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into the ice sufficiently. It was found that the successful mooring to 

 ice depends on Aveatlier conditions and wind direction and speed. 

 On an average, the Edisto^ out of 6 days mooring to ice, had to renew 

 the "dead men" and shift position about every 36 to 48 hours when the 

 ice would break up and start drifting out. 



EXPLOSIVES 



Icebreakers should carry about two hundred 54-pound wrecking 

 mines with at least six mines on deck, forward, together with the 

 wrecking mine outfit. A rigid ladder lightly constructed, must be 

 secured to the forward bulwarks for instant use. If possible the 

 demolition team should be trained before getting into heavy pack. 

 Mines should be lowered to approximately 6 feet below the lower sur- 

 face of the floe. One mine is generally sufficient to crack a floe 100 

 feet across and 30 feet thick. Great care should be exercised in 

 using mines abaft the midships section if the icebreaker carries air- 

 craft, because of damage resulting from falling debris. If such 

 mines are used to liberate the beset vessel or provide swinging space, 

 the aircraft must be covered with a tarpaulin. TNT demolition blocks 

 may be substituted for the wrecking mines but at least six are required 

 to loosen even young ice. In order to place a charge properly, the ship 

 must have an ice drill to place the charge deep enough to be effective. 

 The icebreaker's demolition team should practice blasting as soon as 

 practicable after reaching the pack. The team of the Northioind was 

 so trained that it required only 3 minutes and 36 seconds from the 

 time of nosing into the ice until the charge was exploded. 



ENGINEERING PROCEDURES 



The following engineering notes are based chiefly on the report 

 of U. S. S. Burton Island^ operating in the Antarctic in 1948: 



(a) Ice-breaking operations usually call for full power except when 

 in column astern of another icebreaker. Under these conditions four 

 main generators will generally carry the load, with the two remaining 

 generators on 5 minutes' notice and a full steaming watch maintained 

 in the standby generator room. The main engines are started once 

 each 4-hour watch and run until the lubricating oil of the engine is be- 

 tween 120° and 140° F. (The Northivind found it necessary to limit 

 shaft speed to 100 revolutions per minute when breaking ice with only 

 four generators on the line to prevent overheating.) 



(b) While the engines are secured the warm-up system is cut into 



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