A new breakthrough in marine propulsion?

Hull integration inflow tunnels of the Voith Linear Jet
Hull integration inflow tunnels of the Voith Linear Jet
Turbine Transfers’ 'Trearddur Bay'
Turbine Transfers’ 'Trearddur Bay'
Three main components of the Voith Linear Jet
Three main components of the Voith Linear Jet
Computational Fluid Dynamics showing the VLJ in static bollard pull condition. Red shows positive pressures (thrust) and blue shows
Hull integration inflow tunnels of the Voith Linear Jet
Hull integration inflow tunnels of the Voith Linear Jet

Details from the latest sea trials of a brand new marine propulsion system that its makers claim can dramatically cut operating costs – while producing more thrust with less noise and vibration – will be revealed at Seawork International in June.

Comparative trials of the Voith Linear Jet, which has taken seven years to develop, are currently taking place with twin boats – one fitted with the Voith Linear Jet and the other using conventional waterjet technology.

German propulsion giant Voith hopes that the sea trials will further demonstrate the technology’s outstanding performance as well as cost savings to operators in several marine market segments.

The “world-first” propulsion system has been fitted to the new Turbine Transfers’ ‘Treardurr Bay,” a 21-metre long, seven-metre beam support catamaran designed by BMT Nigel Gee and built by Aluminium Marine Consultants.

Following trials outside the Isle of Wight at the end of last year the Voith Linear Jet received glowing tributes for its speed, handling, power, and efficiency, as well as reduced noise, vibration and cavitation.

Alistair Knowles, marine superintendent of Turbine Transfers, said: “It’s phenomenal…….extremely fuel efficient for a vessel of the size, so able to run at 25 knots under reduced power and with consumption comparable to smaller and less capable boats.”

On noise and vibration he said that measured levels were down to about 62 decibels “and we could chat comfortably without raising our voices even at high speeds.”

On speed Rob Stewart, AMC’s commercial director, said: “Voith originally calculated that it would do about 27 knots but we got up to 32 knots out of it on tests. He added that with a seven-tonne load – half on the foredeck and half behind – “we still got a 30 knot speed with no loss of handling.”

The Treardurr captain, Antony Robson, said of its pulling power: “We’ve measured bollard pull of 12.4 tonnes whereas most jets of a similar size would only give you six tonnes. “

As far as Maritime Journal can ascertain at this early stage, the VLJ works rather like a waterjet in that the aft thrust from the propeller, (or rotor) is exhausted through a set of thrust straightening stator (fixed rotor) vanes and pushed through a slightly tapered nozzle. Unlike a waterjet, who’s inlet is usually flush with the hull in the turbulent water of the hull’s boundary layer, the Linear Jet is fixed partially beneath the hull. This suggests that it will draw more clean, unturbulent water and this may be where much of the claimed benefit is coming from.

One of the benefits of water jets which it is not clear whether the VLJ will share is that they often draw through a scoop grate protected from most debris by horizontal vanes aligned to the water flow. Because of the general arrangement of a conventional water jet the hydrodynamic penalty of this grate is relatively low, so it is usually well worth having as an additional benefit.

The sides of the nozzle appear to be foil shaped to increase pressure in the nozzle and reduce it outside. This presumably means that at this higher pressure the speed of flow through the rotor is reduced with reference to the vessel's speed through the water, thus moving the rotor's likely development of cavitation up to a higher speed. 

Voith Turbo’s marine manager, Mark Harvey believes the benefits of the Voith Linear Jet will interest key figures in the professional marine market including designers and naval architects; owners and operators; marine and technical engineers; and boatyards that see the Voith Linear Jet as important to the type of vessels they build.

“The Voith Linear Jet will be a good choice for CTVs particularly wind farm service vessels. There is a growing market for 15 to 30-metre catamarans to take service crews to wind farms,” said Mark.

The fast ferry market, he said, was also a target. Despite being somewhat flat at present he felt that Voith would find opportunities in the new build market.

“We believe the military and para-military markets will present further opportunities for us with specific services such as coastguards, police, and pilot boats, gaining benefit from the new propulsion system’s additional speed and efficiency.”

Mark said that in broad terms the new propulsion system’s key benefits were lower fuel burn than vessels with water jets or propellers. The high efficiency of the Voith Linear Jet over a wide speed range means that at 25 knots – where water jets are not particularly efficient – the VLJ is significantly better.

For vessels with fixed pitch propellers, he added, the power density becomes an issue above 30 knots, but this was no problem for the VLJ which can be used up to 40 knots without any such issues.

He said that full technical details of the Voith Linear Jet would be revealed at Seawork International on stand A12.

By Jake Frith

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