Sec. 463 



DATA ON SEPARATION, EDDYING, AND VORTEXES 



137 



visible by larger eddies, by chips, boxes, or blocks 

 of wood thrown into the zone, or by floating refuse 

 caught in it. As mentioned previously, these are 

 whirled around in the eddies or drawn back 

 toward the hull, and dragged along with the ship. 

 The floating objects outside the separation zone 

 disappear rapidly astern. Special vantage points 

 are necessary in many ships from which to make 

 these observations, because the entire waterline 

 or waveline in the run is not visible from the 

 topside. 



Under water, where the eddies can not be seen, 

 separation zones are often detected by the marine 

 growths which flourish in these areas of low 

 relative velocity. When the separation zone is 

 free of large air bubbles, paint surfaces that show 

 little signs of wear may be indicators of stagnant 

 regions. If there are, in an area suspected of 

 being in a separation zone, any sea connections 

 which can be shut off from a system and used as 

 pressure orifices, an observed head at or close to 

 the sea connection which is less than the actual 

 hydrostatic head is an indication that a — Ap 

 exists there. Regardless of the exact nature of 



the water flow outside, this — Ap indicates the 

 presence of separation drag if it is on a portion 

 of the hull which faces aft. The — Ap's mentioned 

 here are not to be confused with those which 

 may be developed because of potential flow 

 around certain portions of the ship, described 

 in Chap. 4. 



By far the most satisfactory method of detect- 

 ing separation, determining the extent of the 

 zone, and observing the nature of the flow is to 

 test a model. This can be towed in a basin, using 

 a water box and mirrors for viewing the under- 

 water portion, or it can be run in a circulating- 

 water channel and viewed through large observa- 

 tion windows. Colored threads, strings, and tufts 

 are attached directly to the model surface or to 

 slender pins driven into the model so as to lie at 

 a distance from that surface. The tufts may be 

 attached to appendages, or mounted ahead of and 

 abaft propellers on wire frames which will not 

 affect the flow appreciably. Tufts which wave, 

 which curl this way and that, or which actually 

 point forward are unmistakable evidence of 

 unsteady flow, of approaching separation, or of 



Fig. 46.D Fish-Ete View of a Ship Model with Some Partly Reversed Tufts 



