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waterjets Wärtsilä LJX1720SR axial
waterjets drive the ferry Francisco at speeds up to 58 knots
Jets tailor thrust for niche markets
W
aterjets are based on a pump, which can be grouped from pure axial designs (delivering a high flow at a low pressure) to pure radial designs (generating a low flow at a high pressure). For a high thrust output, a waterjet needs to generate both a high flow through the jet system and a high pressure, dictating a pump with mixed- flow properties.
A unique feature claimed for the Wärtsilä
axial flow waterjet is that it delivers the mixed flow properties
required but in a
pure axial geometry. A significant advantage results as the water follows the optimum flow path straight through the pump instead of partly travelling in a radial direction before exiting at the nozzle.
Axial waterjets primarily target applications with vessel maximum speeds up to 50-55 knots, Wärtsilä explained; above that level axial jets should not be used and a more radial-shaped mixed-flow pump adopted to give better results. For such extremely high speed applications, the group offers its E-series waterjets based on a non-axial pump, which address
72 I Marine Propulsion I April/May 2014
Across the board developments maintain the attraction of waterjets for applications ranging from wind farm support vessels to warships
by Doug Woodyard
applications calling for very high power densities and vessel design speeds up to 70 knots.
Among the merits cited for axial jets are compactness, high efficiency, low weight, wider cavitation margin, higher shaft speed reducing torque, and low forces transferred to the vessel structure. The shaft speed depends on the jet size and the power applied; a small jet at high power can run at 2,000 rpm, a large jet at 200 rpm.
Wärtsilä’s axial pump design is available as a pre-assembled unit for smaller jets up to around 4,500kW; the inlet duct is included in the kit and the unit is delivered on a skid with all auxiliary systems pre-mounted. Larger jets extend to units rated up to 26MW. The US Navy’s Joint High Speed Vessel and
Littoral Combat Ship programmes are among notable Wärtsilä projects.
Last year saw Wärtsilä’s axial waterjet references
extended by a prestigious
installation driving the world’s fastest high speed ferry. The 99m-long catamaran Francisco, built by Incat Tasmania for the South American operator Buquebus with capacity for 1,000 passengers and 150 cars, is deployed between Uruguay and Argentina. A lightship
achieved on trials by the twin GE LM2500 gas turbine-driven Wärtsilä LJX1720SR axial waterjets, although a service speed of 50 knots is adopted for the River Plate crossing. The 22MW turbines are arranged to burn marine diesel or LNG. An impressive power input can be absorbed by the relatively small waterjets, whose compact dimensions allowed installation within the ferry’s transom, saving valuable space.
waterjets in
Effective and reliable control of the manoeuvring the vessel
is assigned to Wärtsilä’s Lipstronic 7000 propulsion control system. The system both controls and indicates the steering angle, bucket position and impeller speed, and can
www.mpropulsion.com speed of 58.1 knots was
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