U boat returning home usually reported (on Coastal Frequency) when they were 48 hours or 36 hours off port - with the groups UKVL or UKUK respectively in single group messages. As boats tended to meet their escort at fixed time of day, it was possible to distinguish on time between messages saying UKVL and those saying UKUK, the whole class of signal being distinguished by frequency, time, length and D/F.
2 group signals from similar positions normally included beacon requests and
were of the form
or UL.. UL..
Reference to beacon requests was sometimes made in Dolphin signals, and Dolphin Signals to escorts occasionally gave the names of boats arriving and made it possible to crib signatures.
Single group signals from near the coast usually reported FK.. (...hours later or earlier than expected).
Signals of one or two groups from about 18 W were normally position reports from boats leaving the Bay.
A chart was kept showing time and decode of all B-bars on the Coastal Frequency and even in June when a new procedure for boats entering the Bay was adopted, it was possible to trace and crib the type of signals associated with it.
In June certain boats were sent to mid-Atlantic to make dummies at regular intervals, at a time when there was little other traffic. Dummy groups were all of the form W.... This information was known from previous Shark decodes.
(e) Mediterranean Signals.
Before Med. U Boats went onto Turtle, there were occasional Short Signals saying UKWM (30 hours off port) if they were single group messages, or giving a position if they were 2 group messages.
The difficulties of compiling satisfactory B-bar menus were due to the need to use links from several messages, every message and often every link having an independent chance of being wrong. Several menus were necessary in cases where there was a limited set of hypotheses with considerable joint chance of being correct. The most convincing menus sometimes came from level starts, but these were unfortunately rare, especially when the traffic was small. The following example, in which a level start of 3 (AlBlC), with one other message D1 was used to bring out 17th March 1943 is an amusing example of B-bar cribbing.