Michel Ebersolt, from the Merimar consultancy in France, considers some aspects of bulb-type solutions to this perennnial problem and draws some interesting conclusions. The Costa bulb, distributed by Maierform in Geneva, enjoyed certain success in the past. According to Maierform, this device should achieve a saving of between 2% and 5% in motive power, as well as reducing vibrations opposite the stern post. Merimar, based in Paris, France, recently ran a series of tests, both in open water and in a cavitation tunnel, to analyse the hydrodynamic performance of this device. This work was carried out on two 'nozzle propeller' assemblies, with and without rectifier fin, and subsequently on a nozzle-propeller assembly with rectifier fin, and with a bulb mounted on this fin (Fig 1). A nozzle configuration was used for these tests, although the arguments employed are also valid for a free propeller.
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