The principle of marine propulsion that makes use of the reaction produced by a jet of water ejected from the stern has developed to the stage of successful practical application. So far however, its adoption has been limited to small high-speed craft for special uses-pleasure boats in particular. This limitation in the application of the hydrojet propulsion system might derive partly from the erroneous belief, based on past experience, that the system is inherently less efficient than the screw propeller. The author undertook matching tests on a 22-foot boat hull adapted for hydrojet propulsion, to compare the performance of three pump units-IHJ 18/3 s, IHJ 24/2 s and IHJ 24/3 s-incorporated in a propulsion system developed in the author's institute. The boat thus equipped had a displacement of about 2 tons; the power was supplied by a 310 If marine gasoline engine. The tests proved the IHJ 24/2s to be the most suitable, with which the craft attained a maximum speed of 29 knots and a propulsive efficiency of about 0.51. It was not possible to compare this performance with screw propulsion, since no tests were carried out on a screw-propelled boat of same size and form. It is usually considered however that a small planing boat with screw of comparative type would have a propulsive efficiency of around 0.45. Thus it may be safely concluded that, in respect of propulsive efficiency, we have been able at least to match, if not surpass, screw propulsion, proving that wider application of jet propulsion is a practical possibility. It remains for the future to analyze the problems still left unclarified, such as optimization of the inlet configuration, through basic experiments, and to develop hull forms particularly suited to hydrojet propulsion. Practical applications in dredgers, fishing vessels and other craft should be developed by fully exploiting the structural and functional advantages of the hydrojet propulsion system.