Dynamic-inlet water jet

Abstract

Jet propulsion is becoming increasingly popular in the field of fast navigation, and is steadily tending towards the «dynamic-inlet» pattern in the range of higher powers. Indeed, the choice of a «slanting-inlet» nozzle (fig. 1), placed on the boat bottom without any protrusion (which might increase draft), meets the particular requirements of small seaside craft. However, the more suitable solution in general is certainly the straight one (figs. 2 and 3), which allows the best recovery of the energy resulting from forward motion. On the other hand, not only sport boating, but also coastal navigation for civilian and military duties show an increasing tendency to high speed, especially in connection with the present success of hydrofoil ships (fig. 3). This report outlines the main points in the design of a «dynamic-inlet» water jet propulsion unit through a generalized adimensional analysis. A specific-thrust optimization method is given, so that the choice of size and shape is merely reduced to considerations of the necessary compromise between efficiency and overall dimensions. Moreover, a procedure is reported in order to estimate working conditions different from the nominal ones, and to optimize the relative characteristic curves. Finally, the results of an experimental investigation carried out on a low power unit are reported. This investigation used a special mobile bench, equipped with a straingauge load cell to provide direct evaluation of the instantaneous thrust at several speeds.