Another recent arrival in the offshore sector is the Voith Linear Jet (VLJ) propulsion system. Successful tests on the VLJ were carried out off the Isle of Wight in the UK at the end of 2014. VLJs were selected as the main propulsion system on the 21m catamaran Trearddur Bay, which was designed for Holyhead Towing Company subsidiary Turbine Transfers by BMT Nigel Gee.
The VLJ is a new type of propulsor combining the best properties of conventional propellers with the best properties of conventional waterjets. At first sight, it looks to be a conventional propeller housed inside a nozzle, but it actually comprises a fully submerged custom-shaped deceleration/acceleration nozzle with a stator section aft of the rotor. The stator section cancels rotor-induced swirl and through that optimises the acceleration of the jet stream and the rudder inflow. The submerged position eliminates the requirement for a long inlet tunnel resulting in linear inflow and outflow and low marine growth sensitivity. The resulting benefits of this are increased efficiency and reduced noise and vibration levels.
The crew transfer vessel the system was tested on will take service technicians to offshore windfarms at sites all around Europe, initially at the Westernmost Rough site off the Humber for Dong Energy. The craft has achieved a trials speed of 30 knots – above expectations when compared to 26.5 knots for a near sister with jet propulsion. The efficiency of the VLJ would allow the craft to achieve the economical service speed of 25 knots required by Turbine Transfers even if the currently installed 10-cylinder 900kW engine was replaced with an 8-cylinder 720kW unit.
“The vessel is achieving more thrust at high speeds and when stopped in the water and pushing on a turbine than with conventional systems while achieving significantly lower noise and vibration levels,” said Alistair Knowles, a marine superintendent at Turbine Transfers. “It also adds to our green credentials through substantial fuel and emissions savings in our operations.”
VLJs are expected to be of interest in the workboat sector, the fast ferry and military and para-military markets. They are also expected to have potential applications on yachts and superyachts. In broad terms, the new propulsion system’s key benefits are lower fuel burn than vessels with waterjets or propellers. The high efficiency of the VLJ over a wide speed range means that, at 25 knots – where waterjets are not particularly efficient – the VLJ is significantly better. For vessels with fixed pitch propellers, power density becomes an issue above 30 knots, whereas the VLJ can be used for speeds of up to 40 knots without any such issues.
Other recent additions to the growing range and type of thrusters include the Verhaar Omega V-POD, an electric propulsion/manoeuvring system that can replace a conventional screw shaftline with rudder. The V-POD is a pod drive with electric motor in an underwater housing. It can be used in combination with a diesel-electric and LNG-electric generator set and is said to have a high level of propulsion efficiency. The company says it overcomes some of the disadvantages of existing pod drives, such as high costs and low efficiency. It is also designed to be easy to install and occupies a minimum of space. Depending on the application, it is available as a pushing or pulling design and can be supplied with or without a nozzle.
Wärtsilä recently passed the final step in securing type approval from Lloyd’s Register for its new WTT11 tunnel thruster. The WTT11 thrusters are electrically driven and can be delivered with a controllable pitch or fixed pitch propeller. Late last year, the company secured approval from classification society DNV GL for the design of its WST-14 thruster, which is part of the Wärtsilä steerable thruster (WST) compact series aimed primarily at tugs, anchor-handling vessels and coastal and inland waterway cargo vessels. The key benefits of the series include superior performance, easy installation, a high level of integration and ice class compatibility.
Article published in the Guide to OSV Propulsion, a supplement to Offshore Support Journal
