540 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.25 



great as 0.7 times the noininal boundary-layer 

 thickness 5 (delta). 



For the ABC transom-stern design, at an 

 estimated x-distance of 489 ft, 5 is 2.8 ft from 

 Fig. 45.1, and 0.75 is 1.96 ft. From Fig. 66.Q the 

 tip clearance at the top of the wheel, arrived 

 at indirectly, is 2.62 ft. This should take care of 

 a certain amount of thickening of the boundary 

 layer under the stern due to fouling, for which 

 there are no rules at present (1955). 



Since so little is known concerning the effect of 

 hull shape and curvature on this nominal thick- 

 ness 5, wake measurements are made on a model 

 at the proposed propeller position (s), extending 

 from a point inboard as close to the hull as may 

 be experimentally practicable, to a region out- 

 board, at least O.IR beyond the far side of the 

 propeller disc(s). 



The nominal boundary-layer thickness S on 

 the short model is greater in proportion to the 

 scale ratio than the nominal thickness on the 

 ship, described in Sec. 6.8 and illustrated in 

 Fig. 6.E. This is compensated for by the inevi- 

 table thickening of the ship boundary layer when 

 the hull surface is roughened by fouhng. The 

 propeller tip circle should, if practicable, be kept 

 outside the boundary-layer region on the model 

 where the wake fraction is 0.25 or more, reckoned 

 preferably by the pitot-tube survey method 

 illustrated in Sec. 60.6. 



Where tip and edge clearances are both in- 

 volved, as for the wing propeller of a twin-screw 

 vessel with long bossings, and where vibration is to 

 be minimized, the combination of theory, model 

 tests, and experience all indicate that fore-and-aft 

 edge clearance is more important than transverse 

 tip clearance. In other words, it is better to move 

 the propeller aft and to cut away the bossing 

 termination as far as possible, than to move the 

 propeller outward, away from the hull [Tomalin, 

 P. G., SNAME, 1953, p. 592]. 



67.25 Baseplane and Propeller-Disc Clear- 

 ances. The baseplane clearance for a screw 

 propeller is determined by the service operating 

 conditions, by considerations of drydocking, and 

 possibly by the fact that the ship may rest on 

 the bottom at certain wharves when the tide is 

 out, as in the Thames at London. For a propeller 

 unprotected by a shoe, say on a twin-screw craft, 

 the baseplane clearance may vary from a mini- 

 mum of 0.2 ft on small vessels to 0.5 or 0.7 ft on 

 large ones [van Lammeren, W. P. A., RPSS, 1948, 

 p. 279]. 



For certain vessels whose maximum or extreme 

 draft is appreciably less than the channel or 

 river depths where they are to operate, the base- 

 plane clearance may be negative. The propeller 

 disc then extends below the baseplane for a 

 distance limited only by the height of the blocks 

 when the vessel is drydocked or hauled out. 

 Only rarely should this exceed 4 ft; probably 5 ft 

 is a maximum. 



When propellers are mounted abreast each 

 other or nearly so their disc clearances may, if 

 necessary, be reduced to mechanical values only, 

 say O.OoD, regardless of the direction of their 

 relative rotation. Indeed, the large 19.5-ft twin 

 screws of the old Atlantic liners Teutonic and 

 Majestic, built in 1889, had a negative disc clear- 

 ance of 5.5 ft [Maginnis, A. J., "The Atlantic 

 Ferry," London, 1892, pp. 186-187; Cassier's 

 Mag., Jan 1897, p. 231; Barnaby, S. W., "Marine 

 propellers," 1900, pp. 64-65]. The port propeller 

 disc was placed 6.25 ft ahead of the starboard 

 disc and the tips of each wheel swung beyond the 

 centerplane, to the opposite side of the vessel, 

 through a large aperture in the centerHne skeg. 

 Contemporaneous accounts of the behavior of 

 these passenger liners, at that time the largest 

 on the Atlantic, make no mention of vibration or 

 other disturbances caused by these overlapping 

 propellers, possibly because of their large diameter 

 and relatively light thrust loading. 



When adjacent propellers are offset by appre- 

 ciable fore-and-aft distances, as on quadruple- 

 screw vessels of normal form, great care is required 

 in establisliing the disc clearance between the 

 outboard and inboard wheels, reckoned by the 

 projection of their discs on a transverse plane. 

 The general direction of flow in way of these 

 offset propellers is first approximated by analytic 

 methods or determined by flow tests on a model, 

 preferably in a circulating-water channel. Follow- 

 ing this, it is necessary to sketch in the probable 

 boundaries of the inflow and outflow jets of each 

 propeller, doing this on a plane passing approxi- 

 mately through the propeller shaft axes and 

 normal to the hull plating in the vicinity. On 

 the principle that, at designed speed, the general 

 direction of flow through the outflow and inflow 

 jets is not modified appreciably by angular 

 differences between the flow direction and the 

 shaft axes, the propeller jets should be resketched 

 to follow the general ship flow. They do not, in 

 general, follow the shaft axes. AVhen so modified 

 the outflow jet of the forward propeUer theoret- 



