550 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 68.4 



Fig. 68. C, apply to types of vessels in the following 

 services: 



(1) Low- and intermediate-speed merchant types, 

 low point of sheer at or close to amidships, 0.50L 

 from FP, sheer at stern about 0.5 of sheer at bow. 

 In average heavy- weather areas the wave speeds 

 are greater than the speeds of these slow vessels. 

 For a considerable part of their running time 

 they are being overtaken by seas, hence the need 

 for rather large sheer at the stern. 



(2) Medium-speed passenger and cargo ships and 

 intermediate liners, low point of sheer at about 

 0.575 to 0.625L, sheer at stern about 0.33 of 

 sheer at bow. These vessels, running at speeds 

 higher than those in (1) preceding, spend less of 

 their time being overtaken by following seas. 

 Furthermore, they often have more than the 

 minimum freeboard in order to have internal 

 space for accommodations. However, at the higher 

 speeds, they pitch more than the slower vessels. 

 With their finer entrances the pitching axis is 

 farther aft. 



(3) Large medium- and high-speed liners, low 

 point of sheer at about 0.65 to 0.75Z/, sheer at 

 stern about 0.2 of sheer at bow. To obtain internal 

 volume and superior wavegoing performance the 

 freeboard is usually exceptionally high. This 

 means that the pitching depth at the stem and at 

 the stern are adequate without using great sheer. 

 The latter may, in fact, be governed more by 

 appearance than by wavegoing, or by the necessity 

 for rapid shedding of the water that may come 

 aboard. 



(4) Tankers, trawlers, fishing vessels, and other 

 low-freeboard craft, low point of sheer at from 

 0.5 to 0.65L, sheer at stern about 0.7 of sheer at 

 bow. An alternative straight-element profile for 

 tankers is indicated in broken lines. There are 

 incUned straight lines at the bow and stern and 

 about half a ship length of straight sheer amid- 



ships, parallel to the baseline. Most of the tankers 

 are in the same category as the intermediate- 

 speed merchant cargo types, running for a large 

 percentage of the time in overtaking seas. How- 

 ever, at these speeds they require more sheer aft 

 than the cargo vessels because their average 

 freeboard is less. The trawlers and fishing vessels 

 heave to or creep along at slow speeds on all 

 courses and in all weathers so that they are over- 

 taken by waves to about the same degree as 

 other ships encounter them. 

 (5) Tug and similar types, lowest point of deck 

 profile at about 0.65 to 0.8L, sheer at stern about 

 0.4 of sheer at bow. Ocean-going tugs may require 

 from 1.5 to 2.0 times as much sheer forward as 

 harbor tugs. To prevent the excessive heels 

 mentioned in Sec. 68.2 it is necessary that the 

 towing bitts of tugs be kept close to the water. 

 This means that the low point of the profile is 

 well aft, in the vicinity of the bitts, and that the 

 sheer aft is the minimum permissible by standards 

 of safety which have been proved in everyday 

 service. 



For sailing yachts the low point of the sheer 

 line is usually much farther aft than for any other 

 type. It can be as far aft as 0.75 times the overall 

 length from the bow. 



All the sheer curves in Fig. 68. C are arcs of 

 second-order parabolas, x^ = —az, where z is 

 directed keelward for -|- values. Their axes are 

 vertical and their vertexes are at the low points 

 indicated by circles. The forward and after arcs 

 belong to different parabolas but they can belong 

 to the same one if the low-point position and the 

 forward and after sheers correspond. New sheer 

 lines are easily drawn by taking constant per- 

 centages of the ordinates shown in the figure, as 

 illustrated for the sheer fine of the ABC design. 



If construction costs are a factor, any part or all 

 of the sheer line may be straight, as shown in 



Fig. 68. B Typical O-Duil Freeboard Ratios for a Whale Catcher 



