Parking a 173m guided missile cruiser is tougher than it sounds. So when the US Army's largest ocean-going vessels need a helping hand squeezing into foreign ports, they call on the biggest and baddest tugboats in the armed forces: the MGen. Nathanael Greene-class large coastal tugs.
There are currently six Nathanael Greene's currently serving with the US Army, all of which have been since 1994. They measure 39m long with a 5m draught and displace 713 tonnes. Each is powered by a pair of 2550 bhp engines which generate a bollard pull of 53 tonnes (bollard pull is the nautical equivalent of horsepower). That power's on par with the largest commercial tugboats available today -- and about 14 tonnes more than what the average tug is rated to pull.
But bollard pull doesn't really illustrate what the Nathanael Greene-class is capable of -- no more than what horsepower rating on your truck suggests about its towing capacity. "The large tug is capable of towing up to five 120-foot (37m) barges loaded to their maximum weight of 733 long tons (745 tonnes) per barge," Chief Warrant Officer Darren Reese explains. "Or a 567 foot-long (173m) guided missile cruiser with a displacement 9600 long tons (9754 tonnes)."
As such, these tugs (and their 24 crew members) are tasked with a variety of oceanic operations, according to the Army Watercraft Master Plan (pdf), including:
...a capability for coastal and ocean towing, docking and undocking operations of large ocean vessels. It also provides trans-oceanic transport of the Barge Derrick 115 Ton (BD 115T). It can recover disabled or damaged Army watercraft assets to include the Logistics Support Vessel. It also supports salvage operations for port clearance during denied and forced entry operations. General purpose duties to include fire-fighting, barge transport/repositioning and salvage capability. Two of the LT 800's special features are the ability to tow five medium barges and can sustain a minimum speed of five knots in Sea State 4 conditions under full tow with the BD 115T.
It can tow "any vessel in the Army inventory," Stars and Stripes declared. "And can even pull an aircraft carrier." And with a service range of more than 5000 nautical miles, the Nathanael Greene sextuplets can do so virtually anywhere in the world. [US Army - Global Sec - Stars and Stripes - Wiki 1, 2 - Hazy Grey - Google Books]