The 109,000 HP Wärtsilä-Sulzer RTA96-C diesel engine is the largest and most powerful, but at 2,300 tons, any warship it's attached to is going to have trouble outmaneuvering jellyfish, much less torpedoes. Instead, the British Navy is relying on a new gas turbine engine that, while only half as powerful as the RTA96, weighs 68 times less.
Known as the MT30, this gas turbine engine has been built by Rolls Royce since 2004 and is based heavily on the Trent 800 aero engine, sharing 80 percent of its parts. While Trent 800s power Boeing 777s, the twin-spool MT30 has found use powering naval frigates, destroyers, and aircraft carriers. Since each engine can efficiently produce between 25 and 40 MW (34,000 to 54,0000 HP) of power and only weighs 24 tons when used in direct drive systems—84 if you include the optional base plate and alternator—it produces a much higher power-to-weight ratio than the lumbering Wärtsilä diesel, as well as every other engine in it's class.
The British Navy's new Queen Elizabeth-class frigates, the HMS Queen Elizabeth and HMS Prince of Wales, will each sport a pair of MT30s outputting 36 MW apiece which will combine to provide two-thirds of the ships' 109 MW energy needs. The engines will be tapped to drive weapons and navigation, the boats' 230 V power systems, and the massive propulsion motors needed to accelerate the 65,600-ton 40-aircraft carriers to their maximum speed of 25 knots. While neither the Queen Elzabeth or the Prince of Wales will be seaworthy until at least 2016, the MT30 can already be found on:
American Zumwalt-class destroyers,
Freedom-class littoral combat ships,
and South Korean Incheon-class frigates.
[Defence Talk - Wikipedia - Defense News - Rolls-Royce - Naval Technology - Image: Rolls-Royce, US Navy, and Naval Technology]