700 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 73.23 



the positions of maximum waterline or extreme 

 beam. Tiiese positions may vary vertically, or 

 girthwise, from station to station on a ship, so 

 the fenders can seldom be led along the Unes of 

 flow to insure minimum drag. Perhaps this is just 

 as well, because most of them lie near the free- 

 water surface. Here the lines of flow under the 

 waves of the Velox system change with speed and 

 with position along the length. Moderate amounts 

 of roUing and pitching also change the direction 

 of the resultant flow accordingly. 



Considering structural, fabrication, first-cost, 

 and maintenance factors, as well as hydro- 

 dynamics, nothing can equal or surpass the 

 heavy fender strake which forms an integral part 

 of the shell plating. It involves practically no 

 increase in drag. It eliminates all discontinuities 

 in the fair form of the hull except the strake 

 edges. It takes care of all problems of leaky 

 connections, unseen corrosion, spray throwing, 

 and the like. It serves best when acting as the 

 outer boundary of an irregular hull section, with 

 some transverse curvature for stiffness, dia- 

 grammed at 1 in Fig. 73. Q. It is adequate, never- 



Sinole- Thickness 

 Fender Stroke 



^ Fender 5tral<e 



Structure is Schematic 

 and ExQoaeroted for Emphasis 



Transverse Curvature 

 Gives This Plate 

 Inherent Stiffness 



Fig. 73.Q Forms of Heavy Fender Steakes 



theless, with possibly some slight increase in 

 weight, for vertical ship sides against vertical 

 walls, sketched at 2 in the figure. 



A heavy fender strake of this type is always 

 made an outside strake. If the seams are riveted 

 or if they are welded a strake is easily removed 

 from the outside for repair or replacement. A 

 fender strake of this kind, standing vertical at 

 the maximum beam of an underwater bulge, is 

 shown by J. L. Bates and I. J. Wanless [SNAME, 



1946, Fig. 14, p. 332]. Diagram 2 of Fig. 73.Q is a 



schematic sketch of a similar fender strake. 



When placing propeller guards it is necessary 

 to take account of the wave profile aft, plus the 

 the usual disturbed-surface layer covering it, in 

 the deepest draft-aft condition, to insure that 

 these guards remain out of water and do not 

 throw spray. 



A rather large-scale drawing of a bow rudder 

 and its close-fitting guard, designed for a cross- 

 channel steamer, is published in The Shipbuilder 

 [(now SBMEB) Jan-Jun 1914, Vol. X, p. 36]. 

 Another one is given by G. de Rooij ["Practical 

 Shipbuilding," 1953, Fig. 495, p. 203]. Twin weed 

 skegs outboard of the twin propellers of a 63-ft 

 motorboat are shown in (American) Motorship, 

 April 1948, page 34. 



73.23 Design to Avoid Vibration of Append- 

 ages. Means of predicting and avoiding the 

 singing of screw propellers are outlined in Sees. 

 23.7 and 70.46. It is equally important that those 

 parts of the main ship hull and those appendages 

 in the vicinity of the propulsion device (s) be 

 designed to have certain natural periods of vibra- 

 tion in water. These .should be different from the 

 exciting frequency of the blades or other principal 

 parts of the propulsion device (s). There are 

 techniques available, similar to those described 

 in TMB Report R-22 of April 1940, whereby the 

 natural frequencies of the parts of appendages in 

 question may be determined on existing ships, or 

 on new ships -prior to the first sea trial. This involves 

 the local attachment of small vibration generators 

 in watertight casings, or the excitation of the 

 appendages in their natural modes, also in water, 

 by connecting the generators and the appendages 

 with long struts. 



Long, slender, and well-streamlined appendages 

 Uke strut arms nevertheless require checking for 

 their susceptibility to resonant vibration. Under 

 operating conditions they may be followed by a 

 vortex street or trail. Existing data (1955) are far 

 from adequate as to the eddymaking charac- 

 teristics of elongated sections, especially in the 

 yawed attitudes to be found during turning and 

 wavegoing. 



A few general rules may be laid down as a 

 help to the designer in this respect: 



(1) A vortex street or trail may be shed from 

 any section, yawed or otherwise, on which there 

 are opposite separation points, and on which 

 these separation points may shift forward or aft 



