Planing Craft 



predicted at a Froude number of 0.4; the apparent absence of interaction for 

 Froude numbers in excess of 0.5 was striking. 



J. B. Hadler of NSRDC presented some comments on trim measurements 

 of "free-to-trim" resistance tests on planing craft. Referring to the work of 

 Sottorf and Schmidt (1933, 1937) on the comparison of geosims with full-scale 

 seaplane floats, the expected scale effect on the viscous drag were observed, 

 but the running trim for the geosims was larger than full-scale trim — which is 

 opposite to what was expected. Tests at DTMB on models of the PT8 ranging in 

 size from 11.1 ft to 5.6 ft also showed that the smallest models ran at higher 

 trim angles for v/\T> 1.75, but that there was good agreement between the 

 various size models at y /\T < 1.75. Mr. Hadler suggests that a research pro- 

 gram be undertaken to explain this difference. The chairman suggested that, if 

 the chine edges of the smallest models were not made with exaggerated sharp- 

 ness, the flow separation would be delayed in the aft regions of the small model, 

 which could result in slightly higher running trim angles. The practice at the 

 Davidson Laboratory, Stevens Institute of Technology, is to sharpen the chines 

 and recess the model walls immediately above the chine line in order to ensure 

 proper ventilation and flow separation from the sides. It was suggested that 

 consideration be given to adapting a standard for model construction which 

 would provide for sharp chines. 



R. Lofft of AEW, Hasler, also reported on "Effect of Scale on Running Trim 

 and Resistance of Planing Forms." Tests were made of 1/6, 1/8, 1/12, and 1/20 

 scale models of a fast patrol boat (model beams of 3.52 ft, 2.78 ft, 1.39 ft, and 

 0.84 ft). An interesting side investigation was to determine the effect of model 

 construction material on the test results. The 1.39-ft-beam and 2.78-ft-beam 

 models were constructed in both wood and wax. Unfortunately, the smallest- 

 scale model was only built of wood. From the test results, Lofft concludes that 

 there is no scale effect for the 1/6-, 1/8-, and 1/12-scale models. The smallest- 

 scale wooden model (1,20) had a lower resistance relative to the larger wooden 

 models, and this was attributed to lack of artificial turbulence stimulation in the 

 small model. The smallest model also ran at a slightly higher trim than the 

 other models. Lofft attributes this high trim angle of the smallest model to its 

 wooden construction. He found that for the other scales, the wood models ran 

 at approximately 1/4° to 1/2° higher than the wax model. 



E. Amble of the Norwegian Ship Model Basin reported on tests to investigate 

 scale effects which arise when testing longitudinally stepped planing-huU models. 

 A test program was carried out in Trondheim using four geometrically similar 

 models where, for the same test Froude number for each model, the Reynolds 

 number varied from 9.275 x 10 ^ to 4.950 x 10^ . A surface-piercing turbulence 

 strut towed ahead of the models was effective in obtaining satisfactory conform- 

 ity in results. From the test results. Amble concludes that the drag-to-lift 

 ratio for the largest model of the stepped hull (B = 391 mm) was 10% less than 

 for the unstepped hull, while, for the smallest model tested (B = 128 mm), there 

 was little difference between stepped and unstepped configurations. Further, 

 Amble states that tank tests of longitudinally stepped planing hulls are much 

 more exposed to scale effect influence than are tests with conventional, un- 

 stepped planing designs. It was suggested from the audience that the sharpness 

 of the edges of longitudinal steps for the smallest model may have been insuffi- 

 cient to assure flow separation and that, further, perhaps, the introduction of 



1666 



