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for an improvement and then proceed with the whole convoy. If, 

 however, such a change is not anticipated, the ships must be conducted 

 ahead one by one, eventually resulting in the speedier advance of all 

 the ships. In this case precautions must be taken to prevent the ships 

 left behind from being damaged by the ice. A more or less homo- 

 geneous mass of slush pressing against the ship creates a kind of 

 cushion, with equal pressure along the ship's entire length. If the ship 

 is near a heavy floe or an ice field, the pressure developed may result 

 in serious damage or perhaps loss of the ship. Heavy ice under pres- 

 sure creates a strain at certain points or over certain sections of the 

 hull. Forced by the closing ice. large blocks of ice in the channel 

 may be crushed against the hull, denting or even penetrating the ship's 

 side. Under these circumstances, the icebreaker .must make a few 

 trips around the ships so as to break the large pieces. Then it can 

 take the ships on one by one without fear that those left behind will 

 be damaged or crushed. 



The most difRcult work for an icebreaker is to conduct ships in 

 motionless young ice with no leads or cracks. Broken ice remains in 

 the channel with the exception of a small amount which goes under its 

 edge. If the channel is to be of considerable length, this brash not 

 only hinders the convoyed ships but also makes the progress of the 

 icebreakers more difficult. If small hummocks are encountered, the 

 icebreaker often becomes wedged in. To avoid wedging and to facili- 

 tate the movement of the ships, it is necessary to break a channel con- 

 siderably wider t'han the beam of the icebreaker. Under such cir- 

 cumstances, the width of the channel must be sufficient for an ice 

 breaker to turn, 100 to 150 yards. To achieve this, a double or triple 

 channel is broken. The double channel is made by the "herringbone" 

 method. 



To break a channel by this method, the icebreaker first strikes the 

 ice at a small angle to port, then backs up and strikes again at an 





Figure 22. — Herringbone method of breaking ice. 



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