548 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 6S.4 



It is mentioned, in the discussion of reserve 

 buoyancy in Sec. 68.2, that a certain minimum 

 freeboard of the intact hull amidships, or at 

 the lowest point of the sheer line, is more often 

 than not regulated by law. This takes into account 

 considerations of the range of transverse meta- 

 centric stability, floodability and damage control, 

 rolling and righting energy, classification and 

 insurance rules, and the like, which need not be 

 gone into here. All these and other features are 

 discussed by H. F. Norton in Chapter II and by 

 J. F. Macmillan and J. P. Comstock in Chapter 

 V of PNA, 1939, Vol. I. Normally the minimum 

 freeboard based upon the considerations set forth 

 therein is sufficient to meet the wavegoing re- 

 quirements for the service of any particular ship. 

 However, the freeboard may be and often is 

 determined by the difference between the hull- 

 girder depth D necessary for strength and rigidity, 

 and the draft H. This is the basis for selection of 

 the freeboard of the ABC ship amidships, ex- 

 plained in Sec. 66.30. 



To this freeboard there must be added sheer 

 at the bow, and generally also at the stern, if the 

 vessel is not to be inundated when pitching at sea. 

 As a rule, the lower the minimum freeboard 

 the higher must be the sheer forward. The free- 

 board for wavegoing, discussed more fully under 



design for wavegoing in Part 6 of Volume III, is 

 therefore usually measured at the bow rather 

 than amidships, although the freeboard for good 

 wavegoing is sometimes reckoned at 0.3 the length 

 from the FP, or at 0.1 that length. The latter 

 procedure is based upon the reasoning that the 

 most objectionable water comes over the side at 

 those positions. 



Fig. 68.A is a diagram for selecting a value of 

 the freeboard at the forward perpendicular in the 

 preliminary-design stage. Its scale of abscissas is 

 dimensional, for the reason that the average 

 steepness ratio of natural waves increases as the 

 wave length decreases. In other words, short 

 waves are steeper than long waves. For a short 

 craft like a fishing boat the ratio of freeboard 

 forward to length must be large while for a large 

 liner this ratio can be diminished considerably. 

 The heavy curved line of the figure is intended to 

 indicate this relationship. It is broken because its 

 position is still tentative. In general, the ships 

 with ratios above the line have proved to be 

 good-to-excellent sea boats in service. Many of 

 those below the line are definitely lacking in 

 freeboard forward. 



The minimum freeboard forward (and aft) is 

 also determined by a combination of minimum 

 freeboard amidships and a sheer height forward 



I I III I l| I I III I II I 

 200 I 250 I 300 



Waterlme Leriolh, meters 



0.09- 



0.08- 



0.07- 



l"iT'1\ 



0.06- 



0.05- 



004-^ 



llllllll 



50 



\ 



n^rr 



\ 



o \ 



T1 



I I II I 



150 



Spot for 

 ABC Ship 



-^. 



T' 



'0.06 



-0.05 



004 



100 2^00 300 400 500 600 700 600 SCO 1000 1100 1200 



Woterline Length in feet 



Fig. CS.A Tentative Freeboard Ratio j'or Ships Traversing the Open Sea 



