Sec. 76.26 



DESIGN OF SPECIAL-PURPOSE CRAFT 



795 



Use" (designations T-AKD-1 and T-AK270), 

 Maritime Reporter, 15 Sep 1955, pp. 11-13]. 



No specific requirements other than the general 

 functions Usted in the two paragraphs preceding 

 are given for icebreakers and iceships because 

 the service varies rather widely. A vessel suitable 

 for breaking a lane and convoying large vessels 

 through the Northwest Passage, for example, is 

 entirely unsuitable for clearing out a small 

 harbor and working around slips. The details of 

 ship handling in various kinds of ice are men- 

 tioned in a number of the references of Sec. 76.27 

 following, but are given in more methodic fashion 

 in the 1948 Edition of the British Antarctic Pilot, 

 Chap. I, pages 42-54, under "Ice Navigation." 



While the process of bucking ice is in no sense a 

 hydrodynamic action there are major hydro- 

 dynamic problems involved. Considering the 

 solution of these problems and the selection of 

 design features for an icebreaker (not an iceship) 

 in somewhat the same order as in Chaps. 64 

 through 68 for a surface ship, the appended list 

 supplements one previously given by D. R. 

 Simonson ["Bow Characteristics for Ice Break- 

 ing," ASNE, 1936, Vol. 48, pp. 249-254]: 



(1) Waterline length 



(2) Normal displacement 



(3) Length-beam and beam-draft ratios 



(4) Engine power 



(5) Thrust available from the propeller(s) at low 

 speeds 



(6) Transverse section shape 



(7) Number and position of propellers 



(8) Forebody shape 



(9) Appendages. 



These items have to be balanced against carry- 

 ing capacity, steering and maneuvering qualities, 

 allowable draft, and economical power. 



The length is limited to the practicable mini- 

 mum so that the vessel can work to advantage 

 in open-water spaces of small size. The waterline 

 length and the weight are also important because 

 they largely determine the downward icebreaking 

 force which can be exerted at the bow when the 

 latter is pushed up on the ice and the vessel lifts 

 forward, usually by an angle less than 5 deg. 



Although there are limited authentic data for 

 analysis it appears obvious from a consideration 

 of the mechanics involved, presented by D. R. 

 Simonson in the reference cited, and by R. Rune- 

 berg in references (2) and (5) of Sec. 76.27, that 



the smaller vessels are not intended to break ice 

 as thick as retiuired for the larger ones. 



More important than the length of an ice- 

 breaker is its length-beam ratio, which must be 

 small for superior maneuverability in driving 

 through leads in the ice. When forcing its way 

 into a harbor or inlet the ship must go around 

 corners, or must back and fill and turn to clear 

 an open space for other vessels. The icebreaker 

 may even have to back up or turn around to free 

 convoyed vessels that have become stuck in the 

 ice behind it [Sokol, A. E., USNI, May 1951, 

 p. 482]. 



Fortunately, the very large beam relative to 

 the length needed for this purpose is also required 

 to clear a lane wide enough for a convoy following 

 behind. This lane can not always be straight, so the 

 corners must be cut off for longer vessels in the 

 rear. 



A study of beam-draft data for a large number 

 of icebreakers (and designs), reveals rather wide 

 variations with weight displacement. Considering 

 only those vessels whose performance is known 

 to be good or excellent, optimum values of L/B 

 ratio with weight W (or A) may be taken from 

 the vicinity of the curved broken line in Fig. 

 76. Q. This ratio increases from about 3.6 in 



■^0 1000 20OO 3000 40O0 5O0O 600O 7000 8000 9000 10000 

 Normal Displacement, tons 



Fig. 76. Q Plot of Length-Beam Ratios fob 

 icebreakebs 



small vessels to 4.5 in the largest vessels of this 

 type. 



A similar study of B/H ratio on waterhne 

 length reveals no clear optimum value, possibly 

 because the load conditions are rather variable, 

 and possibly because the draft needs to be as 

 large as practicable to allow the use of large- 



