Gadd 



SECTIONAL AREA 

 CURVE 



Fig. 11 - Widened bulbous head form matched with the 

 afterbody. The longitudinal dinnensions are compressed 

 by a factor of 1/2. 



the same area to produce a stern draught of half the maximum. The forebody 

 length of 8 ft corresponded to 1.28 times the source length L in Fig. 5, and the 

 beam was 2.5 ft. The initially designed total length was 20 ft, but the stern was 

 found to be so finely cusped that it was truncated by 6 inches to give a model 

 19.5 ft long. 



As will be seen from Fig. 11 the sectional area curve for the resulting 

 model is nearly the same as for the dotted airship form, and is highly asymmet- 

 rical. Wave theory suggests that this must be a bad thing, but the hope was to 

 recoup any losses on this score from a reduction in frictional resistance. Al- 

 though it was intended that the bulbous end of the model should form the bow, an 

 over-rigid dogmatism on this point was felt to be undesirable, and hence the 

 model was fitted with transition -fixing studs at both ends. 



The wisdom of this decision is illustrated in Fig. 12, which shows that the 

 resistance was, for the most part, much less when run with the bulbous end aft. 

 At low speeds it is true the airship-way round shows some advantages, but these 

 are very small. Perhaps even here wavemaking is still significant, or perhaps 

 the viscous drag penalties due to running a sufficiently fine airship form back- 

 wards rather than forwards are smaller than I had supposed. (Such penalties 

 might well be greater for beamier forms.) However, once wavemaking becomes 

 appreciable the bulb-aft condition shows a clear advantage, with a resitance 

 curve well below that for the other condition, though parallel to it for speeds 

 above 5 ft/sec. 



The residuary resistance is thus much less for the model with the fuller 

 end aft, in apparent contradiction to theory, which states that a given source 

 distribution should have the same wave drag run in either direction. The direc- 

 tional effect found in the experiments reinforces Wigley's findings (7) on asym- 

 metrical forms. In the present experiments the wave-pattern resistance was 

 determined from wave -pattern measurements which were fed into computer 



722 



