An increasing number of ships are using water jets instead of propellers for moving vessels through water. Water jets produce a jet of water for thrust. The mechanical design may be a ducted propeller (axial-flow pump), a centrifugal pump, or a mixed flow pump which is a mix of both centrifugal and axial designs. The design also embodies an intake to give water to the pump and a nozzle to direct the flow of water out of the pump.
Such an arrangement of propulsion was once limited to high-speed pleasure craft and other small vessels. But, since 2000, the technology has increasingly been used by high-speed larger vessels, such as military ships and ferries. Such larger craft, can make use of diesel engines or gas turbines and can have an operational speed of up to 40 knots depending on the hull configuration. Amongst the leading suppliers of such water jet propulsion systems solutions include Hamilton Jet, Kongsberg Maritime, Rolls Royce, Wärtsilä, etc.
Water jet propulsion is useful for:
- High-speed vessels. Waterjets have better propulsion efficiency at ship speeds above 25 knots. Due to escalating fuel costs, operators are choosing waterjets that have been optimised for lower speed and fuel consumption whilst improving vessel operation, even covering speeds below 30 knots.
- Shallow draft vessels. With no underwater appendages and small radial dimensions of the impeller, axial waterjets are well suited for rescue vessels, inshore passenger ferries, landing crafts and special workboats.
- High power density. Because of the pressure build-up in the waterjet inlet, the physical dimensions are minimised.
- High level of manoeuvrability. Waterjets have integrated steering and reversing capabilities with fast response.
Types of water jet design
A water jet works by having an intake (at the underside of the hull) that allows water to pass underneath the vessel into the engines. Water enters the pump through this inlet. The pump can be of a centrifugal design for high speeds, or an axial flow pump for low to medium speeds. The water pressures inside the inlet is increased by the pump and forced backwards through a nozzle. With a reversing bucket, reverse thrust can also be achieved for faring backwards, quickly and without the need to change gear or adjust engine thrust. The reversing bucket can also be used to help slow the ship down when braking. This feature is the main reason water jets are so manoeuvrable.
The nozzle also provides the steering of the water-jets. Plates, similar to rudders, can be attached to the nozzle to redirect the water flow port and starboard. This provides water jet-powered ships with superior agility at sea. Another advantage is that when faring backwards by using the reversing bucket, steering is not inverted, as opposed to propeller-powered ships.
An axial-flow waterjet’s pressure is increased by diffusing the flow as it passes through the impeller blades and stator vanes. The pump nozzle then converts this pressure energy into velocity, thus producing thrust.
Axial-flow waterjets produce high volumes at lower velocity, making them well suited to larger low to medium speed craft, the exception being personal water craft, where the high water volumes create tremendous thrust, acceleration and high top speeds. But these craft also have high power-to-weight ratios compared to most vessels. Axial-flow waterjets are by far the most prevalent type of pump.
Mixed-flow waterjet designs incorporate aspects of both axial flow and centrifugal flow pumps. Pressure is developed by both diffusion and radial outflow.
Mixed flow designs produce lower volumes of water at high velocity making them suited for small to moderate craft sizes and higher speeds. Common uses include high speed pleasure craft and waterjets for shallow water river racing.
Centrifugal-flow waterjet designs make use of radial flow to create water pressure. Centrifugal designs are not commonly used anymore except on outboard stern drives.
The benefits of using water jets
Water jet propulsion systems offer many advantages over traditional propeller propulsion, these comprise:
- Being able to deliver higher speeds for the same amount of power.
- Offering lower fuel consumption at a constant speed with less power.
- At constant revolutions per minute, waterjets absorb about the same power regardless of the vessel’s speed, so the engine cannot be overloaded, giving it a longer lifetime.
- Also, waterjets produce less vibration and noise, improving passenger comfort levels. At speeds over 20 knots noise and vibration can be reduced by over 50%.
- Waterjets for naval operations can be built with special requirements concerning signature, shock and other higher requirements.
- Excellent manoeuvrability, allowing boats to be berthed quickly and with high precision.
Examples of vessels using a water jet propulsion
Wärtsilä’s waterjets are being used on Incat’s Francisco, which is the world’s fastest ferry. This vessel links the ports of Buenos Aires and Montevideo. This wave-piercing catamaran achieved 58.1 knots on trials and is powered by two GE LM2500 gas turbines driving through two Wärtsilä LJX 1720 SR axial flow waterjets. Wärtsilä’s axial waterjet series comprises six-bladed stainless steel designs from 510 to 2020 in size, covering the range from 1,000kW up to 27,000kW.
Rolls-Royce has been picked to produce propulsion to two new 40-meter Italian Navy Special Forces crafts. The two vessels, known by the acronym UNPAV, which are eight meters wide, will be powered by three MTU 2000 series diesel engines connecting to three Kamewa S4 water jets, the firm said. The vessels will be used for maritime traffic control, to combat human trafficking, for counter-terror and anti-piracy operations and for evacuations, the firm added.