How a fin and a duct save fuel

ESDs are stationary flow-directing devices positioned near the propeller. These can be positioned either ahead of the propeller fixed to the ship’s hull or behind the propeller fixed either to the rudder or the propeller hub. Today, various energy-saving devices exist on the market, all validated though extensive in-service data or model testing experience. It would, therefore, appear impossible to develop an absolutely new solution to the problem. However, by combining two or more components of already established principles, new developments are possible. This approach offers even more   possibilities by targeting a combination of different types of flow losses.

Mewis Duct

The Becker Mewis Duct from Becker Marine Systems is an example of this as it is based on two fully independent working ESD-Principles:

• The Pre-duct principle first published in 1949 by Van Lammeren. This principle consists of equalization of the propeller inflow by positioning the duct ahead of the propeller. The duct axis is positioned vertically above the propeller shaft axis with the duct diameter smaller than the propeller diameter. The duct is stabilizing the fin effect as well as producing thrust.
• The Contra-rotating propeller principle, well known for more than 100 years which consists of reduction of rotational
• losses in the slipstream by integrating a pre-swirl fi n system within the duct. The chord length of the fin profiles is smaller than the duct chord length, with the fins positioned towards the aft end of the duct. The duct itself acts as a type of endplate to the fins, thus increasing their effect.

FORCE Technology’s Mewis Duct experience

In 2011, FORCE Technology established a close cooperation with Becker Marine Systems, serving as one of the major testing facilities for their Mewis Duct systems. About 10 projects have been successfully tested and verified. These include typically high-block coeffi cient bulk-carriers and tankers with moderate to high propeller loading. The test scopes included two phases; 1) optimization of the pre-swirl fin system (definition of the final setting angle) and 2) subsequent full-speed range comparison of the propulsive performance against the base case (vessel without Mewis Duct). Thus the results serve as a basis for confirmation of the predicted power-saving effect. 

Based on the above experience, FORCE Technology documented predicted power savings in the order of 5-6% (on average). A few recent projects are to be further tested in full-scale sea trial conditions both with and without Mewis Duct aiming at a final verification of power saving. These results will possibly be available at a later stage.