THE UNIVERSAL DRAFTING MACHINE AS A BATTLE APPROACH INSTRUMENT
Lieutenant Commander D. T. Hunter, U. S. Navy
During the strenuous gunnery program of the Pacific fleet in the season of 1919-20, while the writer was on duty as navigator of the U.S.S. Idaho, approaches for the different forms of practice were a part of nearly every day's work.
Being dissatisfied with the various methods of using the mooring board for this problem, on account of the inaccuracy of the circles and graduations, it was decided to try the drafting machine with the ordinary cross-section paper supplied to ships, and the results were uniformly good.
No originality is claimed for the method, but conversations with other officers who have recently been navigators of battleships, developed the fact that none of them had ever seen this particular combination used, so it was thought that it might be of interest to others in the service.
The equipment used in the Idaho was very simple, consisting of an ordinary drawing-board, hinged to a batten which was lashed to some conduit pipes on the conning-tower wall, directly under the target-bearing indicator. The opposite edge of the board was fitted with a single hinged leg, so that the board, when not in use, hung vertically against the wall. A spare drafting machine anchor, secured permanently in the upper left-hand corner of the board, a camp-stool and a rack for pencils, dividers, and stop-watch completed the outfit.
A sheet of cross-section paper, cut from a large roll, covered the whole board, and the long rulers supplied with the drafting machine were graduated to the same scale (inches and tenths) as the cross-section paper. The advantage of the cross-section paper and similarly graduated ruler is the great variety of comparatively large scales which is available for any approach problem. It was the writer's practice to use a scale of 1"'= 200 yards for S.R.B.P., 1"= 500 yards for S.R.D.P., and 1"=1,000 or 2,000 yards for the longer range practices.
The target was represented by a pin driven into the board near the middle of the upper edge, through the intersection of two of the heavy lines of the paper, to facilitate alignment of the ruler while plotting. It is also advisable to plot the towing vessel at the proper distance ahead of the target, for use in case the latter should be temporarily obscured.
The heavy horizontal line, containing the pin, was always taken as the base course of the target and if the practice involved firing on both sides the course was reversed by simply reversing the compass rose of the drafting machine. The required track of the ship for the practice, and all important bearings and points were plotted on the sheet in red ink and prominently labeled.
The operation of plotting involved no new principles, but for the sake of clearness as to the use of the gear it will be described briefly. The ship's course and the bearing of the target are obtained from the target-bearing indicator, and the range from a J.W. talker stationed beside the chart board, so that the ship's position at any instant can easily be plotted by distance and bearing. The method of reaching the initial point may best be illustrated by an example. Suppose it is desired to reach an initial point from which the target will bear 50° to the right of the base course, distant 19,000 yards; base course North (true); target speed 7 knots; ship's speed 21 knots; and that the target now bears 25° (true), distant 24,000 yards.
First orient the drafting machine, plot ship's position and draw a line connecting it to the initial point. Consider the initial point to move forward on the base course at the target's speed and lay off from that point, to any convenient scale, a distance representing 7 knots, in the direction of the base course. With the end of this line as a center and a radius equal to 21 knots (to the same speed-scale) strike an arc to intersect the line joining ship's position and initial point. Press the points of the dividers into the board, lay the drafting-machine ruler against them and clamp the machine. This gives the course to steer (343-1/2° true). Leaving the ruler clamped, lay it through the ship's position and measure the distance to the point where it intersects the horizontal line through the initial point. This is the distance which must actually be run to reach the initial point (15,200 yards) and, dividing by the ship's speed (700 yards per minute) gives the time it will take to cover that distance (21.7 minutes).
The stop-watch may be started when the plotting begins, so that the time consumed in plotting and changing course will not affect the result, but practice will so reduce this time that this is, perhaps, a needless refinement at long ranges. The course and position should, however, be checked repeatedly before reaching the initial point.
At the shorter range practices it is often important to know the ship's advance and transfer for changes of course of less than 90° and a table of advances, transfers and times for changes of course of 15°, 30°, 45°, is very useful. In the case of the Idaho; it was found, during rehearsals of S.R.B.P., that putting the rudder over 15° one way and meeting with 15° of the opposite rudder when the ship began to swing, would bring the ship on a line fifty yards in or out from the target. This will vary for different ships, but a table may be constructed on this principle, to bring the ship any desired distance in or out. Any maneuver of this kind must, of course, be performed before "whistle" if friendly relations with the gunnery department would be maintained.
All navigators have their pet methods and "gadgets" for this sort of problem, but, unfortunately navigators do not get together and compare ideas as gunnery and engineer officers do, and it was with this in view that this article was submitted.