724 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.11 



Transverse Members Should Not Be ^°^^ Wrapper Plate 



Welded to Side Plotinq I '^'l '" °"^ ^'^^^ 



Fig. 74.G Rudder Structure with One System of 

 Welds Parallel to the Flow 



Affected by Hydrodynamics. Any control sur- 

 face, especially on a high-speed vessel, requires a 

 structural design and a fabrication procedure 

 which Avill insure that the selected control-surface 

 section, along the Ime of flow, is achieved m the 

 building of the ship and is maintained in service. 

 The finished boundary plating on a structure 

 that is hollow, as are most control surfaces, must 

 be fair and smooth, Avithout wrmkles or bulges. 

 Further, it must be so stiffened that it maintains 

 its fairness and shape while subjected to many 

 different kinds of loading, — steady, intermittent, 

 and alternating. This is one reason why the thin 

 coverings of airplane wings and control surfaces 

 are attached to ribs or stiffeners which lie in the 

 direction of flow. 



Indeed, the process of welding a rudder assembly 

 may introduce discontinuities in the surface before 

 the ship is completed. Internal stiffening members 

 attached to the boundary plates by inside welds 

 produce ridges in those plates because of shrinkage 

 of the weld metal when cooling. If the internal 

 stiffeners are placed generally normal to the flow, 

 welding gives the boundary plates a washboard 



effect, illustrated schematically in the lowest 

 diagram of Fig. 74. G. The boundary plates of a 

 rudder or a diving plane should be attached 

 primarily to internal structural members which 

 run fore and aft, parallel to the streamlines of the 

 liquid flowing over it, as in the topmost diagram 

 of Fig. 74. G. This is especially true if the conti'ol 

 surface lies within the outflow jet of a screw 

 propeller or other propulsion device. 



If practicable, there should be no joints, 

 welding beads, or the like, transverse to the floAV, 

 anywhere in the entrance of the control surface. 

 This is achieved by wrapping a single plate 

 around the nose and both sides of the entrance, 

 and attaching it to the frame only by fastenings 

 parallel to the flow. These fastenings are spaced 

 as closely as practicable, normal to the flow, so 

 as to make the plating relatively rigid, with little 

 likelihood of panting or bending between supports 

 and of failure through fatigue. 



74.11 Design Notes for Motorboat Rudders. 

 The general principles for the design of the rudders 



TABLE 74.d — Ratio op Rudder Area to Lateral- 

 Plane Abba for Motorboats and Similar Craft 

 The data listed here are translated from the book of J. 

 Baader entitled "Cruoeros y Lanchas Veloces (Cruisers 

 and Fast Launches)," Buenos Aires, 1951, pages 333 and 

 335. The percentages of Group III include, for each case, 

 the total projected area of the two rudders. 



On pages 334 and 337 of the reference Baader gives 

 profiles of thirteen launches, motorboats, and motor 

 yachts, showing the relative positions and comparative 

 sizes of screw propellers and rudders for these craft. 



I. Boats with One Screw Propeller and 



One Rudder {W')Ar/L{H) 



Minimum surface for good steering . 2 

 Normal surface for steering and 



maneuvering 2.5 



Ideal surface for steering and man- 

 euvering 3 



Best surface for going astern ... 4 to 5 



Maximum surface for special cases . 10 



II. Boats with Two Screw Propellers and 



One Rudder (between them) 

 Minimum surface for steering ... 2,5 

 Normal surface for steering and 



maneuvering 3 



Best surface for maneuvering con- 

 ditions 4 to 5 



III. Boats with Two Screw Propellers and 



Two Rudders 

 Minimum surface for steering ... 2 

 Minimum surface for steering and 



maneuvering 2.5 



Advisable surface for steering and 



maneuvering 3 



Best surface for going astern .... 4 to 5 



