Sec. 67.24 



UNDERWATER HULL DESIGN 



537 



sternposts and bossing or spectacle frames the 

 square and blunt endings of forged-steel stern 

 frames have largely disappeared. However, the 

 slopes are too steep and the trailing edges of 

 sternpost castings are in general still much too 

 blunt to eliminate objectionable separation and 

 eddying behind them. Also there is no more 

 excuse to lap shell plating on the outside of a 

 sternpost than to lap it on the outside of a stem. 

 The effect is different but it is an objectionable 

 discontinuity just the same. 



The projecting portions of Thermit welds used 

 to join several cast or forged sections of a stern- 

 post— or a stem — need not be left for reinforce- 

 ment. They can and should be trimmed off to 

 conform to the shape of the adjacent parts. Like- 

 wise, butt welds can and should have the external 

 reinforcements removed. 



The slope angles on the trailing edges of skegs 

 and other major parts should be 15 deg or less, 

 reckoned from the known or the predicted direc- 

 tion of water flow. Drawings of these trailing 

 edges should call for smoothnesses and tolerances 

 of the same order as those required on shaft struts. 



Despite all that is said here and elsewhere 

 about the fining of skeg endings ahead of a screw 

 propeller, experience indicates that some delib- 

 erate thickening of the skeg ending ahead of its 

 termination increases the wake fraction at the 

 disc position. It is particularly beneficial below 

 the shaft axis in a ijormal form of single-screw 

 stern, where the average wake fraction is usually 

 much smaller than above the axis. The greatest 

 thickening can be applied at the bottom, just 

 above the keel, where the upward and aft flow 

 of the water eliminates most of the boundary- 

 layer wake. For this reason the thickening of 

 such a skeg ending is called clubbing. However, 

 the design of a club ending or bulbous skeg, 

 illustrated in Fig. 25.L, is a ticklish procedure. 

 If the thickening is carried too far, it may do 

 more harm in producing vibration than help in 

 improving propulsion [Williams, E. B., Thornton, 

 K. C, Douglas, W. R., and MiedUch, P., SNAME, 

 1950, p. 78]. No design rules have as yet been 

 formulated for this feature. 



As a means of reducing the interference from 

 a skeg ahead of a screw propeller the designer 

 may shorten the skeg drastically, expose the 

 propeller shaft, and support the propeller bearing 

 by a V-strut just ahead of the wheel. This arrange- 

 ment has been in use for many years on motor- 

 boats and larger vessels, such as on the center 



shafts of the German "schncUboote" or high- 

 speed S-boats of World War II. Its use un- 

 doubtedly diminishes the wake fraction at the 

 propeller but it may diminish the thrust-deduction 

 fraction by a greater amount, and it may reduce 

 the periodic vibratory forces from the propeller. 



As a rule, the profiles of major hull and skeg 

 endings are not too important except as they 

 affect aperture clearances, discussed at length 

 in the section following. 



67.24 Aperture and Tip Clearances for Pro- 

 pulsion Devices. The lift load per unit area on 

 the blades of any moderately loaded screw 

 propeller, corresponding to the weight loading per 

 unit of wing area on an airplane, lies within 

 rather narrow limits, say 8 to 13 lb per in". 

 Furthermore, the section shapes within the region 

 of heaviest loading, say from 0.5 to 0.95i2, are 

 quite similar for both narrow and wide blades of 

 a modern screw propeller. On this basis the circu- 

 lation patterns and the pressure fields around all 

 screw-propeller blade elements in the given radius 

 range may be taken as roughly similar, using the 

 expanded-chord length as a reference dimension. 

 Based on these assumptions, the pattern of cor- 

 responding streamlines and isobars is roughly 

 proportional in size to the chord length or blade 

 width. Very approximately, therefore, at least 

 for a not-too-wide range of thrust loading, a 

 point in space one blade width from a blade 

 element on one propeller is subject to the same 

 pressure as a point in the same corresponding 

 position, one blade width distant from a blade 

 element on another propeller. This is an absolute 

 rather than a relative value because, ahead of the 

 propeller at least, the reduced pressure can not 

 drop below the vapor pressure of water. 



The foregoing argument may be a reason for 

 specifying propeller-aperture clearances, defined 

 in Sec. 33.3 and indicated in Fig. 33. D as "upper 

 aft," "upper forward," and so on, in the form of 

 absolute dimensions for a certain range of pro- 

 peller diameters or ship sizes [ME, 1942, Vol. I, 

 Table 1, p. 275; van Lammeren, W. P. A., RPSS, 

 1948, Fig. 73, pp. 127, 278]. It may be a reason 

 even for specifying these edge clearances as 

 functions of the propeller diameter [Ayre, Sir 

 Amos L., INA, 1951, pp. 145-148]. It appears, 

 however, that the propeller-aperture clearances 

 are logically a function of the maximum blade 

 width of the propeller. 



In general, the aperture clearance ahead of 

 the propeller, at the Q.IR, should be equal to, qv 



