Principles of naval engineering - Historic Naval Ships Association

Chapter 2-SHIP DESIGN AND CONSTRUCTIONBASIC SHIP STRUCTUREIn considering the structure of a ship, itis common practice to liken the ship to a boxgirder. Like a box girder, a ship may be subjectedto tremendous stresses. The magnitudeof stress is usually expressed in pounds persquare inch (psi).When a pull is exerted on each end of a bar,as in part A of figure 2-2, the bar is under thetype of stress called tension . When a pressureis exerted on each end of a bar, as in partB of figure 2-2, the bar is under the type ofstress called compression. If an equal butopposite pull is exerted on the upper and lowerbars, as shown in part C of figure 2-2, thepins connecting these bars are subjected to astress at right angles to their length. Thisstress is called shear. When a shaft, bar, orother material is subjected to a twisting motion,the resulting stress is known as torsionalstress . Torsional stress is not illustrated infigure 2-2.When a material is compressed, it is shortened.When it is subjected to tension, it islengthened. This change in shape is called strain .The change of shape (strain) may be regardedas an effect of stress.If a simple beam is supported at its twoends and various vertical loads are appliedover the center of the span, the beam willbend (fig. 2-3). As the beam bends, the uppersection of the beam compresses and the lowerpart stretches. Somewhere between the top andbottom of the beam, there is a section whichis neither in compression nor in tension; thisis known as the neutral axis . The greateststresses in tension and compression occurnear the middle of the length of the beam,where the loads are applied.LONGITUDINAL BENDING AND STRESSESIn an I-beam, the greater mass of structuralmaterial is placed in the upper and lowerflanges to resist compression and tension. Relativelylittle material is placed in the webwhich holds the two flanges so that they canwork together; the web, being near the neutralaxis, is less subject to tension and compressionstresses than are the flanges. The webdoes take care of shearing stresses, whichare sizeable near the supports.A ship in a seaway can be considered similarto this I-beam (or, more correctly, it can belikened to a box girder) with supports and distributedloads. The supports are the buoyantforces of the waves; the loads are the weightof the ship's structure and the weight of everythingcontained within the ship.The ship shown in figure 2-4 is supportedby waves, with the bow and stern each ridinga crest and the midship region in the trough.This ship will bend with compression at thetop and tension at the bottom. A ship in thiscondition is said to be sagging . In a saggingship, the weather deck tends to buckle undercompressive stress and the bottom plating tendsto stretch under tensile stress. A sagging shipis undergoing longitudinal bending—that is, itis bending in a fore-and-aft direction.When the ship advances half a wave length,so that the crest is amidships and the bow andstern are over troughs, as shown in figure 2-5,OAO LOAD LOAD NEUTRAL ^^^^^If ^ f ''r^^ I-BEAM I147.8Figure 2-2.— Stresses in metal: (A) tension;(B) compression; (C) shear.Figure 2-3.— I-beam with load placedover center.147.917