564 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 68.15 



(a) These gases must be discharged into regions 

 of relatively smooth and regular flow, rather 

 than into flow containing large-scale eddies. The 

 discharge must be above and outside of separation 

 zones, else the gases are scattered widely in 

 regions of low velocity or are drawn back and 

 downward by the reversed flow and the eddies. 



(b) Stacks of large diameter, width, and hori- 

 zontal area create their own separation zones, 

 into which the escaping gases are drawn, thence 

 to find their way to the decks below 



(c) Raising the stack top to a great height, 

 projecting far above the top of the turbulent 

 region, will not of itself prove satisfactory because 

 of the soot and dirt that falls upon the ship at 

 relative wind velocities approaching zero [Smith, 

 W. W., SNAME, 1946, pp. 76-77] 



(d) To insure that gases issuing from a stack top 

 in the open are projected far enough into the 

 regular flow to prevent their mixing with the 

 turbulent flow behind the stack or over the ship 

 it is necessary that the stack-gas velocity S, 

 approximately vertical, be at least as great as 

 the relative-wind velocity W r in the open. The 

 velocity S may have to be 1.5 or 2.QiW r if the 

 volume of stack gas is not large. For the worst 

 general service conditions the maximum relative- 

 wind velocity W r may be taken as 40 kt, so that 

 for no contamination the stack-gas velocity 

 should exceed 65 ft per sec if there are adjacent 

 turbulent areas. For fine, fast, high-powered 

 ships the actual relative wind velocity may reach 

 60 or 65 kt, corresponding to 100 or 110 ft per 

 sec, under conditions when the decks and upper 

 works should still be smoke- and gas-free. 



(e) It may under some circumstances be possible 

 to project the objectionable gases into the tip 

 vortex of a long, thin stack shaped like a symmet- 

 rical airfoil. If caught in such a vortex the gases 

 remain there reasonably well, at least until they 

 are downwind far enough to be clear of the ship. 



(f) For fast ships in which the relative wind is 

 generally in the forward quadrant a shield around 

 the forward side of the stack opening or a partial 

 elbow directing the combustion gases aft as well 

 as upward, long used on French men-of-war and 

 built into the German cruiser Prim Eugen and 

 other naval vessels, is a simple means of retaining 

 some of the upward component of stack-gas 

 velocity in an effort to keep the gases clear of the 

 turbulent region. This shield or elbow, however, 

 is properly placed at the forward edge of the gas 

 opening, and not at the forward edge of a stack 



structure that may be larger than the opening, 

 (g) If, for reasons of appearance, tall chimney- 

 shaped stacks are not acceptable, combustion 

 and exhaust gases may be discharged from the 

 after upper corner of a stack casing, as on the 

 liners Constitution and Independence (1950-1951). 



Supplementing (a) preceding, wind-tunnel tests 

 on ship models reveal that set-backs or steps in 

 the forward surface of a multi-deck superstructure 

 give the equivalent of a streamlined forward face 

 if the set-back slope through the upper edges of 

 the various vertical surfaces is not more than 30 

 deg with the horizontal. If the slope is as great 

 as 60 deg, the turbulent separation region above 

 the uppermost deck is about as high and as large 

 as if the superstructure face were a solid vertical 

 wall, with a 90-deg slope. Furthermore, perform- 

 ance on the ship is found to be somewhat better 

 than the model tests predict [Acker, H. G., 

 SNAME, New Engl. Sect., Oct 1951, p. 5]. 



For the reader who wishes to pursue the subject 

 further the following references are quoted: 



(1) Dui-and, W. F., AT, Julius Springer, 1936, Vol. Ill, 



p. 165 



(2) Valensi, J., "Methode des fillets de fumee (maquettes 



d'avions, ailes d'avions) (Method of Smoke Fila- 

 ments using Models of Airplanes and Airplane 

 Wings)," Publ. Sci. et Tech. du Ministere de 

 I'Air, 1938, No. 128, pp. 11-16 



(3) Sherlock, R. H., and Stalker, E. A., "A Study of Flow 



Phenomena in the Wake of Smokestacks," Univ. 

 Mich. Res. Bull. 29, Mar 1941 



(4) Ijsselmuiden, A. H., "Machine- en electrische 



installatie van het m.s. 'Oranje' (Machinery and 

 Electrical Installation of the Motorship Oranje)," 

 De Ingenieur, 7 Jul 1939, p. 79. In Figs. 27, 28, 

 and 29 on this page, there are given three diagrams 

 of stack arrangements tested, apparently in a 

 wind tunnel. For each of the diagrams there is 

 sketched the type of flow found to issue from and 

 to lie above and abaft the stack. 



(5) Squire, H. B., and Troucer, J., "Round Jets in a 



General Stream," ARC, R and M 1974, 1944 



(6) Nolan, R. W., "Design of Stacks to Minimize Smoke 



Nuisance," SNAME, 1946, pp. 42-82 



(7) Sharp, G. G., "Design of Modern Ships," SNAME, 



1947, pp. 462-466 



(8) Valensi, J., and Guillonde, L., "Sur les Formes de 



Carenage de Chemin^es de Navires Propres k 

 Eviter le Rabattement des Fumfees (On the 

 Shaping of Stacks of Ships Intended to Prevent the 

 Settling of Smoke over the Decks)," ATMA, 1948, 

 Vol. 47, p. 173 



(9) Eustaze, S., "Le Rabattement des Fum6es sur les 



Fonts d'un Navire; Essais sur Modeles et Dis- 

 positions Pratiques (The Settling of Smoke over 

 the Decks of a Ship; Tests on Models and Practical 

 Arrangements)," ATMA, 1951, pp. 285-307 



