After years of steady development, waterjet propulsion is on the brink of a new, electric era
This year, Tasmanian shipyard Incat will launch its next high-speed ferry, a 100-m-long wave-piercer for Trinidad and Tobago, cementing in the process what is surely one of the longest-running waterjet partnerships in the industry.
Although better-known for its engines, Wärtsilä has supplied no less than 178 waterjets for Incat-built vessels over a period of 30 years, almost since the shipyard started up in business.
Incat’s relationship with Wärtsilä has endured during a period of steady development in waterjet technology that has seen an increase in power and reliability, coupled with a decrease in weight. Incat’s latest vessel will be driven by four of Wärtsilä’s new-generation waterjets, the WXJ 1200 SrI series, and powered by four main engines, MAN’s 28/33D 16v units each rated at 7280kW.
The ferry’s propulsion will nudge 40 knots as it carries 1,000 passengers including crew and enough lorries to fill 175 lane metres, or 182 cars.
The increasing size and capacity of Incat’s vessels illustrates the progress made in waterjet and waterjet-related technology over the decades. At 1,000 dwt the Saint John Paul II, launched in late 2018 for Virtu Ferries, has 43% more truck capacity than the company’s existing vessel, 15% more passenger capacity, and 7% more car capacity. Accommodating 900 passengers and with room for 23 heavy commercial trailers, the vessel is the largest high-speed catamaran on the Mediterranean. Sailing between Malta and Sicily at a service speed of up to 38 knots, it is powered by four Wärtsilä LJX 1500SR waterjets configured for steering and reverse.
To smooth the vessel’s progress, Saint John Paul II is installed with a Naiad active ride-control system that combines active trim tabs aft and two hull-mounted T-foils. The four generators were supplied by MTU.
Further, in May 2019 Incat booked a contract for a 130 m flyer that will see service between Argentina and Uruguay for ferry operator Buquebus. Incat’s largest-ever catamaran and the world’s biggest aluminium ship, the ferry will weigh in at around 13,000 gt and carry 2,100 passengers and 220 cars at a maximum speed of over 40 knots. LNG-powered, the four dual-fuel engines will drive Wärtsilä waterjets.
Finite element modelling
As waterjet propulsion has advanced, Wärtsilä has found itself competing with several rival manufacturers. “Waterjet technology has largely plateaued over the past few years,” says Incat founder and chairman Robert Clifford. “The efficiency of all the large waterjets are good. For ships operating over 30 knots and up to 60 knots they are the most satisfactory solution.”
Working with local naval architects Revolution Design, Incat has developed craft with uniquely large carrying capacity. The latest generation of ferries boasts a payload almost equal to the vessel’s own weight that, claims Incat, has not been achieved by any other builder of high-speed light craft. A 110-m ship will typically boast a maximum deadweight of 1,000 tonnes, carry 1,200 people including crew and have a vehicle capacity of 595 truck lane metres, plus 210 cars.
The secret to this capacity is finite element modelling, which, among other things, helps optimise global and local scantlings with the purpose of maximising the efficiency and weight of the ship without compromising strength.
Revolution Design applies its own models that run a very accurate analyses of loads in high-stress areas. The result is known as a sea frame – or base vessel – rather like the air frame in the aerospace industry. In short, it is the essential structure, exclusive of whatever fittings the owner wants for the vessel’s ultimate purpose, whether it be ro-pax, pure passenger, freight, offshore or military.
Typically, the waterjets are driven by diesel or dual-fuel engines or, in the case of higher-speed catamarans, dual-fuel gas turbines. A 112-m vessel would be powered by four diesel engines delivering 36MW at a cruising speed of 20 knots under heavy load and 50 knots when light. With a headroom of generally 4.5 m on the vehicle deck and a beam of 30.2 m, heavy road vehicles can easily turn around in the bow and disembark over stern ramps.
The next revolution in waterjets will likely take the guise of hybrid electric propulsion. Some manufacturers are already under pressure from customers for battery-powered boosts to top speed, but also for better low-speed manoeuvring, an important requirement in offshore workboats.
Incat is now exploring the potential for electric propulsion in its big ferries and Mr Clifford believes batteries will find their niche for short duration, moderate-speed routes, for example 20 knots on one-hour journeys. “Higher speeds will require a lot more power and that may not be sustainable with currently available technology,” he says. “Longer routes will depend on the availability of improved batteries; however, that technology is rapidly developing.”
And in line with current developments in Norway, America, France and elsewhere, Mr Clifford sees a possible future for hydrogen power, at least in the longer term. “Hydrogen fuel cells are being studied, but the handling of the fuel at high pressure and the risk will not make hydrogen ships popular in the short term,” he says.
Melbourne-headquartered Doen Waterjets has already developed hybrid-ready units and points out that electric technology will require work on the impellers because they have a fixed relationship between power and rpm. “The trick is configuring it to work efficiently with both the electric motor and the design engine,” points out director of engineering Tim Udvary.
Interest however is growing. “We have had a lot of enquiries, but the majority had gaps in them in terms of what is technically and commercially viable”, says Doen Waterjets international sales manager Bob Khosravi. “Right now, that is dictated mainly by the battery technology.”
A modular approach
In late 2019 Wärtsilä unveiled the WJX series of modular waterjets based on a new axial pump. “[The design] boosts performance with an increased thrust of as much as 3%, while the improved cavitation margins help reduce the environmental impact by lowering noise levels,” explains Wärtsilä general manager of waterjets Leendert Muilwijk.
Wärtsilä describes an axial waterjet as a single-stage, high-performance system that combines mixed-flow properties with an axial construction. The benefits include more space at the vessel’s transom and increased waterjet cavitation margins.
Cavitation is essentially the formation of bubbles – vapour-filled cavities – in places of lower pressure in the water, for instance around impellers.
Naval architects reputedly like axial technology because it means they can pack more power onto a narrower hull, which is inherently faster than a fat-beamed hull.
Why a waterjet?
The waterjet has many advantages over a propeller. The very high efficiency of the waterjet pumps offers higher speeds for the same power, or substantially lower fuel consumption at a constant speed with less power. And power is what it is all about when it comes to the defence of territorial waters, a mission that often requires high-speed chases and tight manoeuvring close to coastlines. Here waterjets come into their own for navies and coastguards which constantly operate in shallow, rock-strewn waters.
Waterjets also give the engine an easier life. At constant rpm, waterjets absorb approximately the same power regardless of the vessel’s speed, so the engine cannot be overloaded, increasing its lifetime.
And finally, waterjets operate with less vibration and noise, improving passenger comfort levels. At speeds over 20 knots noise and vibration can be reduced by more than 50% compared with propeller-driven vessels. And in ferries, that means a lot to passengers.
HamiltonJet makes waves
In late 2019 New Zealand’s HamiltonJet unveiled the HTX52 waterjet and almost immediately scored its first sale. Married to a Caterpillar C32 1450 bhp engine – the biggest in the C32 range – two each of the waterjets will be installed on two 25-m monohull crewboats seating up to 50 passengers. Built by Sam Aluminium in Singapore, the vessels will be deployed off the coast of Africa.
HamiltonJet says the HTX series is more powerful, robust and easier to install than its predecessor. With a maximum power of 1950 kw, top speeds are 4% higher, up to 55 knots. There is a 20% increase in peak bollard pull.
HamiltonJet has long pursued higher top speeds. When the smaller HTX30 was installed in an offshore workboat for the Norwegian Coastal Authority, the expectation was that the single waterjet would hit speeds of 30 knots at 600 bhp; instead, the vessel achieved 36 knots at 600 bhp and went on to 38 knots at 650 bhp.
Because the hydraulics are fully integrated, the new system takes up less inboard space. The compact size means the HTX systems can be installed through the transom late in the building process, which helps reduce costs.
Caterpillar’s engine comes with a compact closed-loop scrubbing system that works by injecting urea into the exhaust stream. A catalyst then converts nitrogen oxide into diatomic nitrogen carbon dioxide and water vapour. “[The technology] minimises total cost of ownership by reducing the diesel and urea expenses to offer the lowest total fluid consumption,” explains Michael Easter, application engineer for Caterpillar Marine. And because the engine uses a turbocharged air system that boosts back-pressure capability, the scrubber can be smaller, an advantage of vital importance in smaller vessels.