the rotors seem pretty small for giving enough power on a such freighter. So it's just a complement of power.
To give you an idea of the power produced by spinning those things at 200 rpm, multiply the area of each cylinders cross section ( silhouette ) by seven. This is the equivalent sail area they replace.
An article on the effectiveness on a much smaller boat fitted with one rotor provides some actual figures
http://www.rexresearch.com/flettner/flettner.htm
Popular Science January 1984
"Power Mode --- Ave. Wind (Knots) --- Ave. Boat Speed (knots) --- Ave Fuel Saving (%)
Rotor-Assist --- 16.1 --- 7.0 --- 44
Rotor-assist --- 12.9 --- 6.0 --- 27
Rotor Sailing --- 17.7 --- 5.3 --- 100
Under rotor power alone, the Tracker reached a maximum speed of 6.1 knots in an 18.4 knot wind and a true wind angle of 122 degrees.
Bergeson is demonstrating the Tracker to fishing-boat owners, talking to large shipping companies, and presenting scientific papers at maritime conferences. And interest is growing. He now has a Navy contract to study the conversion of a military sea-lift ship to rotor-assisted propulsion. He is also conducting similar studies for a number of independent shipping companies, including major oil and cruise-ship companies.
The economic potential certainly is there. Bergeson has calculated that the world’s shipping fleet consumes 730 million barrels of petroleum a year at a cost of $30 billion. If only 20 percent of the world’s fleet adopted sail assist, the savings would be on the order of 91 million barrels a year --- almost $3 billion.
The payback to an owner can be astonishingly quick. The entire rig for the Mini Lace cost $250,000. But the owner’s records show that the sailassit saves $48,000 worth of fuel a year. In addition, average speed is increased by 5 percent, which means that the ship can make more trips. Extra income from this source was $9200. At that rate, the rig would pay for itself in a little over 4 years. But there’s more. On the New Orleans-Jamaica route, where winds are usually unfavorable, the fuel savings was an incredible 36 percent, and the speed was up 18 percent. If the ship were used on similarly favorable routes, the payback would fall to an astonishing 1.7 years. "
