Sec. 52.19 



FLOW PATIERNS AROUND SHIPS 



261 



David Taylor Model Basin, partially analyzed, 

 are presented in Figs. 52.Za and 52.Zb. Sum- 

 marizing these tests briefly: 



I. A model of a destroyer, TMB 3613, was towed 

 and a 3-diml wake survey was made at the pro- 

 peller-disc position. It was then self-propelled by 

 its own model propellers, TMB numbers 2170 

 and 2171. While self-propelled, at a simulated 

 ship speed of 28.6 kt, three additional wake 

 surveys were made, at positions corresponding to 

 0.607, 2.075, and 3.543 propeller diameters abaft 

 the disc position. The net axial and tangential 

 components of velocity (by vectorial subtraction), 

 due to the action of the propeller, as averaged 

 for several radii, are indicated by vectors in the 

 several diagrams of Fig. 52.Za. The thrust-load 

 coefficient Ctl for the conditions given was 0.907. 

 Other pertinent data applying to this test are 

 added to the diagram. 



II. A model propeller, EMB 857, of 9 inches 

 diameter, was mounted at the end of the long 

 shaft ahead of the propeller boat [Bu C and R 

 Bulletin 7, 1933, Fig. 5 on p. 24] and the assembly 

 run at a speed of advance of 3.26 kt. Wake 

 surveys were made at 1, 2, and 3 propeller diam- 

 eters abaft the disc position. The available 

 records do not indicate the thrust-load coefficient 

 at which the propeller was working but it was 

 apparently very high. The axial and tangential 

 components of velocity, due to the action of the 

 propeller in producing thrust in open water, as 

 averaged for several radii, are indicated by 

 vectors in the three diagrams of Fig. 52. Zb. 



III. A series model, EMB 3424, was towed and 

 a wake survey was made at the propeller-disc 

 position. It was then self-propelled by its own 

 propeller, EMB 1884. While self-propelled, at a 

 model speed of 3.00 kt, four additional wake 

 surveys were made, at positions corresponding 

 to 1, 5, 15, and 24 propeller diameters abaft the 

 disc. These data are available at the David 

 Taylor Model Basin but have not been reproduced 

 here. 



Abaft the destroyer model, it is obvious that 

 the longitudinal centerline of the outflow jet does 

 not lie along an extension to the propeller shaft 

 axis but rises rather abruptly. This rise begins at 

 the disc position, increases rapidly, and then coin- 

 cides more or less with the rise in the water which 

 has flowed under the stern, generally parallel to the 

 buttock fines on the model. This change in vertical 

 position with distance abaft the disc is confirmed 



by noting the upward curve in the swirl core or hub 

 vortex trailing a screw propeller on a destroyer 

 model in the circulating-water channel. 



It is difficult, because of the lack of observations 

 in the upper portions of the propeller outflow jet 

 abaft the stern of the destroyer model, to estimate 

 the limits and shape of the cone of diffusion 

 between the outflow jet and the surrounding 

 water. With a net augmented axial velocity in 

 the outflow jet which is roughly 20 to 25 per cent 

 greater than the ship speed, the cone of diffusion 

 is rather thick. Its inner surface, as well as can be 

 determined, is of such slope that the jet core will 

 persist to a distance of 5 or 6 diameters abaft 

 the disc. 



For the model mounted on the propeller boat, 

 the net augmented axial velocity in the outflow 

 jet is about 55 per cent greater than the ship 

 speed. The cone of diffusion is relatively thin, 

 with an inner-surface slope small enough to 

 indicate that the jet will preserve some measure 

 of its identity to a distance of perhaps 15 diam- 

 eters abaft the disc position. 



For EMB model propeller 1884 on EMB model 

 3424, for which no graphic data are given, there 

 are definite signs of augmented axial velocity and 

 rotational velocity in the outflow jet at 15 pro- 

 peller diameters astern of the disc. There are 

 traces of the rotational velocity at 24 diameters 

 astern. 



52. IQ Persistence of Wake Behind a Ship. It 

 is often useful to know the characteristics of the 

 wake left in the path of a body or ship, mentioned 

 in Sec. 11.11, even though the ship propulsion 

 device (s), fike those of the sailboat or the flying 

 boat, may be entirely clear of the water. The wake 

 may involve only the mean residual velocity 

 along the ship track, over a section taken across 

 the body or ship path. It may involve effects of 

 another order such as variations from the mean 

 velocity, the scale and intensity of the residual 

 turbulence, the presence of entrained air, or some 

 other characteristics of interest. As a rule, the 

 transverse surface waves of the Velox system are 

 dissipated, by spreading transversely and by 

 internal viscous damping, long before the distur- 

 bances within the water disappear. 



Quantitative data on the persistence of wake, 

 applying to the mean velocity only, are almost 

 nonexistent. Perhaps the most extensive and 

 refiable are derived from model-testing tech- 

 niques, but the vafidity of stepping these data up 

 to ship size is still uncertain. It is known that the 



