Sec. 6121 



UNDERWATER-HULL DESIGN 



531 



corners is probably less objectionable than 

 I'oiinding the lower edges. 



The transom bottom slope is , measured in a 

 transverse plane, is related to slamming and as 

 such is discussed in Part 6 of Volume III. 



Before the shaping of the transom stern of the 

 single-skeg ABC ship was completed, in the 

 course of preparing the body plan of Fig. 66. P, 

 the transom depth was increased from the original 

 1.5 ft to 2.0 ft. This gave more slope to the lower 

 transom section lines and decreased the prob- 

 ability of pounding or slamming under the stern. 



The planform of the ABC transom stern at 

 the DWL is made slightly convex, with a radius 

 of O.IOL, partly to facilitate angling the vessel 

 into a short berth, not much greater than its 

 length, and partly for the sake of appearance. The 

 transom width at the DWL, projected to the 

 plane of the AP, is 0.33Ba- • 



The transom of the ABC arch-type stern, 

 sections of which appear in Figs. 67. L and 67. P, 

 is deliberately made deeper at the outer corners 

 than the depth which will clear at 20.5 kt in 

 order to avoid the most troublesome problem of 

 fairing the upper parts of the two skeg endings, 

 under the hull. Some separation is certain to 

 exist in either case. It is considered far preferable 

 to fair the hull directly into the upper portions 

 of the two rudders, and to accept eddying abaft 

 the deep sides of the transom, than to permit 

 separation farther forward, nearer the propeller 

 and possibly interfering with rudder action. 



The transom planform of this vessel is made 

 shghtly convex, with a radius of 0.15L. The 

 transom width at the DWL, projected to the 

 plane of the AP, is 0.4455^ . 



In profile, the shape of the transom is generally 

 determined as a matter of appearance and con- 

 struction. In all vessels which may upon occasion 

 be required to run astern at considerable speeds, 



Probable Direction 

 of Flow Leaving a 

 Sharp- Edc^ed Transom ^ 



Terminotina ot the Knuckle 



Fig. 67.Q Diagram of Probable Flow Under 

 Rounded Transom Edge 



the matter of throwing spray or meeting waves is 

 one to be given consideration. A square, vertical 

 transom should apparently be avoided for this 

 reason, yet large vessels with a stern termination 

 of this type have reported no difficulties in service. 

 Taken by and large, the transom stern of the 

 German World War II destroyers of the Narvik 

 class, portrayed in Fig. 67. R, is commended as 



Fig. 67.R Transon Stern on Model of German 

 Destroyers of Narvik Class 



embodying all desirable hydrodynamic features 

 and offering a pleasing and ship-shape appearance 

 without expensive or complicated construction. 



67.21 The Design of a Multiple-Skeg Stem. 

 Design rules for multiple-skeg sterns are available 

 in rather complete form in the technical literature 

 [SNAME, 1947, pp. 130-132]. The historical 

 examples in that reference are supplemented by a 

 quadruple-screw design for English channel 

 service by John Dudgeon [INA, 1873, pp. 88-95 

 and PL VIII]. The hull form illustrated in the 

 latter reference comprises two skegs, with two 

 screws inboard and two screws outboard of them. 

 The tunnel between the skegs extends all the way 

 to the bow. So far as known, no craft of this type 

 was ever built. 



For the design of multiple skegs on a modern 

 craft the rules in the SNAME 1947 reference are 

 considered adequate, when supplemented by the 

 following: 



(1) Consider the use of twin skegs or multiple 

 skegs only on afterbody forms which lend them- 

 selves to this arrangement, or on which beneficial 

 results may be expected. This includes wide 

 ships, or ones with large B/H ratios, where it is 

 difficult to close the waterlines in to the center- 

 plane without large slopes. In general, the after- 

 body should have a prismatic coefficient Cp of 

 0.60 or larger. 



