Abstract
The typical properties of the new stern shape provided with cochlea (helical) channels are displayed by the comparison of many model test results. The test and design methods of the new stern shape, not restricted solely to twin screw hulls, are thoroughly explained.
The manipulation of the wake by means of the channel system leads to the following results:
a reduction of the thrust pulsation of the single propeller blades; a drastic reduction in amplitude of the main frequency of the blade thrust, leading to its complete elimination; a shift to deeper sectors of the propeller disc of the reduced blade thrust maxima; a counterpropeller effect generated by the vortex induced in the wake; a limitation of the increase of the towing resistance due to the wake vortex and a possible reduction below the resistance of the corresponding traditional ship, provided with its unavoidable appendages; a conveyance into the propeller disc of the (viscous) wake having lapped the surface of hulls provided with more than one propeller.
Each one of the above contributions to the saving of propulsive power is not by itself of great significance, but their combined effect leads to a very substantial benefit.
This explains how a power saving of the order of 25% could be achieved on comparative self-propulsion tests performed with a couple of twin-screw models, as well as the recorded drastic reduction of propeller induced hull vibrations.
The available experimental data and criteria are shown, concerning the question to what extent similar benefits can be expected from the adoption of the cochlea channelled stern on any kind of ship.
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