REMOVAL OF THE NORTH SEA MINE BARRAGE 



115 



None of the ships had been damaged, 

 although numerous instances of counter- 

 mining had occurred. 



From the results of these first two days 

 it was obvious that at the present rate of 

 sweeping it would be impossible to com- 

 plete the work within the year ; so Ad- 

 miral Strauss cabled a request to Wash- 

 ington that sixteen additional sweepers 

 be fitted out and dispatched as expedi- 

 tiously as possible. He also made ar- 

 rangements to charter from the British 

 Admiralty twenty newly built steam 

 trawlers and man them with our own 

 crews, these vessels being required as 

 marker boats to enable the sweepers to 

 maintain their positions while maneuver- 

 ing upon the field. 



A BARRIER 260 FEET DEEP IMPENETRABLE 

 FOR SUBMARINES 



By the 10th of May the sweepers were 

 ready to go out again. This time a 

 definite area was to be cleared. 



The barrage was composed of thirteen 

 separate groups of American mines. 

 Each group consisted of from two to six 

 parallel rows of mines, and the mines in 

 each row were laid at one of three 

 levels — upper, middle, or lower — the 

 three forming a complete barrier in a 

 vertical plane to a depth of 260 feet. 



The average group contained five rows, 

 and of these three were laid at the upper 

 level to give the surface barrage the 

 greatest density. The reason was psy- 

 chological : Submarines, knowing the bar- 

 rage was there, would prefer to risk 

 crossing on the surface, even if they 

 knew their chances were less. 



The upper-level mines were now our 

 gravest concern, for the damage done a 

 sweeper by the explosion of one of these 

 would, of course, be far more serious 

 than from a lower-level mine. 



Group 12 (see chart, page 105) was 

 selected to be cleared on this coming op- 

 eration, since it consisted of only three 

 rows of mines, only one of which was 

 laid at the upper level. 



With the danger from countermining 

 reduced to the minimum, the experience 

 gained in sweeping this group might pro- 

 vide a further means of safeguarding 

 the ships before the more dangerous 

 groups were undertaken. 



£\H < 



A GIANT HAEIBUT, WEIGHING MORE THAN 



4OO POUNDS, CAUGHT NEAR THE 



ORKNEY ISLANDS 



In order to reduce the possible effects 

 of countermining still further, each pair 

 of sweepers was to work independently 

 of the others, so that all pairs should be 

 evenly spaced along the length of the 

 field. Then, if an exploding mine should 

 cause others in its vicinity to countermine, 

 the possibility of damaging other sweep- 

 ers than the one pair was very remote. 



The method of sweeping to be used 

 was what is called transverse sweeping — 

 that is, the sweepers were to cross the 

 lines of mines perpendicular to their di- 

 rection, then turn, recross. and so on. 

 This method is much more laborious than 

 attempting to keep a line of mines be- 



