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to assist in penetrating ice wlien the tips of the blades are below the 

 Tinder surface of the ice, both by inducing turbulence to break up the 

 ice and by washing it back out of the way after it is broken. 



The double bottom of these vessels is carried up above the load water- 

 line, the two skins being 15 inches apart. The outer plating is 1% 

 inch high-tensile steel at the waterline, tapering to VA inches on the 

 bottom. The pronounced flare to the underwater sections, resembling 

 that of the Maud, is matched by considerable tumblehome in the top- 

 sides, for the purpose of reducing the fouling of top hamper wlien 

 Avorking around heavy ice or other vessels. The bow of the Wind class 

 has the characteristic sloping forefoot of icebreakers, which acts to 

 slide the bow of the ship up onto ice too heavy to break by t'he 

 forward motion of the ship alone. The weight of the ship thus exerts 

 a bending action on the ice, which is much weaker in tension than in 

 compression and therefore breaks. Since icebreakers must at times 

 back into ice, the shape of the forebody is reproduced in the afterbody. 



In the Wmd class there is a notch in the stern for towing. This is 

 heavily padded to receive the stem of any vessel that has to be towed 

 into ice, thereby eliminating the possibility of the tow running down 

 the icebreaker should it be suddenly stopped by striking an unusually 

 heavy piece of ice. The extra power from the vessel close-coupled 

 astern can also be of assistance in breaking ice. 



The vessels can be assisted in their attack on the ice by reducing stick- 

 ing from static friction through the use of wing tanks and heeling 

 ])umps, which transfer 220 tons of water from one side to the other and 

 produce a 10° roll in I14 minutes. Ballast can also be shifted between 

 fore and after peak tanks to change trim, to assist in backing off ice, or 

 to present the most advantageous angle of attack under different 

 loading conditions. 



Vessels of this class can successfully maintain a speed of 4.8 knots in 

 5-foot thick ice, using the bow propeller, although there is some ques- 

 tion as to value of such a propeller in heavy pack in the open sea. 

 Without the bow propeller, they can open channels in 10-foot broken 

 polar ice at a speed of advance of 1 knot, by backing and ramming. 



CARGO SHIPS 



Modern steel merchant vessels are not suited for unassisted naviga- 

 tion in any but the most open kinds of floating ice. The chief source of 

 weakness is the bow plating, but other structural deficiencies can be 

 gathered from the following extract from the rules of the American 

 Bureau of Shipping: 



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