All those large diesel engines are run on the test bed before they get installed in the ship. They're tested at 100% for a certain period, then at 110% of rated load for another period of time. The full output is developed into a dynamometer...a device that absorbs the full output, measure the torque, and from that plus the rpm the horsepower is calculated. Needless to say, cooling the dyno is a major consideration...the dyno is hydraulic (not friction) with a way to measure the torque level.
Think of the sound of a 100+,000 hp diesel engine inside a building--four turbochargers screaming, the engine running so hard that you can see each individual cylinder head lift as it fires and the tie rods stretch!
Here's some interesting data from one of the makers of these engines, Mitsubishi...the large 12 they make weighs 2030 tons and is 23 meters long and 13 meters high. One model, RTA84T-D, has a top speed of 70 rpm and a ten foot stroke.
http://www.sdia.or.jp/mhikobe-e/prod.../sulineup.html
Ken