530 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.20 



TABLE 67.d — Immersed-Transom Drafts Hu and Corresponding Speeds for a Transom-Submergence 



Froude NUi\rBBR Fh of 5.0 

 Here V/\/gHu = 5.0. The draft Hy is measured in the at-rest condition. The value of g is taken as 32.174 ft per sec^ 



transom. If the ship powers in this range are of 

 little or no importance, the immersion can be 

 deeper if there are other reasons for making it so. 

 If only high-speed operation is of importance, in 

 the range of T„ from about 1.1 or 1.2 to 4.0 or 

 more, f„ from 0.33 or 0.36 to 1.19 or more, the 

 draft Hu at the transom may be a quarter or 

 more of the draft of the ship. If reasons other 

 than water flow and resistance predominate, such 

 as on a floating whale factory with an immersed- 

 stern ramp, the lower transom edge is placed as 

 deep as may be necessary. 



The transom need only be wide enough to 

 prevent separation of flow along the sides of the 

 ship at the waterline. If a transom stern is adopted 

 to achieve useful hull volume and deck space at 

 the stern, it is made as wide as need be. A normal 

 transom stern may therefore have a width of 

 0.8, 0.9, or more of the maximum waterline beam 

 Bwx of the vessel. 



It was at one time considered that the immersed- 

 transom area ratio A^/Ax was the principal 

 parameter in the selection and design of a stern 

 of this type. On the basis that the transom must 

 clear before its benefits are fully reaUzed it is 

 obvious that, for a given area ratio, the immersed 

 draft can change greatly, depending upon the 

 transom waterline beam Bu or the transverse 

 section slopes at or near its lower edge. Since the 

 latter two parameters apparently have no direct 

 effect upon either the performance or the design, 

 provided the beam is great enough to prevent 



separation at the waterline ahead of the transom, 

 the effect of the transom-area ratio is questionable. 

 To be sure, this ratio appears as f r on the section- 

 area curve, an example of which is given in the 

 lower diagram of Fig. 24. F, but the significance 

 of this ratio, as well as the terminal value tn at 

 the AP, remains to be discovered. On high-speed 

 craft the immersed-transom area ratio may be 

 made 0.15 or more, depending upon the desired 

 shape of the buttocks aft. 



The exact transverse shape of the submerged 

 transom is not too important, except as it affects 

 the immersed draft Hu . It can have a vee or a 

 rectangular shape to suit the lines of the run, 

 it can have square or rounded corners in a trans- 

 verse plane to correspond to the lines forward 

 of it, and it can have flare or tumble-home at the 

 sides. Likewise, the planform can be varied 

 throughout rather wide Umits from a flat trans- 

 verse to a V-shape or to a curved shape. So far 

 as known, it is not even necessary that the 

 planform curvature below the designed waterline 

 remain convex to the hull. 



Rounding off the bottom edges of the transom, 

 sketched in Fig. 67. Q, and the more-or-less 

 vertical corners at the sides, in the region below 

 the full-speed wave profile, is sometimes done 

 to ease construction or improve appearance. 

 Although there is definitely an increased drag 

 due to the fringing separation zones at the 

 forward ends of these rounded corners the increase 

 is rarely measurable. Rounding the vertical 



