Sec. 73.19 



FIXED-APPENDAGE DESIGN 



(i'J5 



with some increase in friction resistance. It 

 requires a rather careful preliminary flow study- 

 on a model, to insure that not only the keel 

 proper but the outer flange lie in the streamlines. 

 A flange on the free edge should not carry around 

 the tapered portion at the ends but should itself 

 be tapered to zero where the width-tapering of 

 the keel begins. 



A triangular keel section affords immeasurably 

 greater structural rigidity than a thick flat section. 

 It is preferred where weight and displacement 

 considerations permit. The section can take the 

 form of an acute-angled triangle with a peak 

 angle of not more than 15 deg, without any 

 sacrifice of damping quahties. On large vessels 

 expected to roll heavily the base of the triangular 

 section of a roll-resisting keel may be as great as 

 0.4 or more of the keel width. 



Bilge keels are heavily loaded in an alternating 

 cycle. The extreme forward end is partly unsup- 

 ported unless it is brought to a rather long, sharp 

 point. All in all, the normal bilge keel requires a 

 special structural attachment to render it secure 

 for long periods of hard service. Indeed, the 

 forward sections of roll-resisting keels on fast 

 and high-speed vessels could well be built of 

 heavier scantlings than the rest. 



The attachment of the base of the keel to the 

 hull should be stronger than that of the sides or 

 projecting portions of the keel to the base. This 

 insures that if the keel is overloaded in any way, 

 the shell connections will remain intact. 



73.18 Design of Roll-Resisting Keels for the 

 ABC Ship. The roll-resisting keel traces for the 

 ABC ship, shown on Fig. 66.P, were determined 

 by flow tests on the model, using pivoted flags 

 which projected from the hull for a distance cor- 

 responding to some 3 ft on the ship. From the 

 straightness of the traces as projected on the 

 midsection plane it might appear that they were 

 simply drawn as diagonals on the body plan. It 

 so happens that the flow on either side of the bilge- 

 keel positions straightens itself, as it were, in 

 these regions. This does not always occur, as 

 evidenced by the wavy keel traces on the Mariner 

 class, reported by V. L. Russo and E. K. Sullivan 

 [SNAME, 1953, Fig. 18, p. 127]. Were the crests 

 and troughs of the Velox waves more pronounced 

 along the side of the ABC ship, the bilge-keel 

 trace might well be affected by them. 



The slack bilge of this ship was laid out to 

 permit the attachment of wide roll-resisting keels, 

 for the reasons given in Sec. 66.13. The maximum 



keel width of 3.5 ft, or about 0.0477i?.v , still 

 leaves a clearance of about a foot above the floor 

 line and a foot inside the side of the ship. 



The bilge-keel area was governed in this case 

 only by the rules in the sections preceding and 

 by the endeavor to obtain as much bilge-keel area 

 as possible without sacrificing other features. 

 The keels were carried forward and aft, as 

 shown in Fig. 66. P, to points where they were 

 considered as no longer paying their way. The 

 effective length is some 7 stations (Sta. 7 to Sta. 

 14); this is equivalent to 178.5 ft or 0.35 times the 

 waterline length. 



The taper at the forward ends was made about 

 18 ft long, shown in Fig. 73. N, or over 5 times 

 the depth, exclusive of the structural terminal 

 plate and fairing bar. That at the after end was 

 made some 10 ft long, or about 3 times the depth. 



The width of the triangular keel at the base is 

 1.5 ft, or 0.43 times its maximum depth. The 

 hydrodynamic loads imposed in quenching roll 

 are transferred to the hull as a combination of 

 shear loads and normal loads. The latter pull in 

 and out on the shell in line with the side plates of 

 the triangular structure. Any offset members in 

 this structure, such as side plates of the keel 

 riveted to shell angles, are almost certain to 

 develop high bending moments and eventually 

 to become loose. It is for this reason that the 

 shell angle should be very heavy. The rivets 

 which hold this angle to the shell are placed as 

 close as possible to the force-application lines in 

 the side plates. This means as close as they can 

 be driven to the bosom of the bar. 



73.19 Design of Docking, Drift-Resisting, 

 and Resting Keels. The best design procedure 

 for docking keels, on a large, heavy ship where 

 considerable off-center support is mandatory, is 

 to make them unnecessary by working the desired 

 flat supporting surface into the bottom of the 

 ship itself. In regions where direct support is 

 required and the normal faired lines lie somewhat 

 above the blocking level at the baseplane, the 

 bottom of the ship is brought down deliberately 

 to the level of the tops of the docking blocks. 

 Figs. 67. L and 67.M for the arch-stern ABC 

 design indicate, in the regions near the baseplane 

 at Stas. 14 and 15, how this is done. Although in 

 this ship the hull terminates at the after quarter- 

 point in a flat surface coinciding with the floor 

 line, the shape would be essentially the same if 

 brought down to the baseplane. In many ships 

 this modification involves only a surprisingly 



