696 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 73.19 



small change in form, even when the docking 

 support is offset from the centerplane. The 

 resulting discontinuities are moderate, distorting 

 the flow only slightly and involving little or no 

 added drag. Obviously, at the ends of a ship with 

 the usual deep forefoot and aftfoot the major 

 support area is under the centerline keel. Such 

 a keel is kept down to baseplane level for as 

 great a proportion of the length as possible, 

 consistent with good flow and drag and with the 

 required maneuvering characteristics. In a normal 

 design the flat keel should lie in the baseplane 

 for at least 0.8 of the waterline length, although 

 in special cases this ratio may diminish to 0.7, 

 0.6, or less [Clark, L., ATMA, 1900, p. 361]. 

 For the transom-stern ABC hull this ratio is 

 0.925, indicated in Figs. 66.Q and 66.T, even 

 though the aftfoot is cut away. For the arch- 

 stern ABC design, centerline support is provided 

 for 0.75 times the length plus two rows of side 

 support, under the skegs, each for about 0.15L,rL ■ 



If the hull proper can not be brought down to 

 the level of the baseplane for docking support, 

 the next best procedure is to raise part of the 

 docking-keel level above the flat-keel level. In 

 any one area this can be done by adding standard 

 layers of material, say 4 in or 14 in thick, to the 

 tops of the regular docking blocks. A ship of 

 moderate draft usually has ample clearance over 

 the raised blocking in a dock, even when some 

 compartments are damaged and flooded. The 

 designer may cut out the unwanted or undesirable 

 portion of a deep keel or skeg without sacrificing 

 blocking support by raising the bottom or support 

 surface parallel to itself by a multiple of the block- 

 ing thickness. Figs. 67. M and 67.0 show that 

 under the two offset skegs of the ABC arch-stern 

 design, the level is raised 28 in, or 2.33 ft. Support 

 by keel and skegs is thus provided over 0.913 of 

 the waterline length. 



Blocking-support areas in a small ship should 

 preferably be at least 6 in wide. In a large ship 

 2 ft is the minimum, but 3 or 4 ft is preferred. 



An excellent drift-resisting keel for metal-hulled 

 vessels is a centerline box keel placed underneath 

 the main hull. An appendage of this kind was 

 fitted to the single-hull type of submarines built 

 by the Electric Boat Company during the period 

 1900-1925. This keel was used as a duct for 

 pumping out water tanks along the length of the 

 hull, as well as a means of holding fixed ballast. 

 It was made of two heavy channels with their 

 flanges facing outward. They were riveted to the 



circular pressure hull through their upper flanges 

 and a heavy horizontal closing plate was bolted 

 to their lower flanges. The bosoms of the two 

 channels, on the outside, were filled with blocks of 

 lead bolted in place as more-or-less permanent 

 ballast. These centerline box keels were rugged 

 enough to serve as resting keels when the ship 

 lay on the bottom or as support keels when it 

 was docked. 



There is no practicable limit to the depth of a 

 drift-resisting keel provided it is sufficiently 

 sturdy to take its share of the load when docking. 

 The keel is designed to be of watertight con- 

 struction, with one bottom plate and two side 

 plates, the latter attached to the flat keel plate. 

 If the keel is long relative to the vessel it becomes 

 in effect one of the longitudinal strength members. 

 As such the side plates are attached to the hull 

 in a manner to prevent transverse cracking of 

 both the side plates and the adjacent shell plates. 

 A flanged plate or a channel is used for the bottom 

 member so as to bring the fore-and-aft welds 

 above the lower outboard corners and relieve 

 them of concentrated loads during docking. 



Not only the knuckles or lower corners of the 

 keel but the upper corners should be as sharp as 

 practicable, with reentrant angles not less than 

 90 or 100 deg. The junction of the drift-resisting 

 keel and the hull should not be filleted as is the 

 case of the keep keel on a sailing yacht. The 

 outside and inside corners mentioned are deliber- 

 ately introduced to offer the maximum of dynamic 

 resistance on the advancing side of the keel and 

 the maximum of pressure resistance due to separa- 

 tion on the retreating side of the keel when 

 drifting or sidling. 



The inside of the keel may be filled with (1) 

 some lightweight water-excluding material blown 

 into place, with (2) some inert ballast which will 

 not accelerate corrosion of the metal, or with 

 (3) an inert gas. 



The fine, deep sections of some fishing vessels, 

 resembling those of a deep-keel yacht, have 

 excellent inherent drift-resisting as well as roll- 

 damping characteristics. Body plans and lines 

 are shown in an article reporting NPL tests with 

 models, entitled "Experiments with Herring 

 Drifters" [SBSR, 28 Jul 1938, pp. 103-106]. 



Whether any centerline drift-resisting keel 

 provides sufficient roll quenching to permit elimi- 

 nation of the roll-resisting keels is open toquestion. 

 This depends upon the shape of the transverse 

 section and the length of the lever arm of the 



