Period H. 



Period II lasted from the beginning of July 1940 through 

 March 1941, 



During Period I Allied patrol planes had achieved con- 

 siderable success in forcing U-boats during daylight to travel 

 below the surface where they could be detected and tracked by 

 ASDIC-fitted surface vessels. The resulting short life-expect- 

 ancy of their U-boats forced the Germans in Period II to adopt 

 a complete change of tactics, 



They abandoned the method of attacking from periscope 

 depth during daylight. Instead, Period II saw U-boat command- 

 ers switch to surface attacks at night , Trimmed down on the 

 surface, the U-boats launched their torpedoes under cover of 

 darkness, then used their comparatively high surface speed 

 to escape. The period was also distinguished by the success- 

 ful raids of such submarine "aces" as Prien, Kretschmer and 

 Schepke, Initially , at least, the modified tactics of the 

 enemy were highly successful, 



The Allied anti-submarine defense had also to contend 

 with another problem. The fall of France permitted the 

 Germans to use the French ports. Thus they were able to 

 extend the westward range of their U-boats, while occupancy 

 of French airfields made it possible for them to send out 

 long-range aircraft to spot approaching convoys. Allied 

 merchant vessels were compelled to take a longer route 

 around the north cf England, 



Another disadvantage suffered by the Allies was that 

 this was the period when an Invasion of England appeared 

 possible and ships and planes heretofore available for es- 

 cort and anti-submarine patrol work had to be assigned to 

 defensive stations to guard the south and east coasts of the 

 British Isles, Furthermore , Italian submarines joined the 

 Germans in the Atlantic , 



Allied countermeasures were stepped up, New systems 

 of evasive tactics for convoys were devised. Patrol planes 

 were fitted to carry depth charges, British aircraft launched 

 punishing attacks on U-boat bases in the French ports of 

 Lorient and Bordeaux, 



British scientists produced a helpful countermeasure in 

 an improved H/F D/F, or High Frequency Direction Finder, a 



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