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(1) General. — ^Vessels constructed "^itli special strengthening which 

 is at least as effective as that described in this section will be distin- 

 guished in the record by the words "Ice Strengthening."' 



(2) Special strengthening for navigation amongst ice should cover 

 an area which extends from the stem to the midship three-fifths length 

 (i. e., one-fifth of the length from the stem) between lines which are 

 respectively at least three feet above the load line, and three feet below 

 the light load line. 



(3) Intermediate frames having a strength of at least 75 percent of 

 the strength of the fore peak frames should be fitted over this area ; the 

 intermediate frames should extend from the deck next above the 

 strengthened area to a lower level than the top of the frame brackets or 

 floor plate. 



(4) Side plating should be of midship thickness forward to the 

 strengthened area ; the thickness of the shell plating over the strength- 

 ened area should not be less than 0.6 inch in vessels under 250 feet 

 in length, and need not be more than 1 inch in vessels over 500 feet in 

 length ; the thickness for intermediate lengths may be obtained by 

 interpolation. 



(5) Rudder scantlings, rudder stock, steering gear chains, etc., 

 should all be at least 10 percent above the ordinary requirements of 

 the rules. 



(6) Tailshafts in single screw vessels should have a diameter at 

 least 5 percent and those in twin screw vessels at least 10 percent 

 greater than required by the rules. 



(7) Propeller hlades made of cast iron should not be used. 



(8) Sea connection's should be so arranged as to minimize the risk 

 that attends their attachment to plating which is subject to ice damage. 

 Main injections should be provided with steam connections for clear- 

 ing the strainers. 



Arctic-constructed vessels with icebreaker assistance could penetrate 

 any part of the Bering Sea, according to the experience of the Burton 

 Island in January-February 1949. A 3 : 1 ratio of these vessels to 

 icebreakers could be maintained, as compared with a 1:1 ratio 

 necessary with unstrengthened ships. 



A few cargo vessels, built with an ice-breaking bow, form a class 

 intermediate between the sea-going icebreakers and the strengthened 

 cargo vessels. One of these was the Nascopie, which was operated in 

 northern Canadian waters by the Hudson's Bay Company from 1912 

 until her loss in 1946. 



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