C/W Robertson
    C/W Watson
    U-boat  "U" 249
Top Secret.
Rec. 18.7.44.
    Reg No:  Top Secret 82
    Encl.  ---______________
Current Orders No. 24 of March 1944.
Prowling in greater depth - Experiments have proved that the revolutions for prowling at periscope depth can be increased at greater depths without increasing the possibility of the boat being intercepted, that is, at a diving depth of 20m. 25, 30m. 40, 40m. 55, 50m. 60, 60m. 65 more revolutions than for prowling at periscope depth.  (Increasing to more than 70 revolutions for prowling even at greater depths, is not practical.)
          Further experiments have proved that when proceeding submerged then noise of one U-boat screw is more strikingly audible over a great distance than when both screws are running.  When being chased for purposes of acoustic interception, therefore, avoid running on one shaft only as far as possible.  
          When proceeding at great depth, a hydrophone watch is to be kept for the noise of the screw.  If the noise of a screw is picked up, its revolutions per minute must be reduced, until the noise disappears.  When prowling the number of revolutions is to be selected in such a way that no propeller noises are registered in the multi-unit hydrophone.  The limit of increase to 70 revolutions per minute prowling is made because of the booming which commences with any further increase.  (According to experience, commences at approx. 180 revolutions per minute.)  



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