Principles of naval engineering - Historic Naval Ships Association

Chapter 3-STABILITY AND BUOYANCY4 rCURVE BCURVE ACURVE CCURVE D10 20 30 40 50 60ANGLE OF HEEL.IN DEGREESFigure 3-30.— Stability curve corrected for effects of added weight,free surface, and free communication.8.64becomes greater, then the change is said to beby the bow. Conversely, if the draft aft becomesgreater, the change of trim is by the stern .Changes of trim are produced by shiftingweights forward or aft or by adding or subtractingweights forward of or abaft of the center offlotation.LONGITUDINAL STABILITYLongitudinal stability is the tendency of aship to resist a change intrim. For small anglesof inclination, the longitudinal metacentric heightmultiplied by the displacement is a measure ofinitial longitudinal stability. The longitudinalmetacentric height is designated GM' and isfound fromGM'= KB +BM' - KGr= the moment of inertia of the ship'swaterline plane about an athwartshipaxis through the center of flotationV = the ship's volume of displacementThe value of BM' is very large— sometimesmore than a hundred times that of BM. Thevalues of BM' for various drafts may be foundfrom the curves of form (fig. 3-25).MOMENT TO CHANGE TRIM ONE INCHThe measure of a ship's ability to resist achange of trim is the moment required to producea change of trim of a definite amount, suchas one inch. The value of the moment to changetrim one inch isobtained fromMTI = GM' X W12 LwhereKB and KG are the same as for transversestability BM' (the longitudinal metacentricradius) is equal toBM'=wherewhereGM'= longitudinal metacentric height (feet)W= displacement (tons)L = length between forward and afterperpendiculars (feet)57