FATHOMETER STUDIES 



501 



angle of 2 degrees observed as far as 500 yd behind a 

 15-knot destroyer, and an included angle of 1 degree 

 thereafter. However, a critical examination of the 

 plot reveals such a large quartile deviation that the 

 reality of the differentiation between new and old 

 wakes seems somewhat doubtful. An included angle 

 of l}i degrees for the entire range of observations, 

 with the maximum distance astern of 2,500 yd, gives 

 a fair representation of the plot. The extrapolated 

 initial width of these destroyer wakes is roughly 

 equal to the beam of the vessel, perhaps somewhat 

 smaller. However, the observations do not cast any 

 light on the very large initial divergence of destroyer 

 wakes, revealed by aerial photographs, because the 

 acoustic measurements did not extend to distances 

 less than 100 yd astern. It should be noted that the 

 angle of spread derived from the acoustic measure- 

 ments (13-^ degrees) is in fair agreement with that 

 derived from aerial photographs (1 degree), as re- 

 ported in Section 31.1. It is possible to attribute the 

 difference of }i degree between the two figures en- 

 tirely to the inaccuracies inherent in the respective 

 processes of measurement. 



31.3.2 



Submarine Wakes 



Information on the geometry of submarine wakes 

 is less detailed. Among the measurements made mth 

 the fathometer ranging downward,* an investigation 



Table 2. Wake of submarine at periscope depth. 



of the wake of a fleet-type submarine 309 ft long is 

 reported. At periscope depth the keel is submerged to 

 a depth of 60 ft, the screws to a depth of 48 ft, and 

 the deck to a depth of 35 ft below the surface; the 

 speed was 5.5. knots. Table 2 contains the observed 

 depths. The same information for a surfaced sub- 

 marine of the same class, moving at 7 knots, is given 

 in Table 3. 



The maximum distance of 450 yd appearing in 

 Table 2 is not the upper limit of detectability of the 

 wake at a keel depth of 60 ft, as the observers 

 emphasized. 



The length of the subsurface wake of an S-class 

 submarine,'* running at 6 knots, was found to be 

 about 1,000 yd at a depth of 45 ft, 235 yd at a depth 

 of 90 ft, and 100 yd at a depth of 125 ft. These 

 figures give the distances astern of the submarine 

 over which the wake extends before it becomes un- 

 detectable by the gear used in these experiments. 

 The bow-mounted 24-kc transducer was trained at 

 a fixed bearing of 30 degrees relative to the Jasper, 



Table 3. Wake of surfaced submarine. 



which was following the submarine on a parallel 

 course and then gradually fell back. At creeping speed 

 (2 to 3 knots) the length of the acoustically effective 

 wake is less than 30 yd for a fleet-type submarine, 

 according to recent San Diego observations.' It would 

 seem, then, that the subsurface wake is not a good 

 scatterer at greater than periscope depth, particularly 

 at slow speeds. Analogous experiments * at frequen- 

 cies of 24 and 45 kc were carried out with a fleet-type 

 submarine, ruiming at speeds up to 9 knots and at 

 depths down to 400 ft. According to the observers, 

 during no run was an echo definitely identified as 

 coming from the wake alone. 



From the data summarized in Table 2, it appears 

 that the wake of a fleet-type submarine, running at 

 5.5 knots at periscope depth, extends to the surface at 

 distances astern greater than 100 yd; the single record 

 at a shorter distance of 67 yd, which suggested a com- 

 pletely submerged wake, unfortunately was uncer- 

 tain. Later tests, using the same fathometer equip- 

 ment with the fleet-type submarine USS Trepang 

 (SS412), have led to a general confirmation of the 

 previous results.' The Trepang was running at 8 

 knots at a keel depth of 60 ft, and the wake appeared 



