254 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 52.9 



traces A, B, and C, and by the forward portions 

 of traces L and M, is typical of the flow under 

 models in general. It is possible that it indicates a 

 slowing down in the next-to-the-huU water layers 

 because of the thickening of the boundary layer 

 under the hull. The increase in x-distance from 

 the stem is one cause of thickening; another is the 

 increase in width of the flat portion, with its 

 small (or zero) transverse curvature. 



52.9 Observation and Interpretation of Off- 

 the-Surface Flow Data on Models. It is cus- 

 tomary in circulating-water channels and wind 

 tunnels to observe the nature and direction of the 

 flow over a body or ship surface by watching the 

 behavior of short strings or tufts of yarn. These 

 may be attached not only to the surface directly 

 but to slender pins projecting any required 

 distance from the surface, indicated in Fig. 52. W. 

 Different tuft colors may be used to represent 

 different normal distances from the hull. Indeed, 

 the tufts may be attached to the ends of long, thin 

 wands, moved about by hand to the desired 

 positions. 



Tufts have the disadvantage of shortness, like 

 the streaks from a transverse line of wet paint. 

 However, there is no Umit to the number of them 

 that can be used over the surface of a ship model. 

 They are extremely valuable as nature-of-flow 

 indicators, streaming straight out in a fast, 

 regular (or uniform) flow and waving gently or 

 lazily in a slow flow that is irregular and uncertain. 



Off-the-surface vane and rigid-flag indicators 

 require little in the way of interpretation since 

 they reveal velocity direction only. This is usually 

 sufficient when they are employed to determine a 

 single Une of flow such as a bilge-keel trace. 



Colored inks and dyes may be ejected from the 

 ends of long, thin tubes, moved to any desired 

 position in the water around the model. Figs. 

 46.E and 46.F show ink trails along a model. 



To permit subsequent study at leisure, after 

 completion of the test, flash photographs are 

 made of the tuft positions, like those reproduced 

 in Fig. 52.W, as well as in Figs. 36.F, 36.G, 46.D, 

 46.E, 46.F, 78.E, and 78.F. Similar still photo- 

 graphs have been made at the Experimental 



Fig. 52.W Fish-Eye View of Self-Propelied Model in Circulating- Water Channel, Showing Tufts Carried 

 BY Pins and Tufts on Model Surface 



