INTRODUCTION 



The ability of certain outstanding navigators and tacticians to rapidly and efficiently carry out missions, conduct scouting 

 and search operations, and shift stations within a fleet or other mobile unit has long been known. Although their skill has been 

 described by such terms as "having developed a good seaman's eye," basically their aptitude has been the result of being able 

 to apply the principles of relative movement to the particular problem at hand. Relative movement is an everyday phenome- 

 non. The most familiar example of this is the apparent movement of celestial bodies across the sky. As the globe turns 

 from the West to the East, to an observer stationed on the earth, the celestial bodies appear to rise in the East and set in 

 the West. When two trains on adjacent tracks are moving in the same direction but at different speeds, to passengers on the 

 faster train it appears that the slower train is moving backwards. By movement relative to the faster train and ignoring the 

 actual direction and distance traveled over the face of the earth by both, that is what the slower train is doing. 



The essential difference between the relative movement method of solving problems and the usual navigational plot 

 method, is one of origins. The latter uses a point fixed with respect to the earth and called a "Chart Point." The travel 

 of units, portrayed by lines on the chart used, represents directions and distances actually traversed on the face of the earth 

 or over the ground. Such a diagram, when used in this publication, will be referred to as the "Navigational Plot." The 

 lines representing the travel of units over the ground in this diagram are called "Chart Lines." When several units are being 

 plotted on this diagram, their exact positions for any particular time must be carefully delineated before their positions relative 

 to each other can be found. For a composite picture of the actions of several units, this is excellent; for planning actions 

 in advance, the amount of trial and error involved usually causes much delay, so the relative movement method is to be pre- 

 ferred in most cases. 



The relative movement method uses a moving unit instead of a chart point as the point of origin. The unit so chosen, 

 designated as the "Guide," remains fixed in the Relative Plot, although it represents a ship moving over the earth. The 



FIGURE 1. 



FIGURE 2. 



movement of all other units concerned in the problem is referred to this guide and their travel is portrayed by "Relative Move- 

 ment Lines. " The relative movement line of a maneuvering unit, with respect to the guide, is defined by drawing a line through 

 plotted successive positions of the maneuvering unit, as determined by range finder and compass aboard the guide. These 

 positions are laid off from a fixed point representing the position of the guide in the Relative Plot. The Maneuvering Board 

 has been designed to facilitate this plotting as well as to show graphically all the maneuvers involved. 



In the relative movement method, two diagrams are normally required, the "Vector Diagram" and the "Relative Plot." 

 The "Navigational Plot" may be added as part of the solution or as a check on results. The Vector Diagram is so called 

 because every line in it is a vector and therefore indicates both magnitude and direction. The magnitude of the vector is its 

 length, which applied to the scale in use indicates a velocity, or rate. The direction of the vector is shown by its inclination, 

 an arrow being added to the head of the vector to prevent reciprocal errors. Vector quantities are added and subtracted 

 geometrically by the Parallelogram Law as outlined in standard textbooks. 



The point of origin in a vector diagram is usually a point fixed with respect to the earth, and for convenience is lettered 

 "e." Thus, vectors originating at "e" show direction and rate of travel with respect to the earth, or over the ground. 



In Figure 1, three such vectors are shown, e . . . . g indicates the travel of a unit G, making 10.0 knots in direction 

 000° over the ground; e . . . . w represents the travel of the wind in direction towards 020° at 12 knots over the ground; 

 while e . . . . m shows the unit M on course 070° at a speed of 9.0 knots over the ground. The Wind Vector indicates the 

 direction toward which the wind is blowing, instead of the usual method of describing the direction from which it originates. 

 These vectors, or any others based on the same units of motion, may be combined to form the Vector Diagram. 



Figure 2 shows the combination of the above vectors to indicate concurrent travel. By joining g . . . . m another vector 

 is formed. If we consider m as the head and g as the foot of the resultant vector, g . . . . m gives the direction and rate of 

 travel of unit M relative to unit 6. In the corresponding Relative Plot, G would be stationary while M traversed a line parallel 

 to g .... m and at the rate indicated by the length of g .... m. If, on the contrary, we consider g as the head and m 

 the foot of this vector, then m . . . . g gives the direction and rate of travel of unit 6 relative to unit M, and in the corre- 

 sponding Relative Plot, M would remain stationary while G traversed a line parallel to and in direction m .... g, at the rate 

 IV 



