Sen. 7r,.2r, 



DESIGN OF SPECIAL PlIRrOSF. CRAFT 



797 



a slope of 85 deg should suffice for "all ordinary 

 ice-breakers." H. F. Johnson, in 1946, sa3's that 

 the slopes for the flaring sides of the midship 

 section should be between 70 and 80 deg. 



A pronounced tumble home above the DWL is 

 desirable to prevent the foulmg of top hamper 

 when working around other vessels. One other 

 feature of the abovewater body not to be over- 

 looked at this point is the minimum freeboard 

 when heavily loaded. Entirely aside from wave- 

 going requirements, a reasonable amount of hull 

 extending above the ice level is required to insure 

 that, if held fast in the ice, the ship is not over- 

 whelmed by overriding floes and pressure ridges 

 rising above the water level. Nansen's Fram, 

 although undamaged by lateral squeezing in its 

 drift over the Arctic Ocean in 1893-1896, never- 

 theless was nearly overwhelmed and sunk by 

 ice coming in sLx feet deep over the rail on 3-7 

 January 1895 [Nansen, F., "Farthest North," 

 1897, Vol. II, pp. 47-60]. 



The early, heavily powered steel icebreakers of 

 the 1890's and 1900's were almost mvariably 

 equipped with bow propellers, as were many of 

 those of later years. This was on the theory that, 

 after the breaker's bow had ridden up over a 

 thick ice layer and broken it into chunks, partly 

 by impact and partly by sheer weight, the 

 inflow current to the bow propeller (s) would 

 draw the ice down under the ship. The outflow 

 current would push it aft under the ship, leaving 

 the bow free to break more ice. 



It may be well to remember, however, that 

 when F. E. Kirby of Detroit introduced the first 

 bow propeller on the icebreaking car ferry St. 

 Ignace for the Straits of Mackinac in 1888 [Rune- 

 berg, R., ICE, 1900, Vol. CXL, pp. 109-129], it 

 was to perform an entirely different function. 

 Faced with the problem of getting a ship through 

 ice that was piled in layers all the way down to 

 the channel bed, Kirby held the icebreaker's 

 bow to the ice with a powerful stern propeller 

 while the bow propeller, going astern, forced a 

 current of water into the ice mass ahead to loosen 

 it. When some of it was loosened, the bow pro- 

 peller was set to drive ahead, whereupon the 

 inflow current produced by it drew loose ice from 

 the mass and pushed it aft. After the ship ad- 

 vanced until it was again stalled, the process was 

 repeated. Following Kirby's success, bow pro- 

 pellers were fitted to many icebreakers, whether 

 faced with the same operational problems or not. 

 This is another example of the importance of 



knowing why things are done when new ships 

 are designed. 



It has since been found that when the ice is 

 thin enough to be broken up and drawn down 

 under the bow by the inflow current from the 

 bow propeller, without damaging the propeller, 

 then the latter is of great assistance. When a 

 thick layer of heavy snow hes on top of a rela- 

 tively thin layer of sea ice the abovewater bow 

 banks the snow up in front of it until finally the 

 ship can no longer force its way through. The 

 procedure then is to break up the ice by small 

 increments and to suck both ice and snow down 

 and under the ship, finally ejecting it behind. A 

 bow propeller or propellers are of great assistance 

 here as well. If the ice is so heavy that as the 

 ship rides up on it the bow propeller is struck by 

 huge blocks, these blows are Uable, not only to 

 bend or break the propeller blades but to bend 

 the shaft or to dislodge the thrust bearing inside 

 the ship. 



The meaning of the foregoing is that, under 

 conditions which may change from day to day, 

 the ship needs a bow propeller or propellers or 

 else it is encumbered by them. Although the 

 mechanical problems seem almost insurmountable, 

 especially for such heavy-duty machinery, it may 

 nevertheless be possible at some time in the future 

 to develop a housing bow propeller for an ice- 

 breaker. This might be a 2-bladed affair, made 

 controllable and reversible from within, with 

 nearly flat blades having thick sjonmetrical 

 sections. When not in use the blades could be 

 feathered fore and aft and the whole propeller 

 drawn backward into a shallow recess in a lower 

 vertical portion of the stem. This would support 

 the blades and hub against the impact of heavy 

 blocks of ice striking from ahead. When desired 

 for use the bow propeller with its shaft could be 

 pushed forward a short distance by an internal 

 hydrauhc or equivalent mechanism. With the 

 blades then turned to the desired angle, the 

 propeller would be instantly available for pulling 

 the ship ahead, sucking blocks of ice down clear 

 of the upper part of the bow, or helping to back 

 the ship out of a jam in the ice. 



While the thrust deduction due to positive 

 differential pressures +Ap abaft the bow pro- 

 peller of an icebreaker is of no more than second- 

 ary consideration, the free passage of broken ice 

 through and abaft the wheel calls for the same 

 fining of the hull behind the propeller as would 

 be the case were it used for normal propulsion. 



