(3) Shark and Dolphin decodes especially Shark messages with instructions to send Short Signals.
(4) Reported positions of British convoys.
(5) Positions of A/C sightings from Coastal Command and Aircraft carriers.
(1) and (3) were the most important of these, B-bar cribbing was only possible in a period when most of the Shark traffic was brine read.
From March 10th to May 20th there were U Boat patrol lines in the North and Central Atlantic: about 10 B-bars a day when no convoy was being attacked and on one day when 3 convoys were being attacked there were over 70 B-bars. At the end of May U Boats were withdrawn as their losses were too heavy; new instructions (involving long reencodements) were sent out in profusion, but B-bars slumped to an average of about 6 a day (mostly in the Bay of Biscay).
For cribbing purposes certain types of B-bar signal offered the greater opportunities.
(a) Signals from U Boats in contact with a convoy, identifiable as such by D/F, frequency used and the number of signals being received.
Best cribs included:
|5 groups;||CKSA or CCGG||-Position (2 gr.)||-course||-speed.|
|4 groups;||CKSA or CCGG||-Position (2 gr.)||-course|
|3 groups;||?||-Position (2 gr.)|
If a British convoy was involved an accurate idea of the reported position could be got by studying her course as she reported it. Statistical records were kept to show the frequency of various groups and arrangements and charts constructed to show the signals sent at various stages in a convoy attack and how far their reported position differed from the estimated position.
(b) Fuel Reports.
4-group signals from mid-Atlantic were (in the absence of convoy attacks) usually fuel reports of the form
|Position||UGUI - Number below 120.|
|or||Position||HKUX - Number below 120.|
(c) Reports from boats in the Bay of Biscay.