The following protractor, course indicator, or whatever you may choose to call it, was made to fill the personal need for a quick method of solving the problems of maneuvering and torpedo control, thereby anticipating the prolonged manipulation of dividers and parallel rulers at the chart board, or the no less elaborate fumblings at the torpedo director. The geometry of it is older than Euclid, and the practical use is similar to that of the old Batten- berg protractor and probably to many “gadgets” dear to the heart of their creators.
General Description (See Fig. 1). It consists of a graduated compass circle marked in degrees, with a central spindle on which three arms revolve freely. The narrow target arm and own ship arm revolve around the spindle as diameters of the compass circle. One end of each forms a pointer following the graduated circumference of the compass circle, while the other end, which we will call the Speed End, has marks or Speed Points at distances from the center corresponding to speeds from zero to thirty knots or over.