Pump it up! Bigger and better uses for water-jet technology
Image credit: Company handout
Water jets are no longer used just for thrill-seeking jet-skiers. Now larger boats in a variety of applications are using the technology for propulsion – so what’s powering the trend?
The first thing that may come to mind when you think of water-jet propulsion is the jet skis that are popular with holiday-goers and adrenaline junkies. Traditionally the system has been used in small vessels such as these and the jetboats used by coastguards, the navy and others. Since 2000, however, this technology has increasingly been used by larger vessels such as military ships and ferries. But what makes water jets (also called pump jets) appeal more to operators in these fields than traditional propellers?
Marine propellers, acting in combination with rudders, have been the main way of propelling and positioning a vessel. But in the 1950s, New Zealander Sir William Hamilton developed a practical water jet, initially for use in shallow, fast-flowing rivers where propellers could hit the bottom. The technology is now widely used in a range of marine craft including patrol boats, surface effect ships, hydrofoils and motor yachts.
A water jet creates a propulsive thrust from the reaction created when water is forced in a backwards direction – the effect described in Newton’s Third Law of Motion: ‘every action has an equal and opposite reaction’. In a jetboat, the discharge of a high-velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft’s hull, thus propelling it forward.
In the HamiltonJet design, a ‘reversing bucket’ can be deployed to direct the outflow forwards, creating backwards thrust when required. Steering is achieved by changing the angle of the output nozzle.
Traditionally, water jets have been used in leisure craft, but Martyn Bowden, EMEA & North America sales manager of Finnish firm Alamarin-Jet, notes that “they are being increasingly used in higher value assets as well”. He cites ferries, frigates, submersibles, offshore construction and research vessels as examples
For instance, the US Navy uses water-jet propulsion systems in its Virginia-class fast-attack submarines, since they are quieter than traditional propeller-driven subs, according to the US Naval Institute.
The Red Funnel ferry company operates catamarans equipped with water jets on its cross-Solent passenger route between Southampton and the Isle of Wight. Each vessel has four MTU 2000 series diesel engines and four Hamilton water-jet units to give a service speed of 38 knots.
Another growing area of application for water jets is in marine dynamic positioning systems (DPS), which are used in the oil and offshore wind industries and elsewhere as an alternative to traditional anchors or jack-up barges, typically in deep water or where there are obstructions on the sea floor such as pipelines, cables and drilling risers.
A computer-controlled system is designed to maintain a ship’s position and heading by making use of the engines and thrusters. It achieves this with the help of sophisticated position reference systems (more advanced than those used for navigation), combined with wind sensors, motion sensors and gyrocompasses, which give information to the vessel’s computer relating to the boat’s position and the impact of environmental forces. Based on these inputs, the computer calculates the required steering angle and thruster output for each thruster.
DPS systems enable precision operations at sea. They can be used to keep a vessel’s absolute position at a fixed point on the globe, or relative to a moving object like another ship or an underwater vessel. They may also position the ship at a good angle towards the prevailing wind, waves and current – this is called weathervaning.
In a report published in 2017, Allied Market Research predicted that the total global market for marine dynamic positioning systems would increase from a 2015 value of $1,594m (£1,282m) to $2,127m (£1,711m) by 2022. Thruster systems accounted for 34 per cent of revenue at the start of the period and were expected to maintain their major share (above control systems and power systems) because of their higher cost and long replacement cycle span.
Naval architects, owners and operators are also making use of water jets, since they offer advantages over propellers for a range of vessels and tasks. For example, they enable better manoeuvrability, high acceleration and reduced vibration leading to less stress on engines and transmission systems. Moreover the lack of an exposed propeller makes them safer for use near swimmers or divers.
Another major operational benefit is the ability to operate in shallow water, because water jets do not need projections below the hull, which are vulnerable to fouling or damage from bottom impacts. This, as Alamarin-Jet’s Bowden notes, makes them “ideal for smaller military vessels”.
Another military advantage is the possibilty of low-noise configurations, which are offered by manufacturers such as Wartsila for a reduced sonar signature.
Water jets may be chosen where quick and easy positioning and manoeuvrability are required, such as for a tug guiding a super-tanker or for ferry docking.
In addition, they make it easy for a vessel to change location or weathervane to avoid the effects of bad weather. They are very safe when working above congested seabeds with many pipelines, mooring lines from other vessels or sub-sea structures such as wellheads or risers.
Also, they make it possible to quickly disconnect and sail away in an emergency.
Bowden says: “They are a simple technology, which is effective and efficient, with low mechanical wear and tear, plus ideal for vessels operating at speeds of between 25 to 50 knots.”
At speeds outside the 25-50-knot range, water jets are less efficient than traditional propeller solutions, largely because of distortion in the inlet flow, and usually water-jet installations are heavier than their equivalent propeller-based systems.
Furthermore, water jets cost more than propeller systems and incur higher fuel usage and maintenance costs.
However, we are already seeing further improvements in the pump-jet technology, to improve its efficiency and reduce cavitation problems and also to make it more useful for the increasing number of vessels adopting alternative fuel technologies such as batteries or liquid natural gas.
Principal water-jet design arrangements
There are three common types of design: axial flow, mixed flow and centrifugal flow.
In the case of the axial-flow design, the water jet’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 water jets are by far the most common type of pump.
Mixed-flow designs incorporate aspects of both axial-flow and centrifugal-flow pumps. They produce lower volumes of water at high velocity, making them suitable for smaller, faster craft.
Centrifugal-flow designs tend to be larger and heavier than axial types. They are no longer commonly used except on external outboard sterndrives.
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