776 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.13 



necessitates either an accepted projection of the 

 swinging propellers and rotating blades below 

 the hull or cutups in the hull forward and aft to 

 accommodate these devices. There is generally 

 not room within the vessel, nor would there 

 always be room under it, for retractable pro- 

 pellers of any kind. The cutups involve an in- 

 crease in thrust deduction and a loss of efficiency 

 but there are several successful apphcations of 

 this kind. A ferryboat installation, equally appli- 

 cable to a fireboat, is that on the Virginia ferry 

 Northampton, where a Voith-Schneider rotating- 

 blade propeller under the bow supplements the 

 twin propellers at the stern [Motorship, New York, 

 New York, Aug 1950, pp. 26-27, 43]. 



(10) It may at times be possible to turn a few 

 of the jets in the opposite direction, to balance 

 some of the reactions from the firefighting jets 



(11) Concentrating the monitors near midlength 

 of the vessel avoids the turning or swinging 

 moments set up when monitors near the ends 

 are playing. 



An arrangement of underwater reaction jets 

 might be thought more efficient for holding the 

 fireboat in position because these would impart 

 momentum to some of the surrounding water in 

 the same direction as the jet. It is to be remem- 

 bered, however, that neither force nor pressure is 

 communicated backward through a fiquid jet, at 

 least not for distances more than a few times the 

 jet diameter. The reaction force is all developed 

 when accelerating the water in the nozzle, so the 

 force is made no greater by playing the jet 

 against some fixed obstacle nearby or by directing 

 it into the water. When the nozzle reactions are 

 used to augment the propulsive thrust to get 

 the fireboat free of a hot spot in an emergency, 

 playing the jets horizontally into the air gives 

 the greatest reaction. 



One important feature of fireboat design in- 

 volving hydrodynamics has to do with the 

 efficiency of the monitor nozzles. This is measured 

 by the percentage of the total quantity flow of 

 water that can be delivered per unit time at the 

 greatest possible distance from the nozzle, reckoned 

 on the basis of the quantity rate passing through 

 the nozzle. A jet which disintegrates on its 

 way to the fire or which has a short trajectory is 

 inefficient. Breaking up of the jet in the air, as 

 well as failure of the water to "carry," is a func- 

 tion of the turbulence existing in the water upon 

 its entrance into the large end of the nozzle. 



Low turbulence, a more "solid" jet, and projection 

 of more water to a greater distance is achieved 

 by slowing down, straightening out, and quieting 

 the water immediately ahead of the nozzle. There 

 is insufficient information available in connection 

 with Fig. 76. H to indicate how far the five jets 

 are carrying in that case. Nevertheless, the 

 photograph clearly reveals that a considerable 

 amount of water is dissipated in the form of 

 spray and would probably never reach a fire into 

 which the jets were directed. The reader who is 

 interested in this aspect of fireboat design may 

 consult a paper by H. Rouse, J. W. Howe, and 

 D. E. Metzler, entitled "Experimental Investi- 

 gation of Fii'e Monitors and Nozzles" [ASCE, 

 1952, Vol. 117, pp. 1147-1188]. Improved designs 

 recommended for nozzles and monitors are shown 

 in Fig. 3(c), Table 1, and Fig. 21 on pages 1152, 

 1172, and 1174, respectively, of the reference 

 cited, and in Fig. 76.1 of the present section. 



Jet Diameter Dj 

 Nozzle 



For Further Details See Rq,2I 

 Referenced in the Text 



Fig. 76.1 Improved Design of Fixed Fire-Fighting 



Monitor Recommended by the Iowa Institute 



OP Hydraulic Research 



In the design recommended by the IIHR and 

 illustrated here, there are two honeycombs for 

 removing turbulence, one in the stand and 

 another in the barrel. Flow in the 90-deg corners 

 is facilitated by the guide vanes shown. The 

 nozzle is simpler than, and superior in perform- 

 ance to orthodox designs. 



To obtain an idea of the reaction exerted by 

 the vertical swiveling part of a monitor, compris- 

 ing a nozzle fed from a horizontal 90-deg branch 

 on each side, assume that the barrel fed by these 



