In mid 2006 Wärtsilä Corporation introduced the new, efficient and compact LJX axial water jets for the propulsion of fast vessels with promises of a 25% reduction in transom mounting flange diameter, a 10% overall reduction in weight and a 35% improvement in cavitation margin compared with other waterjets then on the market. These benchmarks were met when the first LJX waterjet units were put through full-scale trials in July 2007. These were LJX 1500 size units of 9000 kW capacity.
The first Wärtsilä LJX 1500 size jets are installed on the diesel-powered high-speed catamaran the "Natchan Rera" built by Incat Australia in Hobart, Tasmania. The 112m-long "Natchan Rera" sails at loaded speeds of about 40 knots and has a capacity to carry 355 cars or 450 lane metres of trucks and 193 cars. Sea trials for "Natchan Rera" were completed exceeding expectations. During the course of the trials while running at 100% MCR (maximum continuous rating), speeds of up to 45.4 knots were averaged with 250 tonnes deadweight on board. Loaded with 600 tonnes deadweight an average speed of 41.9 knots was achieved at 100% MCR - three knots above the contracted requirement.
The available operating margins given by the cavitation behaviour of the pump are crucial for giving the vessel good manoeuvring and acceleration performance with flexible operating capabilities. This is especially valuable as it enables the vessel to continue operation with sustained high power outputs whilst operating with a reduced number of engines and associated water jets. Owing to the increased cavitation margin and extended operational zones, the full power of an engine can be quickly absorbed by the water jet and converted to thrust immediately, well within the new cavitation boundary and far away from thrust breakdown. The result is a smooth and rapid acceleration of the vessel. The increased cavitation margin not only makes for improved acceleration but also better manoeuvring capability at speeds of less than 10 knots.
During the trials it was observed that the jets emitted lower noise levels compared to other jets currently in operation. The reason for this is again the improved cavitation capacity of the pump resulting in both low radiated noise in open water and structural noise in the propulsion train. Clearly for commercial vessels this makes for a smoother ride for the passengers. Other special applications requiring very low noise signatures, particularly at low speeds, will now also benefit.
The compact design of the jet impeller, allowing an optimal shape of the stator bowl in combination with efficient mechanical engineering, proved successful for maximum weight reduction. The predicted 10% weight reduction was exceeded making the Wärtsilä LJX water jets now on average 14.2% lighter than previous designs.
