522 



HYDRODYNy\MICS IN SHIP DESIGN 



Sec. 67.16 



to keep down pressure resistance due to wave- 

 making. It is perhaps even harder to draw in the 

 lines aft so they merge into a long and relatively 

 narrow skeg ahead of the single propeller. Unless 

 the stern is deliberately made wide and flat, 

 incorporating a feature not well adapted to 

 vessels which must operate with a great variation 

 in draft aft, the demand for fine lines at both 

 ends of a medium-speed or fast vessel cuts 

 severely into the waterplane area necessary for 

 transverse metacentric stability. All things con- 

 sidered, it is difficult to fine the designed waterline 

 ending in a single-screw ship of normal form 

 without encountering separation and accepting 

 the drag which comes with it. To avoid surface 

 separation completely means limiting the water- 

 line slopes to a value not exceeding about 12 or 

 13 deg. With beams continuing to increase, this 

 situation is slowly becoming worse. Something 

 needs to be done about it. 



The termination and the slopes of waterlines 

 forward of a single-screw propeller aperture have 

 on many occasions in the past been relatively 

 blunt and heavy, with no consistently objec- 

 tionable effects. This was principally because the 

 powers absorbed by individual propeller blades 

 were small and the transient forces and moments 

 produced when these blades swung through 

 regions of highly variable wake were also small. 

 With higher and higher blade loadings the 

 periodic forces and moments likewise increase, 

 so that turning out a design with greater power 

 than a previous design, or re-engining a ship to 

 accomplish the same purpose, is by no means as 

 simple as making the propeller shaft a little larger 

 and mounting a new propeller to absorb the 

 increased power. 



In the orthodox single-screw stern the propeller 

 is in the same vertical plane as the: 



(a) Arch structure over the aperture 



(b) Rudder 



(c) Rudder horn, if fitted 



(d) Sternpost 



(e) Rudder post 



(f) Shoe projecting from the heel of the ship to 

 carry the lower rudder pintle. 



Any attempt to increase the propeller diameter, 

 to provide more tip clearance, or to leave more 

 vertical clearance in the aperture, runs head on 

 into the situation that many other parts must 

 also be accommodated in the centerplane. 



By shifting the fixed parts into different vertical 



planes where each has all the space it needs, 

 dividing the one large rudder into two smaller 

 ones, and hanging each of them behind an offset 

 skeg, much more room is left for a single propeller 

 on the centerline. Furthermore, in a wide ship 

 it is far easier to work gentle slopes into the sides 

 of twin skegs, especially in their upper portions 

 where they join the hull, than into a single 

 centerline skeg. If a single propeller is mounted 

 in a sort of tunnel between the offset skegs with 

 an arch-shaped roof overhead it can have a 

 diameter 20 to 25 per cent larger than would 

 otherwise be possible. The fact that the tip 

 clearance in this tunnel can be made sensibly 

 constant for more than half-way around the 

 propeller disc means that this clearance can be 

 very small. It can indeed be vastly smaller than 

 is thought necessary on an orthodox single-screw 

 installation to keep vibration down within 

 reasonable limits. This leaves still more room to 

 swing a larger wheel. 



For the ABC design, a propeller-disc diameter 

 of 24 ft was selected as one which would always 

 remain submerged; that is, as one for which the 

 wheel could be kept submerged during the several 

 variable-load conditions by the use of liquid cargo 

 or salt-water ballast in the after peak tanks, 

 coupled with the filling of the tunnel by a solid 

 inflow jet. The tip clearance of 1 ft was estimated 

 to be a reasonable value, considering that it 

 would be constant over at least half the circum- 

 ference. It was not too small to bring the blade-tip 

 fields too close to the hull and not too large to 

 lose the benefit of whatever boundary layer 

 existed on the inside of the arch. 



This reasoning was based upon satisfactory 

 clearances of the order of 0.25 ft for propellers 

 of one-third the diameter on tunnel-stern push- 

 boats. In fact, since only mechanical clearance is 

 required, the tip clearance on a 24-ft wheel might 

 be reduced to less than 0.5 ft, assuming flush 

 hull plating abreast the propeller, truly concentric 

 with its axis. 



For a 20 per cent increase in propeller diameter 

 over that for the transom-stern design, from 20 

 to 24 ft, the disc area is increased 44 per cent. 

 The thrust-load coefficient is reduced 30.5 per 

 cent (1.00/1.44 = 69.5) by this change alone. A 

 comparison of the propulsive efficiencies of the 

 20-ft propeller with the single centerline skeg 

 and of the 24-ft propeller with the arch stern is 

 given in Sec. 78.15. 



The afterbody plan of Fig. 67. L shows how the 



