La propulsion par jet d'eau

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Water-jet propulsion for watercraft, a long-known principle, is finding an extensive range of applications today. Propulsion units of widely varying geometry and performance operate on this principle (Fig. 1), under the two main headings of (l) axial units and (2) hydrojets (Figs. 2 and 3 respectively). Performance of such units can be expressed in simple forms (formulae 1-12) allowing determination of such important factors as pump and ideal propulsion efficiency, internal pressure drop and other significant efficiency-optimization factors (Fig. 4). In the case of fixed-point units, the "merit factor" is optimized (formula 13). Basic operating characteristics of a propulsion unit include variation, reversibility, adaptability and direction of thrust. Some flexibility is required in designing a water-jet propulsion unit for a given application. One has to know the characteristics of its water intake, pump, circuit and discharge nozzle. These are briefly analysed, with indicative flow data. Overall features and performance of axial and hydrojet units are illustrated by examples of applications covering the whole speed range of such designs, i.e. constant, medium and high speeds. The report is intended to allow analysis of the basic features of each type of unit.