Sec.-fS.lI 



WIND-WAVE AND SHIP-WAVE DATA 



173 



at 75 deg and 135 deg true. Their wave lengths 

 are 400 ft, 240 ft, and 120 ft, respectively, with 

 rather moderate steepness ratios of 1/50, 1/40, 

 and 1/30, in the order given. The corresponding 

 wave heights hw are 8, 6, and 4 ft. Other charac- 

 teristics of these waves are given in Table 48. j. 



All components have sinusoidal wave profiles. 

 Their elevations (or depressions) above the 

 assumed quiet water plane may therefore be 

 added algebraically to produce the elevations 

 (considered as a + distance) or depressions 

 (considered as a — distance) of the resultant, 

 above or below the reference plane. 



The first step in the graphic superposition is 

 to draw, to a convenient scale, three transparent 

 contour patterns for the three sinusoidal waves, 

 having straight, parallel lines laid off normal to 

 the direction of travel at elevations to represent 

 1-ft contours, including the crests and troughs. 

 The data for positioning the contour lines between 

 successive wave crests, for various equal sub- 

 divisions of the wave height, are given in Fig. 48. F. 



The three patterns are laid down over each 



other in the desired positions. The resultant 

 pattern is built up on a fourth transparent sheet, 

 superposed on the other three, by adding the 

 elevations algebraically at a multitude of points 

 throughout the field. 



To show how much of an "ilot" (French for 

 "islet") [Pommellet, A., ATMA, 1949, Vol. 48, 

 pp. 589-608] would be formed by three wave 

 crests piled on top of each other, it is assumed as 

 a starter that all three crest lines intersect at the 

 time to . This intersection is the reference point 

 or origin 0, taken to be fixed in space. The three 

 contour patterns, when laid down at the proper 

 angles, are so placed that their crest lines cross the 

 origin. Contours of the composite pattern are 

 then sketched at 1-ft intervals, with the result 

 shown in Fig. 48. G. Those portions of the com- 

 posite waves whose surfaces slope downward and 

 to the right have the contours indicated by heavy 

 broken lines, as though in shadow when illumin- 

 ated by a low sun at about 270 deg true (in the 

 west). 



The validity of this method of superposition 



' y' J 1 1 \ ON N 



Fig. 48.G Contour Diageam for Three Superposed Sinusoidal Waves 



