PRINCIPLES OF NAVAL ENGINEERING 



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COMPRESSION 

 TENSION 



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147.10 



Figure 2-4.— Sagging. 



the stresses are reversed. The weather deck 

 is now In tension and the bottom plating is in 

 compression. A ship in this condition is said 

 to be hogging. Hogging, like sagging, is a form 

 of longitudinal bending. The effects of longitu- 

 dinal bending must be considered in the design 

 of the ship, with particular reference to the 

 overall strength that the ship must have. 



In structural design, the terms hull girder 

 and ship girder are used to designate the struc- 

 tural parts of the hull. The structural parts 

 of the hull are those parts which contribute to 

 its strength as a girder and provide what is 

 known as longitudinal strength . Structural parts 

 include the framing (transverse and longitu- 

 dinal), the shellplating, the decks, and the longi- 

 tudinal bulkheads. These major strength mem- 

 bers enable the ship girder to resist the various 

 stresses to which it is subjected. 



The ship girder is subjected to rapid re- 

 versal of stresses when the ship is in a seaway 

 and is changing from a hogging condition to a 

 sagging condition (and vice versa), since these 

 changes occur in the short time required for 

 the wave to advance half a wave length. Other 

 dynamic stresses are caused by pressure loads 

 forward due to the ship's motion ahead, by 

 pantingl of forward plating due to variations 

 of pressure, by the thrust of the propeller, and 

 by the rolling of the ship. 



Transverse stress results from the pres- 

 sure of the water on the ship's sides which 

 subjects the transverse framing, deck beams, 

 and shellplating below water to a hydrostatic 

 load. Local stresses occur in the vicinity of 

 masts, windlasses, winches, and heavy weights. 

 These areas are strengthened by thicker deck 

 plating or by deeper or reinforced deck beams. 



Panting is a small In-and-out working of the plating 

 at the bow. 



HULL MEMBERS 



The principal strength members of the ship 

 girder are at the top and bottom, where the 

 greatest stresses occur. The top flange in- 

 cludes the main deck plating, the deck stringers, 

 and the sheer strakes of the side plating. The 

 bottom flange includes the keel, the outer bottom 

 plating, the inner bottom plating, and any con- 

 tinuous longitudinals in way of the bottom. The 

 side webs of the ship girder are composed of 

 the side plating, aided to some extent by any 

 long, continuous fore-and-aft bulkheads. Some 

 of the strength members of a destroyer hull 

 girder are indicated in figure 2-6. 



Keel 



The keel is a very important structural 

 member of the ship. The keel, shown in figure 

 2-7, is built up of plates and angles into an 

 I-beam shape. The lower flange of this I-beam 

 structure is the flat keel plate, which forms 

 the center strake of the bottom plating-2 



The web of the I-beam is a solid plate which 

 is called the vertical keel . The upper flange 

 is called the rider plate ; this forms the center 

 strake of the inner bottom plating. An inner 

 vertical keel of two or more sections, con- 

 sisting of I-beams arranged one on top of the 

 other, is found on many large combatant ships. 



Framing 



Frames used in ship construction may be of 

 various shapes. Figure 2-8 illustrates frames 

 of the angle, I-beam, tee, bulb angle, and chan- 

 nel shapes. Figure 2-9 shows two types of 



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TENSION 



COMPRESSION 



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Figure 2-5.— Hogging. 



147.11 



On large ships, an additional member is attached to 

 this flange to serve as the center strake. 



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