Wärtsilä’s new high-performance LJX waterjets outperformed promises during construction and sea trials.
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. LJX water jets in the size range from 1800 to 22,000 kW have been ordered.
The Australian engineering consultancy firm Sinclair Knight Merz Pty Ltd (SKM) was commissioned to undertake a comprehensive test programme to map various jet performance parameters throughout the trials.
Efficient & compact technology
The first Wärtsilä LJX 1500 size jets are installed on the largest, most fuel efficient diesel-powered high-speed catamaran in the world, the “Natchan Rera” built by Incat Australia in Hobart, Tasmania, (hull No.064) for Japanese owners. The 112m-long “Natchen 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 in an impressive three days with all systems performing perfectly and exceeding best 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 – an impressive three knots above the contracted requirement.
Robert Clifford, pioneer of the fast ferry industry and chairman of the Incat group of companies, stated “Wärtsilä has made a significant step forward with this new axial jet design. The delivered power is doing the job expected. The bonus is that more cavitation margin is available throughout the rev range, allowing more power to be used in manoeuvring. More available power means more side thrust and this is important. The weight and space savings are also of benefit. Good news for shipbuilders seeking high performance.”
Carl-Henrik Björk, Wärtsilä’s Vice-President for the cruise and ferry market also remarked “Wärtsilä made a very important step forward by investing in this new technology back in 2005. We were somewhat cautious in our performance predictions all round but were fully confident nonetheless that this new development would bring distinct advantages for the high speed vessel market. We are absolutely delighted with the findings of these test measurements as clearly our goals both for ourselves and our valued customer have clearly been exceeded all round.”
The 35% improvement in operating margins
Full-scale test results with the LJX water jets exceeded all expectations. 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 extraordinarily low 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 10% weight reduction
Once construction was completed, the weights of the units were accurately measured using certified and calibrated equipment. 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.
Sinclair Knight Merz test programme
The test programme undertaken by Sinclair Knight Merz comprised four major sets of tests covering jet performance; shaft torque, power and thrust; pump bowl pressure; and intake duct pressure. Measurements were undertaken during predominantly straight-line running in two sets of tests with four and three engines operating in the speed range of 500–1000 rpm. Various techniques were used for obtaining the accuracy of the measurements.
A thrust breakdown test was undertaken to determine the shaft speed at which jet performance drops at low inlet velocities owing to pump cavitation.
Measurements were also undertaken during a test with one jet operating and the speed incrementally raised until a drop in the power coefficient occurred.
A steering unit test was employed to determine the stress levels at specific locations on the steering post and arm as well as the pressures in the steering cylinders. Measurements were taken during high speed turning tests with starboard and port direction turns.
A manoeuvring test determined the bucket loads during reversing and forward operations.
Link to press release, 3 July 2006: New compact, high-performance waterjets from Wärtsilä
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Notes to the editor:
Wärtsilä enhances the business of its customers by providing them with complete lifecycle power solutions. When creating better and environmentally compatible technologies, Wärtsilä focuses on the marine and energy markets with products and solutions as well as services. Through innovative products and services, Wärtsilä sets out to be the most valued business partner of all its customers. This is achieved by the dedication of more than 15,000 professionals manning 130 Wärtsilä locations in close to 70 countries around the world.