324 RUDDER TRIALS, U. S. S. STERETT. 



APPENDIX IV. 



DESCRIPTION OF HELM-ANGLE INDICATOR AND DETERMINATION OF 



CENTER LINE OF SHIP. 



The continuous record of helm-angles, both on friction tests and tests 

 under way, was obtained by means of a tracing pen and solenoid on the 

 recording gear, electrically connected to the special indicator shown on Fig. 

 1 6, Plate 126. This indicator was fitted as follows. A i-inch mahogany 

 board, cut to proper shape, was set up against the rudder head and bolted 

 securely to the deck. Into this board was fitted a strip of ebonite ^ inch 

 thick and of 6^ inches radial width, sawed to a circular arc of i6§ inches 

 (outer radius) concentric with the rudder stock. Into the ebonite arc, at 

 carefully graduated intervals of 1°, were inserted radial strips of brass ^ inch 

 thick, projecting slightly above the surface of the ebonite. A copper wire 

 ran from a binding post through a slot on the under side of the ebonite arc 

 and was soldered to each of the brass strips. A pointer cut out of |-inch 

 sheet brass was secured by tap-screws to the rudder head, in such a position 

 as to sweep over the strip of ebonite when the helm was put over from one 

 extreme position to the other. A wire was then run from the binding post of 

 the ebonite strip to the battery box, and another wire from a screw contact 

 on the moving pointer to the solenoid on the chronograph, and thence to 

 battery. As the helm was put over in either direction, the brass pointer 

 swept across the surface of the ebonite arc, alternately making and breaking 

 circuit and giving a continuous record of degrees, except on 0° and on 5°, 

 15°, and 25°, on which points the brass contact strips had been omitted to 

 facilitate distinguishing these intervals on the record. 



In general this device was very satisfactory. But on the backing tests 

 and on several other occasions water splashed up on deck, causing a con- 

 tinuous closed circuit over several degrees of helm. It was not difficult, 

 however, on analyzing the records, even in such cases, to determine the 

 helm-angle. On most of the turns, an observer was stationed at the indicator 

 to check the recorded helm-angles. 



For the proper location of the 0° point of the helm-angle indicator, the 

 true fore-and-aft center-line of the ship was established in dry-dock as 

 follows. The half -breadth of the keel was carefully measured and center- 

 punched at a point about 25 feet forward of the stern-post. Then on the 



