IN VIEW of the fact that several of the new fire control installations call for the use of true wind instead of apparent wind, it is expedient to have some method of quickly and accurately determining this factor. The following device, designed and originally used by Lieutenant O. Nimitz, serves the purpose and has the additional advantage of being easily made.
Construction: On a thin piece of board about twelve inches long and six inches wide paste a plain piece of white paper. Draw the longitudinal center line and from a predetermined center lay off—along this line—graduations to represent ship’s speed in knots. Any convenient scale may be used. Construct two dials graduated in degrees. One is to be used as “True Ship’s Course Dial”; the other as “True Wind Dial.” The compass roses, cut from old charts, will serve the purpose very well, provided they are pasted on light metal or cardboard in order to give them the requisite strength. The Apparent Wind Arm and the True Wind Arm are made of light brass or some other similar metal. It is essential that the graduated side of these arms run through their pivot centers and that the graduations be laid off along this side from the pivot center. The scale used must be identical with that previously used for ship’s speed. An excellent method of satisfactorily graduating these arms is to cover the metal arm with a strip of adhesive tape and to paste a piece of paper on the tape. The graduations can then be made with India ink. Pivot the True Ship’s Course Dial and the Apparent Wind Arm, the one above the other, and in such a manner that each may be turned independently of the other, about the previously determined center from which the own ship’s speed graduations were laid off. Pivot the True Wind Dial and the True Wind Arm about a common movable center in such a manner that each may be turned independently of the other. If the ship’s speed in knots is shown by mere line graduations, a thumb tack may be used as a common center for these two. If small holes are drilled for these graduations, it is possible to use a peg of the proper size for a center pivot.
The scale to be used, the size of the dials and the length of the arms can be made to suit the existing conditions. Under ordinary circumstances it will probably be sufficient to allow for a ship’s speed of not more than 40 knots and for a wind of not more than 50 knots.
Operation.—Turn True Ship’s Course Dial until it gives the ship’s true course when read against the inscribed longitudinal center line. Swing the Apparent Wind Arm around until its graduated side gives the direction of the apparent wind when read against the True Ship’s Course Dial. Place the common center of the True Wind Dial and the True Wind Arm over the graduation representing the ship’s speed. Turn the True Wind Dial until its reading against the longitudinal center line is identical with the leading of the True Ship’s Course Dial. Swing the True Wind Arm around until its graduated side intersects the Apparent Wind Arm at the graduation representing the force of the apparent wind. Then the True Wind Arm read against the Apparent Wind Arm will give the force of the true wind and read against the True Wind Dial will give the direction of the true wind. In the illustration the following factors are used:
Ship’s course 60°,
Ship’s speed 25 knots,
Apparent wind direction 90°,
Apparent wind force 20 knots,
which give:
True wind direction 190° (—),
True wind force 12½ knots.
The construction of this device can.be somewhat simplified by drawing the ship’s course dial on the paper and graduating it for relative bearing. The True Wind Dial and the True Wind Arm can then be fastened together so as to move as a single piece. In this case the zero graduation of the dial should coincide with the intersection of the graduated side of the True Wind Arm and the circumference of the True Wind Dial. When so constructed the relative bearing of the true wind is read off of the True Wind Dial against the longitudinal center line.