328 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 58.4 



the trim by the bow increase with the displace- 

 ment-length quotient or fatness ratio. 



(3) At low and moderate speeds, below a T, of 

 1.0, F„ of 0.3, both bow and stern settle, the bow 

 somewhat more than the stern, for the reasons 

 given in Sec. 29.2 of Volume I 



(4) Vessels having Cp values higher than 0.65, 

 with full ends, level off at a T, of 1.0 or just below. 

 For T, values in the range 1.0 to 1.2, there may 

 be oscillations or perturbations in the trim values. 

 At greater values of T^ the stern may be expected 

 to drop much more than the bow. 



(5) As the speed is increased beyond a T, of 1.0, 

 for vessels with Cp values of 0.65 and below, the 

 bow settles more slowly. It reaches its lowest level 

 at a Tj of from 1.05 to 1.30 (averaging about 1.15) 

 and then rises rapidly. The bow reaches its at-rest 

 level in a T, range of 1.3 to 1.5, beyond which it 

 continues to rise. 



(6) The stern settles more and more rapidly 

 beyond a T^ of about 1.1 or 1.2. Thereafter it 

 settles much more rapidly than the bow rises, so 

 that the ship as a whole continues to settle while 

 the trim by the stern is rapidly increasing. 



(7) At a T, of about 1.7 to 1.8, the stern is 

 settling less rapidly than the bow is rising, so that 

 bodily settlement reaches its maximum. The 

 stern does not change level much beyond a T^ 

 of 2.0, while the bow always rises with increase 

 of speed. As a result the vessel is rising bodily at 

 speeds beyond a T^ of about 2.0. 



(8) The center of gravity of ordinary (non- 

 planing) vessels rarely rises to or above its original 

 at-rest level at any practicable speed. Since the 

 effect of the passage of the vessel is to depress the 

 water immediately surrounding it, there may 

 be an impression, at very high speeds, that the 

 vessel does rise above its original level. 



(9) Vessels of special form and planing craft, 

 when driven to their designed high speeds, do 

 rise bodily. Their behavior is described and illus- 

 trated in Sec. 29.3, on pages 415-417 of Volume I. 



58.4 Data on Sinkage and Change of Trim in 

 Shallow and Restricted Waters. The general 

 subject of change of level and trim, at various 

 speeds in shallow and restricted waters, is dis- 

 cussed in Sees. 18.7 and 35.7 of Volume I. Only 

 sufficient information is given here to enable the 

 marine architect to predict the sinkage and change 

 of trim in confined waters in quantitative terms. 

 These are important because of the extremely 

 limited bed clearance with which large vessels 

 transit certain canals, channels, and shoal areas, 

 and the desire to maintain the highest practicable 

 speed while doing so. 



Fig. 35. D on page 530 of Volume I reproduces 

 some trim data given by D. W. Taylor for a 

 scout-cruiser model. Fig. 58. D indicates the 0-diml 

 sinkage of bow and stern for three depth-draft 

 ratios h/H over a wide range of speed-length 

 quotient T^ and Froude number F„ . Paulus gives 

 similar data for the German torpedoboat S119 in 



TABLE 58.a — Characteristics of Prototypes for Which Trim Data are Presented in Figs. 58. D and 58.E 



