796 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.26 



diameter propellers with tip submergences great 

 enough to clear surface ice. 



With these small L/B ratios and relatively deep 

 drafts it is natural that the block coefficients 

 should be low and the displacement-length 

 quotients or fatness ratios high. The Cb values 

 are of the order of 0.5 and the 0-diml fatness 

 ratios range from 8 or less to about 15. 



Like a tug, the icebreaker has waterlines, both 

 above and below the DWL, which are well 

 curved throughout. These enable it to follow 

 desirable leads or to extricate itself when tem- 

 porarily caught. 



The ship often has to back off, stop, and gain 

 the maximum possible speed ahead in a short 

 distance so that its momentum can be added to 

 the thrust forces to push the ship's bow up over 

 the ice. This means an ample reserve of power 

 and propellers that are large enough to develop 

 very high thrust values. In certain kinds of ice 

 the friction of a thick pack may exceed the water 

 friction around the hull, especially at low speeds. 

 On the basis that it is usually desirable to 

 break the ice without too much charging or 

 ramming, the thrust to keep the ship going at 

 low speeds in ice (say 3 kt) and at high real-shp 

 ratios or /-values is a function of the propeller- 

 disc area, the propeller power, and the overall 



Normol Displocement. tons 



Fig. 76.R Plot of Power-Displacement Ratios for 

 Icebreakers 



efficiency of the power plant. At low speeds, this 

 efficiency is only from 10 to 25 per cent. Simonson 

 (in the reference quoted) gives an approximation 

 of 1 (long) ton of thrust from the propeller(s) per 

 100 horses of indicated power in the engines. 



When Scotch boilers and reciprocating engines 

 furnished the only available propeUing power, 

 their large volume and heavy weight made it 

 possible to get only limited indicated power 

 within an icebreaker hull. This is beheved the 

 reason why the data of Fig. 76.R show powers 

 much less than those considered necessary by 

 modern standards, represented by the sloping 

 design fine. The latter indicates roughly a dimen- 

 sional ratio of about 2.35 horses dehvered at the 

 propeller (s) for each (long) ton of weight dis- 

 placement, although a ratio of 2.5 is not unduly 

 high. The former should give an unspecified but 

 adequate free-running speed for any icebreaker, 

 regardless of its duties in ice-free waters. 



The transverse sections necessary in an ice- 

 breaker to prevent crushing of the ship structure 

 as a whole by pack ice put under pressure over 

 large areas are peg-top in form. They taper inward 

 and downward rather sharply from a level above 

 the designed waterplane, following Colin Archer's 

 design for the famous Norwegian polar expedition 

 ship, the Fram, first used by F. Nansen and later 

 by R. Amundsen [Nansen, F., "Farthest North," 

 Harper, New York, 1897, Vol. I, pp. 57-71. On 

 the plate opp. p. 60 there are an inboard profile, 

 a deck plan, and two sections of the Fram]. 

 Archer designed this hull in the form of a vertical 

 wedge which, when subjected to lateral pressure 

 from large floes of moving ice, causes the vessel 

 to be lifted bodily and hence to avoid crushing 

 of the hull and ultimate destruction. The close 

 resemblance between the midsection of the 

 Fram, designed over 60 years ago, that of R. E. 

 Peary's Roosevelt, designed in the early 1900's 

 [Mar. Eng'g., May 1905, p. 193; Zeit. des Ver. 

 Deutsch. Ing., 8 Jul 1905, p. 1135; Scientific 

 American, 15 Jul 1905, pp. 47-48], the midsection 

 of the U. S. Coast Guard icebreakers of the 

 Northwind class [Johnson, H. F., SNAME, 1946, 

 pp. 112-151], and the body plans of many other 

 icebreakers, old and new, are proof of Archer's 

 excellent reasoning and designing abiUty. 



R. Runeberg [ICE, 1900, p. 122] gives examples 

 of early icebreakers (prior to 1900) with trans- 

 verse section slopes at the vicinity of the DWL 

 of from 79 to 90 deg. He says that there is no 

 need to make the slope less than 79 deg and that 



