542 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.28 



in the 2-diameter regions just ahead of and just 

 abaft the disc. Fig. GT.RI indicates that this con- 

 dition is not met in the ABC arch-stern design. 



A single-screw stern having an exceptionally 

 thin skeg, with the after part cut entirely away, 

 leaving the propeller supported by a projecting 

 stern tube, is illustrated and described by H. Waas 

 [STG, 1952, Fig. 9, p. 209, and Fig. 18, p. 214]. 

 If found practicable in any particular case this 

 is one way to reduce thrust-deduction and lateral 

 vibratory forces at the same time. If the after 

 end of the stern tube needs support, it can be 

 provided by a V-shaped strut assembly. 



Paddlewheels mounted abreast parallel water- 

 lines or in way of the maximum waterline beam 

 are admirably placed for eliminating practically 

 all resistance-augment forces. In fact, it is quite 

 possible, with a pair of paddlewheels amidships, 

 for the —Ap field to extend forward into the 

 entrance and the +Ap field back into the run. 

 The differential pressures then generate thrust 

 forces in the ahead direction and the thrust- 



deduction fraction is negative. It is unwise, 

 however, to rely upon this unless it is confirmed 

 by a model test. 



The reduced-pressure field ahead of a stern- 

 wheel extends forward of the point of immersion 

 of the foremost blade for a distance estimated as 

 twice the maximum depth of blade immersion, 

 or dip, as it is called. For this and other reasons, 

 set forth in Sec. 72.11, the buttock endings of a 

 sternwheel vessel are given an easy slope, and 

 possibly reverse curvature, below the level of the 

 wave profile at designed speed. 



The first approximation of the resistance 

 augment to be expected at the designed speed is 

 found by the transverse-area method, described 

 in Sec. 60.9. The method of performing this 

 operation for the transom-stern ABC design is 

 illustrated in Fig. G7.V. 



67.28 The Final Section-Area Curve. When 

 all principal parts of the underwater hull, ex- 

 cluding the appendages, are worked out suffi- 

 ciently to indicate the distribution of volume 



Weiqhted Areo 

 Area Foctor 



2555 

 15 



170.53 _ 



- 170.33 

 OMZ 



Predicted Value oft, from Fitj.eo.P, is 0. 135 

 Fio. 67.V Weighted-Area Diagram for ABC Transom Stern, for Prediction of Thrust-Deduction Fraction 



