Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERING1;;=Tttf + +COMPRESSIONTENSIONFigure 2-4.— Sagging.H147.10the stresses are reversed. The weather deckis now In tension and the bottom plating is incompression. A ship in this condition is saidto be hogging. Hogging, like sagging, is a formof longitudinal bending. The effects of longitudinalbending must be considered in the designof the ship, with particular reference to theoverall strength that the ship must have.In structural design, the terms hull girderand ship girder are used to designate the structuralparts of the hull. The structural partsof the hull are those parts which contribute toits strength as a girder and provide what isknown as longitudinal strength . Structural partsinclude the framing (transverse and longitudinal),the shellplating, the decks, and the longitudinalbulkheads. These major strength membersenable the ship girder to resist the variousstresses to which it is subjected.The ship girder is subjected to rapid reversalof stresses when the ship is in a seawayand is changing from a hogging condition to asagging condition (and vice versa), since thesechanges occur in the short time required forthe wave to advance half a wave length. Otherdynamic stresses are caused by pressure loadsforward due to the ship's motion ahead, bypantingl of forward plating due to variationsof pressure, by the thrust of the propeller, andby the rolling of the ship.HULL MEMBERSThe principalstrength members of the shipgirder are at the top and bottom, where thegreatest stresses occur. The top flange includesthe main deck plating, the deck stringers,and the sheer strakes of the side plating. Thebottom flange includes the keel, the outer bottomplating, the inner bottom plating, and any continuouslongitudinals in way of the bottom. Theside webs of the ship girder are composed ofthe side plating, aided to some extent by anylong, continuous fore-and-aft bulkheads. Someof the strength members of a destroyer hullgirder are indicated in figure 2-6.KeelThe keel is a very important structuralmember of the ship. The keel, shown in figure2-7, is built up of plates and angles into anI-beam shape. The lower flange of this I-beamstructure is the flat keel plate, which formsthe center strake of the bottom plating-2The web of the I-beam is a solid plate whichis called the vertical keel . The upper flangeis called the rider plate ;this forms the centerstrake of the inner bottom plating. An innervertical keel of two or more sections, consistingof I-beams arranged one on top of theother, is found on many large combatant ships.FramingFrames used in ship construction may be ofvarious shapes. Figure 2-8 illustrates framesof the angle, I-beam, tee, bulb angle, and channelshapes. Figure 2-9 shows two types of'3E m. -j3Transverse stress results from the pressureof the water on the ship's sides whichsubjects the transverse framing, deck beams,and shellplating below water to a hydrostaticload. Local stresses occur in the vicinity ofmasts, windlasses, winches, and heavy weights.These areas are strengthened by thicker deckplating or by deeper or reinforced deck beams.S\TENSIONCOMPRESSION\\\Figure 2-5.— Hogging.147.11Panting isat the bow.a small In-and-out working of the platingOn large ships, an additional member is attached tothis flange to serve as the center strake.18