Sec. 73.20 



FIXED-APPENDAGE DESIGN 



697 



centerline keel from the rolling axis. It appears 

 well established, however, that any downward 

 projection on the centerplane, for a normal beam- 

 draft ratio, has a beneficial effect in quenching roll. 



For vessels of nearly rectangular section and 

 large beam-draft ratio, say 4 or more, two drift- 

 resisting keels fitted at the lower corners of the 

 hull serve also as roll-resisting and as docking 

 keels. In this case, the lateral spread is made 

 large, at least 8 times the keel depth, to avoid 

 interference and to make both keels fully effective 

 when subjected to transverse flow. 



Resting keels on a submarine may or may not 

 project below the keel, depending upon the design 

 requirements. They need not even be flat on the 

 bottom, if there is an advantage in a special shape. 



The sides of docking and resting keels flare 

 outward above the support surface, and they 

 meet the hull so that the reentrant angle is greater 

 than 90 deg. This feature is illustrated in Figs. 

 36.N and 73.0. 



Docking and resting keels offer the minimum 

 of resistance when they follow the flowlines under 

 the bottom. This means that under a flat-bottomed 

 ship they should diverge slightly from forward aft, 

 indicated by the directions of the under-the- 

 bottom flowlines of Fig. 52.V. This divergence is 

 of no particular consequence in a dock equipped 



l>=Heiqht of One (or Two) <^ tMo'y Be Vertical on Outsid; 



Shallow Dockintj 



Keel to Reduce 



Resistance 



Upper Levels 

 of Dock in 



Blocks 

 Omitted m Wq>1 of 



.ing , 

 locks LL 

 m WqvJ , 



I Alternative Method of 

 Knucklina Hull and 

 Eliminating Docking 

 Keel Altogether 



Cutup Equal to the Total Height of a 

 Whole Number of Docking- Block 

 Heights, such as 14, 28, or42 inches, 

 to be Cribbed^ Up in This Region 



NormaT^Level of Tops of Blocks 



4 



Side Elevotion o^ a Docking or 

 Resting Keel, Showing Venting 

 Notch or Passage. Bilge Keel 



May Be Similar 



Large- Scale Section 



Hull 

 Vent^ 



Fig. 73.0 Design Sketches for Docking Keels 



to take vessels of large horizontal area which may 

 have these keels. A ship with a rather large rise 

 of floor may have well-curved flowlines; the 

 docking or resting keels preferably follow them. 



The direction of flow toward and away from 

 docking and resting keels is known rather accu- 

 rately from model tests and remains fixed for all 

 steady-state straight-line travel. For this reason 

 the ends are invariably fined on both sides of the 

 keel. The bottom surface is carried out to a 

 point, to give the maximum support surface, 

 then may be cut up rather sharply, as .shown at 6 

 in Fig. 73.0. 



Notches, deep and wide enough to permit 

 making their boundaries securely watertight, may 

 be worked into the upper portions of deep keels. 

 These help in the venting of air bubbles which 

 find their way under and inside the keels and 

 which might work their way up into injection 

 openings. The venting openings are necessary in 

 submarines which have main-ballast flood valves 

 or flooding openings below them. Excess air from 

 the blowing of these tanks can not be trapped 

 below the keels. 



73.20 The Design of Fixed Stabilizing Skegs 

 or Fins. The design of fixed skegs or fins intended 

 primarily to act as stabilizing surfaces, excluding 

 in this case stabilization against roll, is based upon: 



(1) The damping forces and moments required to 

 be exerted by them in the angular motions of 

 pitch and yaw. Damping of the translatory 

 motions of heaving, surging, and sidling is seldom 

 necessary and is excluded here. 



(2) The hydrodynamic lift exerted by these 

 surfaces, and the corresponding restoring moments 

 developed by them, when the ship axis departs 

 from the normal straight-ahead motion axis and 

 the surfaces run at a finite angle of attack. This 

 may be an angle of either yaw or pitch. Good 

 design requires that the fins or skegs be placed in 

 a region of relatively smooth flow, as distinguished 

 from a region of eddying in a separation zone. In 

 general, the lift force developed by them should be 

 zero for the normal attitude and normal operating 

 condition of the vessel. 



In airplanes the disposable weights usually are 

 too small to permit achieving equilibrium by 

 moving them forward and aft. Stabihzer surfaces 

 with adjustable angle of attack are often provided 

 in the tail to produce compensating moments for 

 maintaining trim balance. In submarines pro- 

 vision is made in the liquid trimming system for 



