798 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.26 



Indeed, the bow propellers of the ferryboat and 

 of the icebreaker, although installed for entirely 

 different normal functions, involve many of the 

 same hj^drodynamic principles in their action. The 

 ferryboat bow propeller often comes in handy for 

 clearing loose ice out of the head of its slip. 



For the designer faced with the problem of 

 considering one or two bow propellers, or of 

 designing an icebreaker with them, a most 

 useful document is SSPA Report 20 by H. F. 

 Nordstrom, H. Edstrand, and H. Lindgren, 

 entitled "Model Tests Avith Icebreakers." It was 

 published m 1952 and is entirely in English. 



Whether a bow propeller is fitted or not, D. R. 

 Simonson mentions in the reference cited earher 

 in this section that it is often necessary to fill 

 out what would be the forefoot, just above the 

 baseline, m order to obtain sufficient displacement 

 volume forward. The resulting bow profile cor- 

 responds somewhat to that of an icebreaker with 

 a bow propeller. The vertical stem portion 

 extending for a distance above the keel performs 

 another useful service in that it prevents a 

 vessel with a constant slope of about 30 deg in 

 the bow profile from riding up so far on a deep 

 accumulation of ice cakes that it can not be backed 

 off. The vertical portion of the stem acts also as 

 a cutter to loosen up the lower layers of ice in 

 deep windrows. Fig. 76.S shows the vertical 



portion of the stem abaft a bow propeller position. 

 This portion should be retained, as previously 

 noted, even without a bow propeller. The peg-top 

 underwater sections, the tumble home above the 

 waterline, and the massive bossings are also well 

 shown in the figure. 



The forward buttocks should be sloped as 

 much or more than the bow profile, extending 

 back to the section of maximum beam, so that 

 the whole forward part of the ship acts effectively 

 to break ice. The characteristics of the forebody 

 should be duplicated as much as possible in the 

 afterbody since the vessel will be required to 

 break ice when backing. 



The follo\ving is quoted from page 254 of the 

 Simonson reference: 



"It is desirable to work the same angles into the 

 buttocks of the fore body to obtain equalization of lifting 

 forces when the ice carries past the bow without breaking 

 clear of the hull. As for the frame sections, they should 

 show a marked flare at the waterline to relieve the crushing 

 force of the ice." 



It is obvious that vessels having V-shaped 

 midship and other sections similar to those of 

 icebreakers, with their large bulge radius, possess 

 httle in the way of roll-damping characteristics. 

 Without roll-quenching devices of some kind 

 they roll deeply and heavily, to the great detri- 

 ment of their habitability. When the beam-draft 



Fig. 76.S Bow Quarteu View of Hull of an Icebreaker of the Wind Class 



