Germany: Hamburg-based Becker Marine Systems has launched the new Mewis Duct. Based on hydrodynamic statutes, the duct combines very consequentially different theories of fluid dynamics.
Final true scale tests have proven the enormous fuel saving potential of the product of up to ten percent.
Description of the new power-saving device
Becker's research found that the wake field of full-form vessels, such as tankers, reduces the propeller's propulsion efficiency. The water flow velocity has such an unfavourable characteristic that the propeller does not get an uniform water flow.
The Mewis Duct is a propulsion improvement device for full-form ships (e.g. tankers, bulk carriers and multi-purpose vessels).
The improvement of the propulsion efficiency of these ship types will contribute largely to the reduction of polluting emissions and to the saving of carbon fuel in shipping.
Ships are propelled by combustion engines whose power output is dimensioned by the ship's hull resistance, the propeller efficiency and the desired speed range.
The Mewis Duct reduces power requirement by improving the water inflow toward the propeller to achieve a higher overall propulsion efficiency. The Mewis Duct consists of two strong fixed elements mounted on the vessel: a duct positioned ahead of the propeller together with an integrated fin system within.
The duct straightens and accelerates the hull's wake into the propeller and also produces a net ahead thrust. The individual placed fins have a stator effect by generating a pre-swirl in the counter direction of the propeller's operation, recovering the rotational energy from the propeller slipstream.
The achievable power savings from the Mewis Duct depend on the propeller thrust loading, from three percent for small container vessels up to ten percent for large tankers and bulk carriers.
Optimisation
CFD techniques are used in the optimisation process with validation of the results against appropriate model tests.
CFD analysis
Model tests
Full scale realisation
Manufacturing
Installation
Ready for sea trials
Sea trial results
According to Becker, the power-saving predictions from its CFD-calculations and from the research institute match with the sea trial measurements.
The shaft power and speed trial measurements on the Grieg vessel were carried out from an independent company.
Pure sea trial measurements can not be compared directly to the model tests, said Becker. The conditions of the ship and the environment must be corrected to standardised conditions, this was done by The Hamburg Ship Model Basin (HSVA).
The sea trial measurements were carried out in light loading conditions. With an installed Mewis Duct at a ship speed of 16.45 knots, Becker saved approximately 4.5 percent propulsion power.
When Becker increased the ship speed to 17 knots a power reduction of approximately six percent was measured.
This corresponds at 17 knots an estimated cost saving with Mewis Duct of US$23,000 per year at a bunker price of US$460 per tonne at 220 days of operation per year.
Greenhouse gases were also considerably reduced. Becker obtained a 50,000-tonne reduction in nitrogen oxide and a 1,600-tonne per annum reduction in carbon dioxide.
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