Thursday, August 13, 2009

USS Freedom lcs1 waterjet propulsion

USS Freedom (LCS 1) is the first US Navy Littoral Combat Ship in an entirely new class of Navy surface warships. The ship is designed for littoral, or close-to-shore, operations and to provide access and dominance in coastal-water areas.

The Littoral Combat Ships (LCS) will employ waterjets.waterjets have not been used on larger ships until recently. There are many advantages of waterjets. The most prominent advantage is the shallow draft of the system. Waterjets do not have appendages (such as propellers, shafts and struts, or rud- ders) that extend below the waterline. This minimizes the risk of damaging the propulsion gear from grounding or from hitting a submerged object, and it also reduces the maintenance require- ments. As a result the boats can operate close to the shoreline, land on a beach for deployment of troops or equipment, or even run over submerged logs or sandbars without damaging the propulsion equipment.They present some clear advantages for warships. Waterjets deliver rapid acceleration and can sustain high speeds. Waterjet-powered ships are extremely maneuverable and can stop quickly. They offer simplicity. The flow is constant in a single direction. Engine loading is constant, regardless of vessel speed, and waterjets do not overload the engines. There may be no need for a gearbox. Astern propulsion is applied by means of deflectors that TDert the jetstream for- ward. Precise station keeping can be maintained with waterjets. Waterjets were chosen for LCS to provide high speeds in shallow waters, where the LCS will operate to combat asymmetric anti- access threats in the littoral regions of the world. Two variants of LCS are being built. Lockheed Martin has delivered the USS Freedom, a semi-planing monohull design built at Marinette Marine in Wisconsin. General Dynamics is building a trimaran, the USS Independence, at Austal USA in Mobile, Alabama. Both will have diesels and gas turbines, and both will employ waterjets. The General Dynamics LCS has four steering and reversing waterjets, while the Lockheed Martin LCS has two steering and reversing and two booster jets. Both ships displace about 3,000 tons and up to 4,000 tons fully loaded. This will make the two LCS combatants the largest naval water- jet-powered warships. While the two versions have taken different naval architectur- al approaches to the mission, both “seaframes” will carry mission modules that can be reconfigured to adapt to each ship’s combat mission assignment. USS Freedom is powered by two Rolls-Royce MT30 36 MW gas turbines and two Fairbanks Morse Colt-Pielstick 16PA6B STC diesels. The seaframe is based on the Fincantieri-built, Donald Blount-designed high-speed yacht Destriero, which holds the record for the fastest transatlantic crossing (60 knots). The 378-foot Freedom has a steel hull with aluminum super- structure. The two 36 MW gas turbines and two diesel engines power four large Rolls-Royce Kamewa waterjets. Four Isotta Fraschini Model V1708 ship service diesel generator sets provide auxiliary power. USS Independence, the slender stabilized trimaran monohull built by the General Dynamics team, has an overall length of 418 feet, maximum beam of 93 feet, and full load displacement of 2,637 tons. The seaframe is based on Austal’s design for the Benchijigua Express passenger and car ferry. Two General Electric LM2500 22 MW gas turbines and two MTU 20V8000M90 9100 kW diesel engines are the prime movers, powering four large steering and reversing Wärtsilä-Lips 2 X LJ160E and 2 X LJ150E waterjets. With all propulsion flat out, the Wärtsilä-Lips waterjets together expel roughly 27,000 gal- lons of seawater per second exiting from the jet nozzles at a speed around 90 mph. The trimaran variant built by General Dynamics will also have a retractable azimuth thruster. CONCLUSION One design is not optimum for all situations. Cruise ships with large portions of their itineraries at low power benefit from elec- tric drive. Fast ferries, which go to full throttle as soon as they clear the breakwater and remain at full throttle until they reach the next port, would be at a disadvantage with electric drive. There are advantages to a mechanical drive system. Mechanical drive systems are more efficient compared to electric drive sys- tems in terms of their ability to transmit energy from the prime mover to the propulsor.


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