736 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 14.21 



in the forwaid portion of the rudder. Usually 

 there is not adequate room for the stock and its 

 bearings if this is done. The hull is so thick at 

 the after end of the aperture that it is not possible 

 to take advantage of a balance portion abaft the 

 stock, where the flow would be ver}' disturbed. 

 The customary solution is to mount the stock at 

 the extreme after end, with or without a pintle 

 and bearing at the keel level. A rudder of this 

 type is sho^\^l by G. de Rooij ["Practical Ship- 

 building," 1953, Fig. 495, p. 203]. 



74.21 General Design Rules for Bow and 

 Stem Diving Planes. Bow and stern diving 

 planes on submarines suffer from certain design 

 limitations which should be but are not yet over- 

 come: 



(a) The bow planes, almost invariably required 

 to rig in or house within the fair hull lines, are not 

 easily supported when given great span and a 

 large aspect ratio. Furthermore, their inner ends 

 can rarely lie close against the hull, mth a small 

 gap throughout the complete range of rise and 

 dive angles. 



(b) The port and starboard stern planes, placed 

 across the propeller-outflow jet(s), can almost 

 never he with their inner ends close against the 

 hull, or close to each other, principally because 

 of the triangular gap necessary to s\ving the 

 steering rudder between them. It, too, must lie 

 withm or close to the outflow jet(s). 



(c) To produce the maximum possible vertical 

 forces for a given weight and size of installation 

 the planes are often worked far beyond the normal 

 breakdo"wn range of a symmetrical hydrofoil. 



An effective compromise to meet all these 

 conditions calls for an aspect ratio of approxi- 

 mately 1.0, with no cantilever or image effect. 

 In other words, the diving planes are made 

 roughly square in planform. It is assumed that 

 they do not benefit in lift by being close to the 

 hull or to some large vertical surface. 



The non-housing diving planes of a submarine 

 are so near the surface, when the vessel is not 

 awash or submerged, that in a heavy sea they 

 are subject to severe impact in the form of wave 

 slap. This applies to both bow and stern planes. 

 A good design requirement, admittedly formu- 

 lated on a not-too-scientific basis, is that these 

 planes shall withstand as a working load an 

 impact pressure of 1,000 lb per ft" over their 

 entire horizontal area. 



A somewhat similar but possibly less drastic 



requirement could be imposed on rudder mstalla- 

 tions in Avhich any part of the rudder rises above 

 the water surface during wavegoing. 



74.22 Centra-Features for Diving Planes. 

 Whether placed in the outflow jets of propellers 

 or not the diving planes of submarines rarely 

 work in flows that are symmetrical about the 

 mean plane of the blades. First, the flow in any 

 vertical plane is rarely symmetrical ^\dth respect 

 to the submarine axis. Second, it is rarely possible 

 to locate the diving planes in symmetrical posi- 

 tions relative to that axis. The flow at any plane 

 position usually has some vertical component of 

 velocity. The neutral plane angle must be ad- 

 justed accordingly or the leading edge of the 

 plane must be bent or twisted to point into the 

 direction of flow. This bending, similar to that at 

 the leading edge of a contra-rudder, prevents the 

 center of pressure from lying too far forward of 

 the plane axis. 



A pair of diving planes, placed in a symmetrical 

 position abaft the propeller of a single-screw 

 submarine and twisted, contra-fashion, serves to: 



(1) Recover rotational energy in the outflow jet 

 and convert it to useful thrust 



(2) Compensate partly for the unbalanced re- 

 action exerted by the submarine propelling plant, 

 in the manner described by Sec. 73.21. 



74.23 Setting Neutral Control-Surface Angles. 

 When rudders are offset from the vertical plane 

 of symmetry, either as parts of single-rudder or 

 multiple-rudder installations, the water almost 

 never flows past them in a direction parallel to 

 that plane, ■with the ship moving straight ahead 

 at a steady speed. To achieve equal turnmg effects, 

 right and left, with equal amounts of right and 

 left rudder angle, each rudder must be placed 

 carefully at zero angle in its neutral position. 

 This adjustment is made for the condition when 

 the propulsion de\'ices are working; all other 

 conditions are assumed to be normal. 



The neutral position is determined with relative 

 ease in a self-propelled model test by any one of 

 several methods. However, if there is a possibility 

 either of laminar flow or of separation on the 

 rudder, due to its form or to the shape of the hull 

 m the vicinity, there may be some scale effect, 

 because of unexpected shifts in the transition or 

 separation points. If so, the model predictions 

 are uncertain when applied to the ship. 



It is diflScult if not impossible to determine 



