Driveline of the VLJ. Click to enlarge.

Voith suggests that the optimal application envelope for the VLJ is between 25 and 40 knots. Within this range, neither a propeller nor a traditional waterjet provides a 100% satisfying solution, Voith says. The efficiency of propellers starts a steady decline at higher ship speeds when cavitation, noise and vibration levels all increase.

Traditional waterjets only become efficient at speeds above 35 knots, and plunge in efficiency for speeds below 30 knots. By contrast, the VLJ delivers a constant high efficiency up to 40 knots. Outside this speed range other features like the reduced draft versus a propeller or a higher bollard pull can also make the Voith Linear Jet the preferred option.

For each sway, surge or yaw motion of the ship, there is only one optimum power setting of main engines, rudders and (bow) thruster; every other power setting will result in unnecessary fuel burn, time lost in port, increased harbor disturbance or in anything else but the desired ship movement, Voith notes.

The thrust allocation algorithms developed for the VLJ deliver the excellent maneuvering properties common for jet boats. The thrust allocation algorithms are optimized for specific vessel parameters such as wind and water surface area; rudder surface and profile; gearbox properties and for the presence of auxiliary propulsion systems such as (bow) thrusters. The master can control the ship with a single hand, pointing the joystick in the direction the ship has to go. Manual control—bypassing the joystick—is possible to always keep the master in control.

The propulsion system performed convincingly all along the line in its sea testing and surpassed the expectations of the vessel owner, the Welsh company Turbine Transfers Ltd, a subsidiary of workboat operators Holyhead Towing Company.

Crew transport vessel equipped with two VLJs. Click to enlarge.

The VLJs are the main propulsion system of the modern DNV-Classed 21m offshore service catamaran, which was designed for Turbine Transfers by BMT Nigel Gee and built by AMC. The Crew Transport Vessel will take service technicians to offshore wind farms at sites all around Europe, initially at the Westernmost Rough site off the Humber for Dong Energy. The CTV has achieved a trials speed of 30 knots, above expectations, compared to 26.5 knots for a near sister with jet propulsion.

Even if the current installed 10-cylinder 900 kW diesels were replaced by 8-cylinder 720 kW diesels, the economical service speed of 25 knots required by Turbine Transfers would still be met with the VLJs.

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. Furthermore it improves our green credentials through substantial fuel and emission savings in our operations.

—Alistair Knowles, Marine Superintendent at Turbine Transfers

The Voith engineers designed and developed the VLJ exclusively by computer, applying the CFD (Computational Fluid Dynamics) method. With a tradition of almost 150 years of hydro power development, Voith was one of the first adopters of the CFD technology.

Voith has been in the marine industry since the early 1920s with its unique Voith Schneider Propeller (VSP). The Voith Linear Jet paves the way for the entry of Voith propulsion systems into the market of fast vessels. Target markets are faster ferries, yachts, workboats of the offshore industry and coastal protection vessels.