390 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 61.3 



13 14 15 16 17 18 19 20 21 22 23 24 25 26 



Ship Speed, kt 



Fig. 61. a Resistance and Trim Data fob a Torpedoboat in Water of Varied Depth 



There are, in the literature, a considerable number 

 of typical graphs giving the variation of resistance 

 and power with speed for ships running in shallow 

 water. They are derived in part from tests on 

 models and in part from ship trials. One set of 

 curves, derived from a model test and illustrating 

 the principal features, including the change of 

 level at bow and stern, is reproduced in Fig. 35.D. 

 Another set, derived from the trial results of a 

 ship, is adapted from a set of graphs given by the 

 German naval constructor Paulus in Fig. 2 on 

 page 1872 of reference (4) in Sec. 61.22. The latter 

 data are presented in Fig. 61. A. Others are to be 

 found in the references cited in the list at the 

 end of this section. 



Paulus' data for the German torpedoboat 

 SI 19 reveal that the maximum trim by the stern 

 occurs at a speed shghtly less than that for which 

 the increase in indicated power Pi (over the deep- 

 water P,) is a maximum. Also that the trim 

 becomes less than the deep-water trim at a speed 

 very slightly higher than that for which the 

 shallow-water Pi drops below the deep-water P, . 

 The trims at the higher speeds, in all depths of 



water, are only slightly less than the maximum 

 trims by the stern at the lesser (critical or near- 

 critical) speeds. Unfortunately, Paulus does not 

 give the lines or even the principal dimensions of 

 torpedoboat SI 19 forming the subject of his 

 investigation. 



Most of the model-test data referred to in the 

 first paragraph of this section are suspected of 

 giving resistances and powers that are too high, 

 because of the restricting effect of the model- 

 basin walls, added to the effect of the false bottom 

 or other device used to simulate the shallow water. 

 For example, in reference (34) of Sec. 61.22, 

 where the shallow-water drag tests were intended 

 to simulate behavior in open water of 13 meters 

 depth, the hydraulic radius R,i works out as 

 only about 9.5 m. For unhmited water of 13 m 

 depth, Rh is also 13 m. A more classic example is 

 W. Froude's full-scale towing tests on the Grey- 

 hound in the early 1870's, unquestionably made 

 in water that was too shallow for the size of the 

 vessel [Robb, A. M., TNA, 1952, p. 449]. 



61 .3 The Quantitative Effect of Shallow Water 

 on Ship Resistance and Speed in the Subcritical 



