classified as hydrodynamic noise, It is pro- 
duced by cavitation and turbulence about the 
hull, bubbles striking the sonar dome, wave 
slap, and quenching. The noise level depends 
upon hull design, sonar dome design and lo- 
cation, and heading with respect to the sea. 
Its intensity increases with increasing ship 
speed and sea state, 
Under sea state 0 conditions the noise 
produced by flow about the dome and hull 
would be the major source of hydrodynamic 
noise, but rolling and pitching caused by 
higher seas bring the other sources into play. 
Cavitation is produced by violent motion of 
the bow, while bubbles, either trapped or 
created by bow motion, cause crashing 
sounds when they strike the sonar dome. 
The sound produced by waves striking the side 
of the ship is obvious even at low speeds, and 
quenching occurs when the bow and sonar dome 
clear the water during severe pitching, Each 
of these sources cause periodic noise, and 
under proper conditions they may make 
listening impossible a large percentage of 
the time. 
Little unclassified information on hydro- 
dynamic noise levels as a function of ship 
speed and sea state exists, nor is it even 
adequately known, It is sufficient here, 
however, to say that as ship speed and sea 
state increase, hydrodynamic noise rapidly 
becomes the dominant source of undesirable 
background noise considerably exceeding sea 
state 6 ambient noise at relatively high fre- 
quencies, It can be, and has been to some 
extent, reduced by streamlining the sonar 
dome and other protrusions and by hull de- 
signs which improve the ship's seakeeping 
capabilities. In spite of this, however, the 
operation of hull mounted sonar at high speeds, 
especially in high seas, can only be acconr 
plished at the expense of detection ranges such 
that it is of questionable value in the vicinity 
of sea state 6, 
229 
As in the case of the other noise sources 
discussed, a prior knowledge of wave con- 
ditions to be encountered will allow avoidance 
of unfavorable conditions or time for tactical 
changes for the best utilization of ships and 
systems under adverse sea and sound condi- 
tions, 
Radar 
In the detection of snorkeling of sur- 
faced submarines, airborne radar has the 
advantage of providing precise range and 
bearing values as well as a high search rate. 
In a manner similar to that of the surface 
backscattering of acoustical energy, electro- 
magnetic energy is also backscattered by a 
rough sea surface, !! In a phenomena known 
as ''sea clutter" transmitted energy is re- 
flected back to the radar from waves causing 
a bright spot in the center of the PPI scope, 
the extent of which increases with antenna 
altitude and sea state. | 
Unlike the backscattering of sound from 
the sea surface, the returned electromagnetic 
energy is dependent upon the relative bearing 
and direction of the reflecting waves with 
reception being better when looking in the 
direction of wave propagation. If sound 
experiences this effect the results are 
obscured by continual fluctuations in 
intensity. Fig. 3 shows the effect of sea 
state and look-direction on the blip-scan 
ratio of an AN/APS-1l5a radar at an altitude 
of 500 feet. As indicated, the effectiveness 
of airborne radar is considerably restricted 
by state 3 seas, and of dubious value at sea 
state 4, Some improvement has been made 
through the introduction of the doppler 
principle, but sea state is still a major 
deterrent to the use of airborne radar for 
surfaced submarine detection, as well as 
for detecting submarine snorkles or periscopes. 
