Warships with waterjets
New naval applications for waterjets
By Edward Lundquist
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High-powered waterjets are now being employed on warships such as the U.S. Navy’s Littoral Combat Ship (LCS).
While not a new form of propulsion, waterjets have not been used on larger ships until recently. 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. The 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 divert the jetstream forward. Precise station keeping can be maintained with waterjets. “At high speed a boat will have a much-reduced turning radius, while at slow speed the boat with twin-jets will turn on the spot or move directly sideways without the need for a bow thruster.”
Waterjets draw water from beneath a vessel. The water passes through an impeller, and is expelled as a high-pressure “jetstream” that delivers the thrust to propel the vessel.
The impeller increases the head that is then transformed into speed in the nozzle, providing the transformation of potential into kinetic energy, says Darren Savoye of Bollinger Shipyards. When it flows out of the nozzle it is close to atmospheric pressure. The jet flow is thus not expelled at high pressure, but at high speed. “The difference between inlet and nozzle speed is what gives the momentum,” Savoye says.
The advantages to waterjets are many, says Tony Kean of HamiltonJet, <?xml:namespace prefix = st1 ns = “urn:schemas-microsoft-com:office:smarttags” />Christchurch, New Zealand. “The most obvious is shallow draught – with no appendages (such as propellers, shafts and struts, or rudders) extending below the waterline, there is very little risk of damage to the propulsion gear from a grounding or striking a submerged object. This means boats can operate very close to the shoreline, land onto a beach for deployment of troops or equipment, or run over submerged logs or sandbars without damaging the propulsion equipment. The jet is also largely immune to problems from floating debris such as ropes, nets or weeds, particularly at high speed. At slow speed these may be sucked into the jet unit but are unlikely to cause damage and can easily be removed. This shallow draught feature results in reduced corrective maintenance requirements.”
According to Kean, manoeuvrability is enhanced because the waterjet is constantly pumping in one direction only, with manoeuvring controlled by directing the thrust forces as they exit the waterjet. “This means there is no gearbox to ‘clunk’ your way through and you have thrust forces available at all times – even when stationary. Reverse and turning thrust is very high and because the waterjet is constantly pumping in the same direction, transfer from forward to zero-speed to reverse is very quick.”
Waterjets are reliable. Like propeller driven ships, there is still a shaft, but it turns the pump impellor at a constant speed as opposed to a much larger screw. Drive shafts, gear boxes and engines receive less stress, prolonging their service lives. The entire propulsion system requires less maintenance.
Waterjets are more efficient at higher speeds, says Kean, particularly in multiple drive installations such as catamarans. With no underwater appendages, there is no increase in hull resistance as speed increases or more drives are added. “Efficiency is also provided over a wider speed range than with propellers. Because waterjets operate independently of the body of water under a boat, they cannot overload an engine if the boat is overweight, towing or battling heavy seas and weather.”
A fast vessel needs a relative higher amount of power than a slow vessel, says Marit Holmlund-Sund of Wärtsilä in Vaasa, Finland. “Waterjets can pack a relative large amount of power in small dimensions. With a conventional propulsor this would result in relative large propeller diameters.”
A clean hull design, free of appendages, delivers greater speed. “Since ship resistance accelerates fast in relation to ship speed the absence of appendages becomes increasingly important with the rise in ship speed,” says Holmlund-Sund.
Bollinger’s Savoye says that the waterjets do rely on the waterjet under the boat in the boundary layer that is determined by the wake fraction. “It is this effective use of the boundary layer of water to build up pressure in front of the impeller. The thrust is equal to <?xml:namespace prefix = v ns = “urn:schemas-microsoft-com:vml” /> and thus relys on Vi.”
(Note: pQ is the mass flow rate i.e. density (p) multiplied by volume (Q) and Vj is the jet outlet velocity and Vi is the inlet velocity.)
Waterjets eliminate the screws and rudders that make launch and recovery or small boats and unmanned vehicles more dangerous and difficult. “Waterjets offer less danger to people or marine life in the water – important during rescue, man-overboard, or diving situations,” Kean says.
Just as props can harm marine life, larger waterjets could potentially harm marine life if ingested and discharged through the jet.
Because they have no turning props, waterjets impart a very different acoustic signature, one that is less susceptible to sonar or acoustic mine detection. Lower noise and less vibration delivers a more quiet and comfortable ride for passengers. “Also because they operate above the waterline they can help reduce vessel wash and have less impact below the surface – an important factor in many inland waterways,” Kean says.
Cost can be an initial disadvantage of waterjets. They are not inexpensive to purchase or maintain. “Jets are manufactured from stainless steel that is a more expensive than the copper alloys normally used for other propulsors,” says Wärtsilä’s Holmlund-Sund. But waterjet lifecycle costs are relatively lower. Waterjets are less prone to impact damage and reduced engine stress results in less engine maintenance and longer engine life.
Waterjets are not impervious to clogging or fouling in shallow or debris-filled water, but there are various means to alleviate that problem, so that it isn’t problematic.
Not all waterjets are quiet, either. Performance sometimes comes at the expense of speed, says Dr. Saul Lanydo, president of Rolls-Royce Marine. Lanyado’s company is prime contractor for the U.S. Navy’s Advanced Electric Ship Demonstrator project featuring the Rolls-Royce AWJ-21 waterjet, which will operate four to five metres below the waterline to reduce noise and wake signatures. The concept also aims to reduce space and weight required for propulsion. The 40-metre AEDS, known as Sea Jet, is a quarter-scale ship, based on the DD(X) hullform, that operates on lake Pend’Oreille, Idaho. Sea Jet was built by Dakota Creek Industries of Anacortes, WA.
Sea Fighter, the HSV ships, and both LCS seaframes employ standard commercial waterjets that discharge above the water line, according to Erik Larsen of Rolls-Royce. “The AWJ-21 underwater discharge design is focused on reducing ship signatures, noise and wake, while at the same time delivering improved efficiency and manoeuvrability.”
Large waterjets for naval applications are currently made by Wartsila Lips of Cheasapeake VA and Wartsila Propulsion Drunen The Netherlands and Rolls-Royce Kamewa of Kristinehamn, Sweden. Gearboxes for waterjet applications are manufactured by Reintjes of Hamlen, Germany, as well as Renk, of Augsburg, Germany, and ZF Marine of Nottingham, UK.
A gearbox is not always required for waterjet propulsion, and this simplicity is a desired trait. However, in some cases where diesels and gas turbines are combined, or where power is taken from one engine and applied to another waterjet, some kind of coupling assembly is required. Clutch couplings from MAAG Gear, based at Winterthur, Switzerland, are used in such combinations.
The majority of surface combatants today have controllable-reversible pitch propellers, in a powertrain with a gearbox and either gas turbines, diesels or both. New designs are using the prime movers to develop electricity, then powering propellers or waterjets.
Littoral Combat Ships will employ waterjets
Two variants of LCS are being built. Lockheed Martin is building USS Freedom, a semi-planing monohull design, at Marinette Marine in Wisconsin. General Dynamics is building a trimaran at Austal USA in Mobile, Alabama. Both will have diesels and gas turbines, and both will employ waterjets. Both ships displace about 3,000 tonnes, with up to 4,000 tonnes fully loaded. This will make the two LCS combatants the largest naval waterjet-powered warships.
Some ships, like both LCS designs, use directional waterjets for steering and propulsion, with non-directional booster jets for additional speed when needed. In addition Wärtsilä-Lips also offers several special designs such as booster jets with a high crash stop installation installed on four MEKO Corvettes at 22mW and waterjets supplying thrust in all directions (360 degree) used in the US Navy Improved Navy Lighterage System (INLS) program.
While the two versions have taken a different naval architecture approach to the mission, both “seaframes” will carry mission modules that can be reconfigured to adapt to the ship’s combat mission assignment.
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.
The General Dynamics LCS has four steering and reversing waterjets, while the Lockheed Martin LCS has two steering and reversing and two booster jets.
USS Freedom (LCS 1) is powered by two Rolls-Royce MT30 36MW 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 115-metre Freedom has a steel hull with aluminum superstructure. Two Rolls-Royce MT30 36MW gas turbines and two Fairbanks Morse Colt-Pielstick 16PA6B STC diesel engines are the prime movers, powering four large Rolls-Royce Kamewa waterjets. Four Isotta Fraschini Model V1708 ship service diesel generator sets provide auxiliary power.
USS Independence (LCS 2) The slender stabilized trimaran monohull proposed by the General Dynamics team has an overall length of 127.8m, maximum beam of 28.4 metres and full load displacement of 2,637t. The seaframe is based on Austal's design for the Benchijigua Express passenger / 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 US Gallons 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 Azi Thruster
Experience drawn from high-speed ferries
Several large ferries have reliably employed waterjets to provide high-speed operations, and correspondingly more revenue generating trips than the slower ferries they replaced. Some of these ships are serving as prototypes for naval applications.
Australian shipbuilders Austal and Incat have both built high-speed catamarans that have been used as car and passenger ferries.
There are a number of fast ferries in service or building that employ waterjets. Their experience is helpful in matching the proper waterjet system for the naval requirement.
Austal built the 86-metre fast ferry H/F Villum Clausen, operating with Bornholmstrafikken in Scandinavia. The catamaran, powered by GE two LM2500 aeroderivative gas turbines and four Rolls-Royce Kamewa 112 SII waterjets, covered 1,060 nautical miles at an average speed of 44 knots, setting a world's distance record in 2000, according to Richard Williams of Austal, Henderson, Western Australia.
Austal also built the 127-metre diesel-powered trimaran auto ferry Benchijigua Express for Fred. Olsen, S.A., now providing service between the Canary Islands. Benchijigua Express is built to the same basic hull design as the General Dynamics USS Independence LCS design. The trimaran operates with four MTU 20V 8000 diesel engines, rated at 9,100kW. The pair of engines in the after engine room power a Rolls-Royce Kamewa 125 SII steerable waterjet. The pair in the forward engine room together power a Rolls-Royce Kamewa 180 BII booster waterjet.
Williams says Austal is currently the largest builder of large, high-speed waterjet powered aluminium vessels.
Australian shipbuilder Incat and its U.S. subsidiary Bollinger Shipyards, has built several high-speed waterjet vessels for the U.S. military. Based on successful catamaran ferry designs, Joint Venture (HSV-X1) was chartered to the US Army TACOM and operated by the Navy and then Army. Joint Venture displaces 1,740 tonnes fully loaded, is 96 meters long, and can achieve speeds up to 48 knots. The catamaran uses four Caterpillar 3618 marine diesel engines with four Lips LJ150D steerable waterjets. The Army liked the HSV concept so much, it chartered another wave piercing catamaran for the Theater Support Vessel Advanced Concept Technology Demonstrator (ACTD) role. Named Spearhead (TSV)-1X, while the Navy chartered Swift (HSV 2) to support the Mine Warfare Command and perform LCS experimentation. The 98-meter Spearhead is powered by four Ruston 20RK270 marine engines, driving four Lips LJ120E waterjets through Reintjes gearboxes. The 98-meter Swift is fitted with four Caterpillar 3618 high density diesel engines and four Wärtsilä-Lips water-jets that allow speeds in excess of 47 knots at lightship and 39 knots fully-loaded up to sea-state 3, with a range of 4400 nautical miles at 40 knots and 6500 nautical miles at 25 knots.
Austal made available by lease the 101-metre Westpac Express (HSV 4676) for intra-theater use by the III Marine Expeditionary Force in the Pacific theater of operations. Westpac Express has four Caterpillar 3618 diesels, rated at 7200kW each, and four Rolls-Royce Kamewa 125 SII waterjets. Westpac Express can operate at speeds up to 37 knots.
Italian shipbuilder Rodriquez Cantieri Navali, Messina, Sicily, has constructed an 82-metre monohull ferry, Aquastrada, that will be able to carry up to 1246 passengers with a maximum payload of 56 cars or 22 cars and 110 metres of truck lanes. The four MAN B&W Diesel Ltd 18VP185s (rated at 3700 kWb each), driving Lips waterjets through Reintjes gearboxes, will be located at the aft end of the aluminum ship, to optimize interior volume for vehicles. Fully loaded, the ferry will make 39 knots and the quadruple VP185 engines will drive Wärtsilä Lips LJ91E waterjets through Reintjes gearboxes. Waterjets permit the ferry to rotate 360 degrees around its centre; move laterally for mooring; and stop from full in less than four ship lengths.
Rodriquez Cantieri Navali also constructed the monohull Princess for Arab Bridge Maritime Company in Jordan. Princess employs four Wärtsilä Lips LJ91E waterjets, each linked to one 3920 kW diesel engine to reach speeds up to 41 knots.
Fincantieri's Riva Trigoso shipyard in Genoa built the 1,000-tonne MDV 3000 Jupiter-class Ro-Ro fast ferries Aries and Taurus, the biggest fast ferries in the world, for Italian state-owned operator Tirrenia. Four MTU 20V 1163 20V TB73 L units rated 6,500kW each and two GE LM 2500 systems rated at 22,000kW each are connected to the largest steering water jets ever built. This class has two gas turbine-driven booster waterjets and two diesel-shaft powered wing steering waterjets.
Combat applications for waterjets
The Swedish Visby-class multipurpose patrol combatant is a 620-tonne ship with a top speed of 34 knots. Visby employs a CODOG arrangement comprised of four Honeywell-Vericor TF50A gas turbines and a pair of MTU 16V 2000 N90 diesels, powering a pair of Rolls-Royce Kamewa 125 SII waterjets to deliver 21m460 shp. The Swedish navy also has waterjet experience with the 420-tonne (fully load) Goteborg class guided missile patrol craft, and other smaller craft.
South Africa’s Valour class frigates, built by Blohm + Voss to the MEKO A-200SAN design, employs a combination of screws and waterjets, called a ‘Waterjet and Refined Propeller’ or WARP® propulsion solution, with three shaft lines. A single GE LM2500 gas turbine (delivering 26,824 shp) is combined with a pair of MTU 16V1163 TB93 diesels, rated at 7,940 bhp each, powering a pair of Lips controllable-pitch propellers outboard and a single Lips LJ2 10E waterjet on the centerline. The first of four of these ships joined the South African fleet last year.
“The Wärtsilä-Lips LJ210E waterjet is the largest reversible waterjet ever built and is so far unique in its kind. The high speed crash stop installation of this jet reverses within 3 seconds after activation roughly 10,000 US gallons of seawater per second, enabling the 3500T vessel to crash-stop from the top speed within three ship’s lengths. The reversing installation can be fully engaged at the maximum output of the gas turbine by use of hydraulic accumulators giving instant power to activate the balanced crash stop-reversing bucket,” says Ruppert Dubbison of Wärtsilä.
Norway’s Skjold surface effect ship (SES) has a CODOG arrangement with two Rolls-Royce Allison 571-KF9 gas turbines, a pair of MTU 12V183 TE92 diesels, a pair of MTU 6R183 TE52 diesels for auxiliary power, and two Rolls-Royce Kamewa 80S2 waterjets. The 280-tonne Skjold can reach speeds of 55 knots. Norway has also built a class of SES minehunters and minesweepers built by Kvaerner Mandal. Larger than Skjold, at 470 tonnes, but much slower at 13 knots top speed, the Oksoy and Alta class of mine warfare ships have two MTU 12V 396 TE94 diesels ( 4080 hp ), a pair of Kvaerner Eureka waterjets ( rated at 4160 hp ), and two MTU 8V 396 TE54 diesels ( 1880 hp ) for auxilliary power.
Wärtsilä-Lips provided waterjets for the U.S. Navy’s Improved Navy Lighterage System (INLS) causeway and warping tugs to Marinette Marine Corporation. In this application, speed is not important. Quite the opposite. The warping tugs are required to move causeway sections or other non-propelled objects into position or to maintain precise position with a 24,000lb bollard pull requirement .
Whereas, the causeway ferries had a 10+ knot speed requirement along with station keeping, tracking and turning requirements says Dick Boardman of Wärtsilä. Both the warping tug and the causeway ferry waterjets were to be of the same design and power for both applications with 360-degree outlet nozzle which is a unique design feature when you think about waterjets. Other special requirements stated that no propulsor components could extend below the keel and stringent weight limitations were applied. For this application, Wärtsilä supplied 2 X LJ90DT waterjet thruster designs for each warping tug and causeway ferry powered by 822HP Cat diesel engines. The program was to include as many as 96 vessels. “The waterjets were the best solution to the requirement that involved shallow and beached operations where debris ingestion was likely,” Boardman says.
Waterjets are versatile. “Our jets are made for virtually any type of vessel application, including military,” says HamiltonJets’ Kean. “ We don't make any jet units specifically for military use. For example the same jet used in a high-speed tourist ferry is also installed in a pleasure cruiser and in a logistics support boat used by the UK navy. Regardless of the type of vessel it is weight, power input and required speed that determines the size of jet required.”
Captain Edward Lundquist, U.S. Navy (Retired) is a senior technical director and naval analyst for Anteon Corporation, Washington, D.C.