Sec. 6S.15 



Troce of Surface Above Which the Flow 



ABOVEWATER-FORM LAYOUT 



More or Less . Undi sturbed 



563 



Uniform Air I" low 

 From Ahead 



Edd\(inq and Reversed Flow Occurs Aboft Almost All Projections and Exposed Objects ond Aboft tht Abrupt Endincjs of Deckhouses 



Fig. 68.L Air-Flow Pattern Over a Passenger Ship, from a Model Test 



condition but they illustrate vividly the back 

 flow, the eddies, and the extreme irregularity 

 of the air flow in general. 



For the many smaller ships, and some large 

 ones [Swedish tanker Oceanus, SBSR, 2 Dec 1954, 

 p. 736; The Motor Ship, London, Dec 1954, pp. 

 374-375], on which the deckhouses and other deck 

 erections (except possibly for a raised forecastle) 

 are all the way aft, there is a possible major shift 

 of the center of pressure of the hull and upper 

 works which may affect maneuverabiUty in a high 

 wind. Apparently this offers no real problem in 

 operation, aside from learning initially how the 

 ship behaves and controlling it accordingly. 



It is customary, for many ship designs, to 

 build a simple, inexpensive model, usually called 

 a drafting-room model, embodying the hull 

 above the DWL and all the principal deck erec- 

 tions and exposed parts in their proper shapes, 

 sizes, and locations. Fig. 54. B shows such a model. 

 It is an easy task, for which techniques are well 

 developed [Nolan, R. W., SNAME, 1946, pp. 

 46-60], to mount this model in a large wind 

 tunnel at various angles to the relative wind 

 and to determine the nature of the flow over and 

 around it. A hght thread or tuft, carried by a 

 thin wand which is manipulated by hand and 

 placed at selected points, indicates instantly the 

 type of flow to be expected there. Multiple tufts 

 of contrasting color, attached to the model at 

 many points, are readily photographed for record, 

 at any test condition. Indeed, such a model may 

 be tested in the open provided a platform or 

 deck is available, over which a uniform wind is 



blowing. A long fish pole, carrying the roving 

 thread or tuft, enables the experimenter to stand 

 far enough from the model so that his body does 

 not interfere with the uniform air flow near the 

 model. 



Sir Victor G. Shepheard describes the results 

 of wind-tunnel tests made at the NPL, Tedding- 

 ton, upon such a model of the British Royal 

 Yacht Britannia [INA, 7 Apr 1954, pp. 11-13]. 

 In this case white smoke filaments were used, 

 which were found to photograph satisfactorily. 



It is possible to make exactly the same type of 

 flow test on an upside-down model of the upper 

 works attached to a large horizontal surface board 

 and suspended in the water of a circulating- 

 water channel. Jets of water from a suitable 

 pump are caused to issue from the stacks at a 

 velocity having the proper ratio to that of the 

 overall stream, corresponding to the relative- wind 

 velocity. Colored dye injected into the water 

 streams from the stacks gives a true and vivid 

 indication of the paths of the exhaust gases on 

 the full-scale ship, complete with swirls, eddies, 

 and the like. 



Methods for calculating the air drag and wind 

 resistance of ship hulls, upper works, and deck 

 erections are given in Chap. 54. 



68.15 Design of Facilities for Abovewater 

 Smoke and Gas Discharge. Since World War II, 

 a considerable amount of research and develop- 

 ment on methods of keeping combustion and 

 objectionable exhaust gases and soot clear of 

 passenger and operating spaces on ships has 

 revealed that: 



