Sec. 76.27 



DESIGN OF SPECIAL-PURPOSE CRAFT 



799 



ratio is moderately large they also roll sharply 

 unless the polar moment of inertia about the 

 fore-and-aft rolling axis is large. 



The usual type of roll-resisting keel is vulnerable 

 in the ice. Any type of retractable projection is 

 almost as vulnerable. Plate-type keels, as dis- 

 tinguished from those of triangular section, 

 may be added at the start of each voyage and 

 used to quench roll until such time as they are 

 bent over or stripped off by the ice. 



Most icebreakers carry fuel and other liquids 

 in certain wing tanks connected by pumps. 

 Shifting these liquids from one side to the other 

 produces enough heehng moment to rock the 

 vessel and help free it from the ice. If sufficient 

 power and weight could be spared to make this 

 shift in the order of half the natural rolling period, 

 the active roll-quenching tanks thus formed 

 would go far toward making the icebreaker a 

 more hvable ship in the open sea. 



Protection against large blocks of ice striking 

 a single propeller at the stern is afforded by fitting 

 one or more fins, generally horizontal, ahead of 

 the propeller, projecting on each side from the 

 centerline skeg [M. S. Kista Dan, SBSR, 19 Jun 

 1952, pp. 778-779; 25 Dec 1952, p. 831; M. S. 

 Theia, The Motor Ship, London, Jan 1954, p. 458]. 

 Since the flow of water along such a skeg is aft 

 and upward, the fins should lie in the natural 

 streamhnes, as determined from model lines of 

 flow and as checked by tufts or equivalent 

 methods. A "ladder" of three or four fins, one 

 above the other, parallel to the shaft hne and 

 extending forward for perhaps a propeller diam- 

 eter, might have a beneficial propulsion effect 

 in producmg more nearly axial flow in ice-free 

 water. Such a layout, however, requires careful 

 checking in the design stage, preferably on a 

 model in a circulating-water channel. The 

 trailing edges of all such fins require fining to 

 avoid separation and eddying ahead of the wheel. 



Rudders of icebreakers and iceships are in- 

 variably completely submerged. They are pro- 

 tected after a fashion by horns, preferably 

 integral with the hull, projecting downward 

 heyond the rudders and below their tops, so as to 

 break up the ice when going ahead or astern and 

 prevent blocks from wedging themselves between 

 the top of the rudder and the hull. To meet the 

 exacting maneuverability requirements they . 

 should be larger than normal but are often just 

 the opposite, to make them less vulnerable to 

 damage from the ice. 



Underwater inlets and discharges for the cooling 

 water of heat exchangers are often shut off by 

 ice cakes resting across the openings or by broken 

 ice lodged in the strainers. It is customary to 

 provide a water box inside the shell to which the 

 circulating lines can be connected. Water dis- 

 charged into this box is cooled by contact with 

 the shell before it is used over again. Similar 

 water boxes are fitted on vessels required to 

 operate in very shallow water where the outboard 

 connections could be plugged temporarily and 

 where the system could be filled with mud, silt, 

 or sand. 



Published model-test data on icebreaker hulls 

 are rather meager. There is available to the de- 

 signer SSPA Report 20, published in 1952 by 

 H. F. Nordstr5m, H. Estrand, and H. Lindgren, 

 entitled "Model Tests with Icebreakers." This 

 is abstracted in SBSR of 5 June 1952, pages 

 726-728; the Swedish report is, however, entirely 

 in EngUsh. 



76.27 Tabulated Data and References on 

 Icebreakers. The first tabulation of dimensions, 

 characteristics, and other data on icebreakers 

 was made by R. Runeberg in references (2) and 

 (5) of the fist in the latter part of this section. 

 These data are contained in a single table on 

 page 285 of reference (2), pubhshed in 1889; 

 otherwise the small tables of data are somewhat 

 scattered throughout both references. 



The first really comprehensive tabulation 

 appears to be that of H. F. Johnson [SNAME, 

 1946, pp. 112-151]. A large 3-page table on pages 

 114-116 contains 44 entries for 39 icebreakers and 

 iceships. In the same year (1946) I. V. Vinogradov 

 pubhshed in Moscow a Russian book entitled 

 "Vessels for Arctic Navigation (Icebreakers)," 

 which has a rather complete list of tabulated data 

 on pages 22-23 and 26-34. The ships described 

 in these tables date from 1871 to 1938. The book 

 appears to cover the theoretical and analytical 

 aspects of icebreaker design rather well, and it 

 contains a great deal of miscellaneous tabulated 

 data. At the time of writing (1955) only the table 

 of contents has been translated into English. 

 The Library of Congress number is VM451.V5. 



Table 76.f contains some dunensions, propor- 

 tions, and characteristics of modern icebreakers, 

 supplementing the 1946 list of H. F. Johnson. 

 These data were gathered from pubhshed sources 

 so they are incomplete and in many cases in- 

 consistent. This is due partly to a lack of strict 

 definitions of displacements, powers, and other 



