The VLJ nozzles are optimised such that they generate high static thrust while at the same time providing a consistently high level of efficiency over the entire speed range from approx. 20 to more than 40 knots (illustration: Voith)
The 21-metre Trearddur Bay, a Crew Transport Vessel (CTV) owned by Welsh ship operator Turbine Transfers, has been transporting personnel, material and tools to offshore wind farms for slightly more than a year. She is the first CTV propelled by a Voith Linear Jet (VLJ) with the power being transmitted from two 900 kW diesel engines. The innovative Voith propulsion system is perfectly suited to the CTV requirements – namely high static thrust, high efficiency over the entire speed range, excellent manoeuvrability and low noise, vibration and emissions. Fast, safe, comfortable and efficient transport even in rough seas requires all these qualities to come together.
The VLJ combines the advantages of propellers with those of waterjets. The VLJ nozzles are optimised in order to generate high static thrust while at the same time providing a consistently high level of efficiency over the entire speed range from approx. 20 to over 40 knots. The VLJ is therefore ideally suited for ships with a mixed operating profile from slow cruising to traveling at higher speeds. Its high propulsion efficiency results in low fuel consumption while at the same time ensuring a high degree of operative flexibility. The efficiency of conventional propellers declines steadily at ship speeds in excess of approx. 25 knots when cavitation increases. At the same time, noise and vibration levels rise. Water jets by contrast are highly efficient for applications with speeds in excess of approx. 35 knots but plunge in efficiency at lower speeds.
The VLJ only has few movable parts made of high-quality materials making it robust and reducing maintenance requirements. The hydrodynamically optimised rotor encapsulated by the nozzle reduces noise and vibration emissions over the entire speed range – advantages that are frequently praised by the Trearddur Bay crew.
A CFD analysis tailored to each ship ensures optimum VLJ integration into the vessel hull. The result is high propulsion efficiency over the entire speed range combined with a shallow draft. At high speeds, the nozzle improves course stability. The stator aft of the rotor linearises the rudder inflow and enables safe and precise manoeuvring even in rough seas. Specific thrust allocation algorithms have been developed for manoeuvres at low speeds. They are adapted to the relevant vessel parameters and enable the integration of manoeuvring aids such as bow thrusters. The VLJ can also be combined with hybrid or electric propulsion systems instead of diesel engines, lending itself not only to workboats but to a wide variety of applications such as yachts, fast ferries or coastguard vessels.