Towing exercises are a Coast Guard staple, but how about towing the largest ship in the world. The French recently did it successfully, and gCaptain has the photographs. That little tow boat is 80 meters (262 feet). The towed vessel is 396 meters (1299 feet) and 250,000 tons.
Reportedly the towing vessel Abeille Bourbon was able to tow her at 10kph (about 5 knots) using only half power.
Chuck Hill's CG Blog 
Calm weather and having a automatic towing machine with a long tow line probably really helped.
Engine power is not the big problem, its having a towing rig which can handle any shock caused by two ships, one much larger then the other moving in even slightly different directions. A mismatch between the two vessels which the towing rig can’t compensate for and by by tow line and rig
Thanks for the reply. It does show how little power is required to move even a very large ship at slow speed.
FYI this is the largest container ship in the world, and the largest high sea tug boat (if that makes sense… it is its name in French) in Europe.