Sec. 7S.17 



MODEL-TESTING PROGRAM FOR A SHIP 



893 



Groph of — 5 ■ 



/ 



-r- 



7 



JOodel Resistance, 

 Bare Hull I 



o With Stimulation -6 

 + Without Stimulation 



1.0 1.4 1.8 2.2 2,6 3.0 34 3.8 4.2 4.6 5.0 5,4 56 6.2 

 Model Speed V, kt 



Fig. 78.0 Plot op Total Resistance R^ and of 

 Ratio RtIY"^ for TMB Model 4505 



The off-surface vanes mounted along the bilges 

 hkewise showed no uncertainty as to the proper 

 water direction. The bilge-keel traces deduced 

 from them are remarkedly fair. 



Takmg first the transom-stern design, repre- 

 sented by TMB model 4505, and analyzing the 

 speed-power differences between the values calcu- 

 lated on paper and those predicted from the model 

 tests, certam features stand out: 



(1) It was both conservative and optimistic to 

 estimate, as was done in Sec. 66.9, that the resist- 

 ance would be no lower and no higher than that 

 of the Taylor Standard Series hull of the same 

 proportions. In the first place, it was difficult to 

 assess the effect of the transom in advance. In 

 the second place, it was known that a sizable 

 bulb was to be used at the bow but it was ques- 

 tionable whether this bulb would be beneficial 

 at the low T, of 0.908. Actually, the effective 

 power P^ is about 3.5 or 4 per cent lower than the 

 TSS effective power at that T^ . 



(2) The wake fraction w of 0.30 used for the 

 transom-stern hull in the shaft-power estimate 

 of Sec. 66.27 was derived from the best available 

 data on the self-propulsion of single-screw models. 

 It was admittedly optismistic; indeed, since a 

 value of only 0.190 was achieved in the model 



self-propulsion test, the estimate was unduly 

 optimistic. The calculation of Sec. 60.8, using 

 K. E. Schoenherr's formula [PNA, 1939, Vol. II, 

 p. 149], was not made until after the hull form 

 had been completely fashioned and a stock 

 propeller had been selected for the first SP tests. 

 Even then, the predicted value of w was 0.255, 

 considerably higher than the test value of 0.190 

 from Figs. 78.Nb and 78.Nc. 



(3) The estimated thrust-deduction fraction I of 

 0.20, used in Sec. 66.27, is close to the value of 

 0.30(0.7) = 0.21, derived by Eq. (60.vi) from K. E. 

 Schoenherr's simple formulas [PNA, 1939, Vol. 

 II, pp. 149-150] on the basis of using a streamlined 

 rudder. When the transom-stern ABC hull had 

 been fully shaped, and it was known that a 

 contra-rudder was to be fitted, a smaller value of 

 t was selected. Takmg the coefficient 0.5 hsted 

 for contra-rudders in Sec. 60.9, the new ^-value 

 would have been 0.255(0.5) = 0.128. Utilizing 

 the stern shape of the hull by the method illus- 

 trated in Fig. 67. V, and taking account of the 

 rudder, the predicted thrust-deduction fraction 

 from the upper graph of Fig. 60. P was 0.135. 

 There appeared to be little justification for antici- 

 pating a low ^-value of 0.07, as was actually 

 derived from the self-propulsion tests. Neverthe- 

 less, the hull efficiency -(]„ of 1.161, estimated 

 by using a wake fraction of 0.255 and a thrust- 

 deduction fraction of 0.135, is remarkably close to 

 the value of 1.148 derived from the self-propelled 

 model-test data of Fig. 78. Nb. The hull efficiency 

 of 1.143, estimated at an early stage of the design 

 and set down in Sec. 66.27, is Ukewise remarkably 

 close to both these figures. 



(4) The estimated Pe/Ps ratio or r;? value of 

 about 0.74 of Sees. 66.9 and 66.27 was admittedly 

 somewhat random and even more hopeful. The 

 higher -qp of 0.761 from the model tests may be 

 due to the care with which the afterbody and 

 skeg were shaped, in an effort to achieve good 

 flow to the wheel. On the other hand, it may be 

 ascribed to the use of: 



(a) The contra-fashioning of the rudder-support 

 skeg and the rudder, which had not yet been 

 decided upon when the power estimates of Sees. 

 66.9 and 66.27 were made 



(b) A propeller diameter somewhat larger than 

 the average. A diameter of 0.7 the draft, one of 

 the simple rules, would have been 0.7(26) = 18.2 

 ft, whereas the corresponding diameter was 20.51 

 ft for the stock propeller used. 



