Sec. 67.22 



UNDERWATER-HULL DESIGN 



535 



the propulsion of the ABC vessel is to be effected 

 as economically as the present state of the art 

 permits. A twisting of the skeg ending to give 

 a contra effect is therefore indicated for the single- 

 skeg transom stern. The skeg waterlines above the 

 shaft are fined to reduce the thrust deduction and 

 it would be easy to work a considerable degree 

 of deflection into that structure. However, above 

 the shaft axis the aperture clearance is deliberately 

 made large, so that the benefit to be derived from 

 prerotation is doubtful. Further, the lower part 

 of the skeg, below the shaft axis, is cut away so 

 far that any deflection effect would be lost on the 

 propeller. A contra-shape is to be worked into the 

 fixed rudder horn, above the shaft axis, and there 

 appears to be room for enough shaping of the 

 horn to remove much of the jet rotation. Taking 

 all factors into consideration, therefore, it is 

 decided not to provide a contra-guide skeg 

 ending for the transom-stern afterbody. 



Nevertheless, as an example of the rules given 

 herein, a contra-guide ending is laid out for the 

 upper half only of the single skeg of the ABC 

 ship, producing the form shown in Fig. 67. T. 

 This embodies a more nearly vertical profile 

 above the shaft, with less aperture clearance 

 than for the symmetrical skeg ending. 



The radius of the propeller bearing boss shown 

 in Fig. 66. P, the body plan of the transom-stern 

 design, is L75 ft. The maximum offset value, at a 

 starting level of x' — 0.1 or R = O.l^Max above 

 the propeller-shaft axis, is taken as 0.85 times the 

 boss radius, or 0.85(L75 ft) = 1.4875 ft. Assuming 

 a constant P/D ratio of 0.98 for the propeller, 

 the blade angle 4> at O.IK is 72.225 deg and sin^ </> 

 is 0.90682. This is a sort of reference value cor- 

 responding to the x' = 0.1 offset of 1.4875 ft. 

 For example, at x' = 0.7, <^ is 24.019 deg and 

 sin^ (^ is 0.16569. Then the offset at x' = 0.7 is 

 (0.16569/0.90682)1.4875 ft = 0.2718 ft, as Hsted 

 in the table on Fig. 67. T. At x' = 1.0, where </> 

 is 17.325 deg, sin' <f> is 0.08868 and the offset is 

 (0.08868/0.90682)1.4875 ft = 0.1455 ft. 



The slope of the median line bisecting the angle 

 between the terminal portions of the level water- 

 lines on each side of the skeg ending is limited to 

 the order of 0.175. This corresponds to just under 

 10 deg, as reckoned from the ship centerplane or 

 the construction plane of the skeg. These limits 

 are admittedly rather arbitrary, to comply with 

 the requirements of the paragraph following. 

 The median-line slope varies from this maximum 

 value just beyond the radius of the propeller- 



bearing boss to a much smaller value at 1.0 or 

 1.1 times the propeller radius. 



Since no separation, or semblance thereof, must 

 occur in way of the inside of the skeg deflector, 

 especially in front of the upper propeller blades, 

 there is another practical limit to which the 

 trailing portion may be bent. The slope of any 

 waterline through the offset portion of the skeg 

 ending, assuming this waterline to be genei'ally 

 in the plane of flow, should under normal circum- 

 stances not exceed 18 deg or, at the most, 20 deg, 

 especially at the shallow drafts. This is admittedly 

 a rather indefinite rule which requires amplifying 

 and checking from actual ship designs known to 

 be successful. Values which are definitely out of 

 bounds can be determined from vessels where 

 separation, vibi'ation, and air leakage are known 

 to exist. For the ABC ship, Fig. 67. T, it is possible 

 to hold the maximum median-line slope at the 

 12-ft WL, at about x' = 0.16, to some 8.8 deg, 

 with a maximum slope on the convex side of 18 

 deg at the same level. 



The use of a contra-guide skeg ending is 

 approached with caution when the waterlines 

 (or flowlines) leading up to the forward edge of 

 the propeller aperture in a skeg have a slope 

 already approaching the limit beyond which 

 separation may be expected at that level, indi- 

 cated in Sec. 46.2. Superposing the deflector shape 

 upon a symmetrical skeg ending diminishes the 

 waterline slope on one side but greatly increases 

 it on the opposite side. 



The augmented slope on the outer or convex 

 side, away from the deflection, may easily become 

 greater than the critical slope for separation. 

 Since the blunter waterlines are generally to be 

 found above the shaft axis, it is wise, under these 

 circumstances, to limit the deflecting portion of 

 the skeg to the region below the shaft boss, 

 leaving the upper portion symmetrical, with 

 equal waterline slopes on each side. 



In view of the limited pressure, and the low 

 pressure gradient available on the convex side 

 of a twisted skeg or stern, the water on that side 

 is not easily changed in direction. This means 

 that a relatively long time, coupled with a 

 relatively long distance, of the order of 0.5 to 1.0 

 times the propeller diameter, may be required to 

 impart to it any appreciable transverse component 

 of velocity without risking separation. To take 

 care of this situation, the asymmetrical slopes of 

 the contra-ending are to merge gradually into 

 the symmetrical waterline slopes ahead of them. 



