Gadd 



pV2L2 



1-2 



O 8 



0-6 



04 



0-2 - 



OI5 0-2 



DESIGN POINT 

 9Vv2-2 " 



0-5 



Fig. 6 - Calculated wave resistance curves 

 for the two head forms 



The models as made had L = 6 ft, and the junction with the afterbody was at 

 10 ft from the bow. Figures 7 and 8 show the experimental results obtained for 

 the net force on the head of the uniform density model and the pressure force on 

 its rear bulkhead as functions of speed. The pressure involved in Fig. 8 was 

 measured at the centre of the bulkhead. It is possible that some variation of 

 pressure occurred on the bulkhead close to the hull surface, and this would in- 

 troduce errors in Fig. 8. However, it is expected that these would be small 

 since the gap, 1/4 in., was small compared with the local boundary layer thick- 

 ness of about 2 in. It can be seen from Figs. 7 and 8 that each of the force 

 components oscillates more wildly than their sum, in Fig. 9, representing wave 

 drag plus friction drag. Also shown in Fig. 9 are the theoretical friction drag 

 Dp, based on wetted area and the ITTC line, Eq. (5), and the sum of Dp and the 

 theoretical wave drag D^^,, derived from Fig. 6. It will be seen that the lower 

 speed hump is bigger experimentally than theoretically, the reverse of what usu- 

 ally happens on a complete ship model. However, the addition of the oscillatory 

 components. Figs. 7 and 8, may lead to inaccuracies, especially at the lower 

 speeds where the forces are smaller, so perhaps too much attention should not 

 be paid to this discrepancy. More significant probably is the fact that the ex- 

 perimental humps and hollows are displaced to the right of the theoretical ones, 

 in exactly the same way as for complete models. Thus this displacement seems 

 unlikely to be the result mainly of viscous effects, but rather to arise from the 

 inadequacies of the approximations of the potential-flow problem. 



However, as was mentioned above, the agreement between theory and ex- 

 periment was not necessarily expected to be as good for the resistful uniform- 

 source form as for the quasi-bulbous form. The results corresponding to Fig. 

 9 are shown for the latter form in Fig. 10. 



718 



