672 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 72.14 



small down slope give satisfactory performance 

 and avoid the added complications of the auto- 

 matic flap. C. E. Ward pointed out, many years 

 ago, that the design of a tunnel-stern craft should 

 be such as to enable the vessel to pivot longi- 

 tudinally about the fore-and-aft position of the 

 propeller(s) as the loading changes [SNAME, 

 1909, p. 100]. In other words, the draft at the 

 propeller position should remain more or less 

 constant with changes in the displacement 

 volume and the trim. If, instead, the draft at the 

 stern can be kept nearly constant, the tunnel 

 remains closed at its after end to the same degree 

 and a tunnel flap is not necessary. 



One problem in twin- or multiple-screw tunnel 

 sterns, where a propeller is mounted so close to 

 the side that the hull plating forms virtually one 

 side of the tunnel, is that air is liable to be drawn 

 into the inflow jet because of the reduced pressure 

 there. When the ship or tunnel side does not 

 project far enough below the actual waterline to 

 form an adequate pressure barrier, air is sucked 

 under and into the propeller. Large chunks of air 

 striking the propeller produce objectionable noise 

 and vibration. L. A. Baier and J. Ormondroyd 

 report that "vicious stern vibration" on a twin- 

 screw towboat, resulting from air leakage of this 

 kind, was corrected by adding vertical stream- 

 lined fins outboard of the propellers [Third 

 Midwestern Conf. on Fluid Mech., Univ. of 

 Minn., Jun 1953, p. 406]. 



The necessary thrust-producing area ilo is not 

 easily obtained with a single screw propeller 

 whose diameter is limited by a shallow draft. 

 Tunnel-stern craft are, therefore, usually designed 

 to be driven by two, three, or four screw propellers. 

 There is, however, the case of the tunnel-stern 

 tugs built for Yukon River service in 1898, with 

 six screw propellers, each 3.33 ft in diameter, on 

 a total beam of only 32 ft [Mar. Eng'g., Jun 1898; 

 ASNE, Aug 1898, Vol. X, pp. 740-745; Ward, 

 C. E., SNAME, 1909, pp. 105-106]. 



72.14 Hull Surfaces Abreast Screw Propellers. 

 The hull surface in way of the small tip clearance 

 provided under the roof of a tunnel should be 

 flush, free of seam laps and butts, and preferably 

 without projecting rivet points and welding 

 heads; in other words, as fair and smooth as 

 good workmanship can make it. The finished 

 tunnel illustrated by A. R. Mitchell [lESS, 

 1952-1953, Vol. 96, Fig. 15 on p. 155; INA, Jul 

 1952, Fig. 6 opp. p. 147], with its protruding 

 strut-arm pads, doublers, and hoisting eyes, is 



an example of what not to do. The hoisting 

 fittings can be of the recessed type, similar to 

 those illustrated in Fig. 75.F. The doublers can 

 be converted to thick, single-layer shell plate 

 and the strut-arm connections can be entirely 

 within the hull. 



The strake or ring of plating abreast the wheel 

 is preferably made heavier than the rest, as 

 illustrated for the arch type of stern of the ABC 

 ship in Figs. 67.0 and 73.F. It is to be held in 

 position securely by substantial internal framing. 

 Careful fitting of any access hatch over the pro- 

 peller is necessary to insure a flush surface in 

 the roof. Bolts, nuts, and other securing devices 

 for this hatch are to be kept clear of the tunnel 

 surface. The curved under surfaces of all tunnels, 

 both ahead of and abaft the propeller positions, 

 are to be fair, with sufficient stiffness to remain so 

 and to avoid panting and vibration. 



The Germans have proposed for small tunnel- 

 stern craft that the portion of the tunnel roof 

 directly over the screw propeller be made of 

 resilient instead of stiff material. In other words, 

 rubber rather than steel [STG, 1952, Figs. 6 and 

 7, pp. 207-208]. There is a structural advantage, 

 and possibly a lessening of the vibratory forces 

 if the hull boundary in way of the propeller tips 

 yields with the pressure variations in the blade 

 fields. The effect of a yielding boundary on the 

 continuity and other characteristics of the water 

 flow is not known well enough to justify the use of 

 a resiUent boundary as more than an experiment. 



72.15 Powering of Tunnel-Stem Craft. The 

 naval architect and marine engineer designing a 

 self-propelled shallow-draft vessel with screw 

 propellers must face the fact that, in the present 

 state of the art, enclosing any appreciable sector 

 of the tip circle within a tunnel recess reduces 

 both the propulsive coefficient -qp and the effective 

 propeller thrust T{\ — t) at low speed. This is 

 undoubtedly because of excessive thrust-deduction 

 forces on the tunnel roof(s) for relatively long 

 distances ahead of and abaft the disc positions. 

 In any case, available published data, principally 

 those of A. R. Mitchell [lESS, 1952-1953, Vol. 

 96, pp. 125-188], reveal that rarely if ever is it 

 safe to employ a value of -qp for speed and power 

 predictions greater than 0.50. Indeed, Mitchell 

 goes so far as to state that: 



"Generally speaking, it is most unmse to guarantee 

 a specific speed when the depth of water under the keel 

 is less than the draught of the vessel" [INA, Jul 1952, 

 p. 152]. 



