874 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 78.8 



Fig. 78.D Stern-Quarter View of Transom-Stern 



Model op ABC Ship, Showing Lines op Flow 



BY Chemical Means 



at the hull surface mclude practically the entire 

 length of the model. The hues should begin as 

 far forward as Sta. 1 if practicable, but in any 

 case not farther aft than Sta. 2. They should 

 extend all the way to the stern. 



Fitting-room photographs are to be taken of 

 the hull to show, as graphically as possible, the 

 lines of flow resulting from these tests. The usual 

 projection of the flowhnes on the transverse plane 

 is also to be made on a body plan. 



For determining the proper positions for the 

 roll-resistmg keels, flag indicators are to be 

 mounted along the bilges on the model, extending 

 from the side for a distance equivalent to about 

 3 ft for the ship. 



Figs. 78. C and 78.D are bow and stern views 

 of TMB model 4505, inverted from the positions 

 as photographed, showing the surface lines of 

 flow as revealed by a chemical indicator. The 

 traces derived from the model, when measured, 

 are those shown in the heavy hues of Fig. 66.R in 

 Sec. 66.28. Fig. 78.C shows clearly how the water 

 at the sides of the bow sections sweeps around 

 and flows along under the bottom. The upward- 

 and-aft flow alongside the centerline skeg is 

 indicated clearly m Fig. 78. D. 



78.8 Flow Observations with Tufts; Sinkage 

 and Trim ; Wake Vectors. Before any appendages 

 are added to the arch-type stern, flow observa- 

 tions between and around the skegs are to be 

 made in the circulating-water channel, using 

 tufts attached to the model. It is particularly 

 desired to learn whether there is any evidence 

 of separation along the roof of the tunnel or of 

 cross flow under the bottoms of the skegs. 



Flow observations with tufts are to be made on 

 both models in the circulating-water channel 



when driven at the model point of self-propulsion 

 by the stock propeller. Still flash photographs 

 suitable for reproduction are to be taken from 

 alongside and from underneath both sterns. 

 These are to be supplemented by motion-picture 

 photographs if any unusual flow conditions are 

 encountered. 



It should be possible to swing the double 

 rudders of the arch-stern model to observe the 

 change in flow -with rudder angle. 



Smkage and trim measurements are to be 

 taken at the FP and AP on both models through 

 the complete range of speeds covered in the tests. 

 These data are to be recorded when both the 

 bare-hull and the mth-appendages resistance 

 tests are run. The bare-hull data are required for 

 comparison ^vith pubhshed data on sinkage and 

 trim. The data mth appendages are to enable the 

 full-scale thrust measurements on the ship to be 

 corrected for the weight component of the shaft, 

 propeller, gear, and other rotating parts [SNAME, 

 1934, pp. 151-152]; see also Sec. 59.16. 



The wake vectors at the propeller-disc positions 

 are to be determined for both types of sterns. In 

 view of the small hull clearance under the transom 

 stern and the still smaller tip clearance inside the 

 tunnel of the arch stern, consideration is to be 

 given to the use of the 5-orifice spherical-ended 

 pitot tube, with its smaUer head, for determining 

 the wake vectors, in place of the 13-orifice tube 

 with the larger head. In any case, the head 

 should not be so large as to interfere mth or 

 modify the floAV it is intended to measure. 



For the transom-stern model the wake observa- 

 tions are to be extended outward to cover a half- 

 beam at least as wide as that of the ship section 



Fig. 78.E Underwater Profile of Arch-Stern 

 Model, TMB 4505-1, in Circulating- Water Channel 

 The dark irregular streaks represent flow positions of 

 tufts of dark yarn attached to the model surface. 



