Scr. 76.11 



DESIGN OF SPECIAL-PURPOSE CRAFT 



777 



branches is of 6-in diameter, and that the nozzle 

 orifice, slightly larger than 2 in, produces a 2-in 

 diameter jet. As the same water passes through 

 both, Fjet = (672')FB.rrei = 97^ . From the 

 energy equation [Rouse, H., EH, 1950, pp. 65-66] 



2g 



J^TPB ^ 



n 

 29 



{9Vnf 

 2g 



(76.i) 



Assume further that the acceleration of gravity 

 g is 32.174 ft per sec", that the salt water being 

 pumped has a specific weight w of 64 lb per ft^, 

 and that the pumping pressure in the barrel is 

 150 psi. Then 



/ 2gps fQ4 



\w(9' - 1) V 



64.35(150)144 



64(80) 



= 16.5 



whence Vj = 9(16.5) = 148.5 fps. As the feeding 

 branches come into the barrel at right angles, 

 AV is also 148.5 fps, reckoned in the jet direction. 

 Then F„ , the force on the vertical-swiveling 

 nozzle assembly, is 



F„ = pQAV 



= 1.9905 



r i48.57r(l)^ 1 

 L 144 J 



148.5 = 958 lb. 



The horizontal reaction on the fireboat is, 

 roughly, the nozzle-assembly reaction times the 

 cosine of the angle of elevation of the nozzle 

 above the horizontal. The transverse reaction on 

 the boat is the horizontal reaction times the sine 

 of the angle which the nozzle makes with the 

 boat centerline. 



It is sometimes necessary to hold a fireboat 

 reasonably stationary in space in a tidal current 

 or flowing river. The propulsion devices turning 

 over slowly furnish the necessary force to breast 

 the current but there remains the problem of 

 holding the craft transversely against the water- 

 jet reaction forces if the monitors are playing at 

 right angles to the current direction. 



An expert boat handler can possible yaw the 

 fireboat slightly toward the direction of the 

 fire and balance the transverse yawing force 

 against the combined nozzle reaction forces. 

 However, the design situation here is exactly 

 comparable to one frequently met with in sub- 

 marines, where it is necessary to hold the vessel 

 at about zero trim and at a given depth against 

 relatively large amounts of positive or negative 

 buoyancy. For negative buoyancy this is done 

 by giving the bow planes a slight rise angle and 

 the stern planes a slight dive angle; in this position 



the leading edges of both sets of planes point 

 upward and both are exerting positive Uft. By 

 adjusting the angles, the moment of these lift 

 forces balances the moment of the negative 

 buoyancy. 



Tipping the submarine over on its side, in 

 imagination, produces the fireboat equipped with 

 both bow rudder and stern rudder. Swinging the 

 leading edges of both rudders toward the fire 

 produces a lateral force to counteract the nozzle 

 reaction force. It is to be remembered, however, 

 that the hft produced by the bow rudder, without 

 the benefit of a propeller outflow jet, varies 

 directly as the first power of the rudder angle 

 and as the square of the water velocity past it. 

 A 4-kt current might easily give the required 

 lateral force forward, with not too great a rudder 

 angle, whereas this might be difficult to obtain, 

 with a large rudder angle, in a 1-kt current. 



Appended is a brief list of references relating to 

 fireboats, supplementing those mentioned at the 

 beginning of this section: 



(i) Parsons, H. de B., "Fire-Boats," Cassier's Mag., 

 May 1896, pp. 28-45. This article carries a con- 

 siderable amount of historical data and many 

 illustrations. 



(ii) Parsons, H. de B., "American Fii'e-Boats," SNAME, 

 1896, pp. 49-64. Tables I, II, and III give detailed 

 data on 24 fireboats dating from 1875 through 

 1895. 



(iii) West, C. C, "Centrifugal-Pump Fire-Boats," 

 SNAME, 1908, pp. 211-228 and Pis. 116-121 



(iv) Seattle Fireboat Alki, Diesel Prog., Mar 1949, p. 

 43 



(v) Robison, D., "Milwaukee's New Fire Boat Deluge," 

 Diesel Prog., Oct 1949, pp. 24-25. The overaU 

 length is 96.58 ft, the waterhne length is 93 ft, 

 and the molded beam is 23 ft, with a draft of 6.75 

 ft. 



(vi) Houston Fireboat Captain Crotty, MESR, Dec 1952, 

 pp. 86, 112 



(vii) New Fireboat John D. McKean for New York City, 

 Mar. Eng'g., Feb 1953, pp. 79-80; Sep 1954, pp. 

 54-58; SBSR, Int. Des. and Equip. No., 1955, 

 pp. 68-69. The Lqa is 128.75 ft, the L^l 125 ft, 

 the B 30 ft, the D 14.25 ft, and the H^^^ is 9.25 

 ft; there are 2 screws. 



(viii) "The World's Fire-Fighting Boats," SBSR, Int. 

 Des. Equip. No., 1956, pp. 61-65. 



76.14 Distinguishing Design Features of Self- 

 Propelled Dredges. A self-propelled suction 

 dredge, while digging, is an example of a ship 

 acted upon by other than hydrodynamic (and 

 aerodynamic) forces. This is because practically 

 all of them draw up the bed material through one 

 or more drag pipes, inchned aft and downward. 



