The Becker Mewis Duct ® - Challenges in Full-Scale Design and new Developments for Fast Ships

The need for energy efficiency in ship design and operation has been continuously increasing over the last few years. Consequently there is a growing interest in energy-saving devices (ESDs) that aim to improve ship propulsive efficiency. The Becker Mewis Duct ® (MD) is such an ESD. The main effect of the Becker Mewis Duct ® is the reduction of energy losses of the propeller. Since its launch in 2008, the Becker Mewis Duct ® power saving device has experienced extraordinary success. To date over 100 have been delivered, with about 300 on order. Model tests for the Becker Mewis Duct ® have shown an average power saving of 6.3%. The design of the Becker Mewis Duct ® is largely based on CFDmethods with model tests remaining a core element of the overall process. The scaling of the model test results and the design of the Becker Mewis Duct ® for full scale operation are sometimes subject to questions or discussion. A part of the paper will deal with the approach used in the design of the Becker Mewis Duct ® to handle this problem. It will also describe special difficulties encountered when using CFD for the prediction of fullscale stern flows for slow full-block vessels. The intention is to raise questions and to increase the interest in this subject to enhance potential ways forward. As a result of customer demand, during the last two years the MD concept has been extended for application to faster vessels with lower block coefficients, such as container ships and reefer vessels. This has resulted in a new product, called the Becker Twisted Fins ® (BTF). Conceptually the BTF is similar to the MD, consisting of a radial series of pre-swirl fins encased by a heavily optimised pre-duct. In addition there is a series of radial outer pre-swirl fins fitted with winglet type end plates, and all fins are twisted for optimal pre-swirl generation. Faster hullforms present special challenges for the development of power-saving devices. The paper will describe some of these challenges, the extensive use of computational fluid dynamics and present results of initial propulsion and cavitation model tests.