of course, thanks, that makes sense. So it's a cubic relation. That would make the energy harvesting potential extremely strongly dependent on the wind strength that the device experiences in practice. So one would need to read the claims of power output very carefully with that in mind.