The paper describes an electronic speed control which has been fitted to one of the two ship-model testing tanks at the National Physical Laboratory. A description of certain aspects of ship-model testing is included in a discussion of the reasons for fitting electronic control and of the performance required of such a control. The control system is a particular application of closed-loop automaticcontrol technique. A general description of it includes full details of those features which are special to this particular application, and a mathematical analysis of the design. Some typical test results obtained are shown in the form of graphical records and Tables.