Sec. 78.17 



MODEL-TESTING PROGRAM FOR A SHIP 



895 



Arrows Indicate -Desiqned Speeds in Each Case 



I r I I I I I I "1 



^G. Shor^ Design, ^193 EX- Ships 



IZ 13 14 15 16 17 18 19 20 Zi 22 

 3hip Speed, kt 



Fig. 78.P Comparison of Propulsive Coefficient 

 vp FOB Models of ABC Ship and Other Designs 



there are plotted similar values for the ABC 

 arch-stern model, for the TMB Series 60 model 

 having a Cb of 0.60, and for three types of mer- 

 chant vessels of outstanding performance, from 

 the period 1930-1955. 



Incidentally, the Telfer merit factor of Eq. 

 (34.XXV) of Sec. 34.10 of Volume I, based upon 

 the model predictions from tests of the transom- 

 stern model, is 



M = 0.61 



= 0.61 



TF(long tons)F^(kt) 



L(ft)Ps (horses) 

 16,573(20.5)' 



510(13,243) 

 = 12.895 



For a r, of 0.908, an F„ of 0.2704, and an Fl 

 of 0.07312, this value is well above the meanline 

 of Fig. 34.1, from which a value of about 9.5 was 

 used for the first shaft power approximation of 

 Sec. 66.9. 



To return to an analysis of the behavior of the 

 transom-stern ABC model, a reduction in calcu- 

 lated shaft power of the magnitude indicated in 

 the foregoing is comforting from the point of view 

 of first cost and operation. However, it means to 

 the designer that a new weight estimate is in 

 order, with machinery and fuel weights, as well 

 as overall displacement, that are appreciably 

 diminished. For a project that is being done 

 thoroughly it may even mean a revision of the 

 prehminary design and the running of additional 

 model tests, on the basis that a revised hull of 

 shghtly smaller volume will be as efficient as the 

 first hull. 



However, before any radical steps are taken, 

 the following items are to be considered: 



(I) The procedure for running the model self- 

 propulsion test does not take full account of the 

 increased roughness effect to be encountered on 

 the ship as compared to that on the model, even 

 when the ship hull is clean and new 



(II) A re-reading of the items previously listed 

 in this section indicates that all the differences 

 between the original calculated or estimated per- 

 formance of the ship and the actual performance 

 of the model are in favor of the ship design as 

 represented by the model. 



In other words, practically everything that has 

 been done to develop the ship, from the time that 

 it had principal dimensions and proportions only, 

 has made it better than the "phantom" ship of 

 the Taylor Standard Series form having the same 

 proportions. To assume that all these favorable 

 factors will carry over into the full-scale ship on 

 trial, and at the magnitudes indicated by the 

 model test, is expecting a great deal. 



Just how much of the indicated reduction in 

 shaft power can be carried into the next stage of 

 the preliminary design should perhaps be deter- 

 mined only after the model is modified as de- 

 scribed in Sec. 78.18, and after the modified 

 model is tested self-propelled with the new 

 propeller designed in Chap. 70. 



The lower graph of Fig. 78. Q reveals that for 

 the transom-stern ABC design the increase of 

 resistance for appendages, over most of the speed 

 range, averages about 5 per cent. This corresponds 

 to the estimate made earlier in the present section. 

 However, in the vicinity of 22 kt ship speed, the 

 effective power with all appendages is lower than 

 the effective power in the bare-hull condition! 

 Since the ship will probably never run at this 

 speed, except at light load when the immersed 

 hull has a different effective shape, the reason for 

 this anomaly is somewhat of an academic matter. 

 The variations with speed in the ratio of (1) 

 effective power with appendages to (2) bare-hull 

 effective power shown by Fig. 78. Q indicate that 

 the appendage resistance is probably related to 

 wavemaking around the ship. Further than this 

 it has not been possible to account for the unusual 

 feature described. 



It is considerably more difficult to analyze the 

 performance of the arch-stern design, TMB model 

 4505-1, because of its unusual hull shape and 

 the almost total lack of reference data for such 

 a form. Despite the drastic change in shape of the 

 run from the normal transom stern, and an 



