Principles of naval engineering - Historic Naval Ships Association

Chapter 3-STABILITY AND BUOYANCYaNGLE OF HEELEXflGGERATEDFOR CLARITYFORCE OF GRAVITYFORCE OF BUOrflNCY147.44Figure 3-28.— Diagram showing virtualrise in G.FigureA PARTIALLY FULLBCSHALLOWALMOST FULL8.613-29. — Pocketing of free surface.in the compartment from wl to Wj^lj, the breadthof the free surface is reduced and the free surfaceeffect is thereby reduced. A similar reductionin free surface effect occurs in the almostfull compartment shown in part C, againbecause of the reduction in the breadth of thefree surface. As figure 3-29 shows, the beneficialeffect of poclceting is greater at largerangles of heel.The effect of pocketing in reducing the overall free surface effect is extremely variableand not easily determined. In practice, therefore,it is usually ignored and tends to providea margin of safety when computing stability.Most compartments of a ship contain somesolid objects, such as machinery and storeswhich would project through and above the surfaceof any loose water. If these objects aresecured so that they do not float or move about,and if they are not permeable, then the freesurface area and the free surface effect is reducedby their presence. The actual value ofthe reduction (surface permeability effect) isdifficult to calculate and, like the value ofpocketing, if ignored when calculating stabilitywill provide a further margin of safety.Swash bulkheads (nontight bulkheads piercedby drain holes) are fitted in deep tanks anddouble bottoms to hinder the flow of liquid inits attempt to remain continuously parallel tothe waterline as the ship rolls. They diminishthe free surface effect if the roll is quick, butthey have no effect when the roll is slow. Aship taking on a permanent list will inclinejust as far as if the swash plate were not there.When a fore- and -aft bulkhead separating twoadjacent compartments is holed (ruptured) sothat any flooding water present in one is freeto flow athwartship from one compartment tothe other, a casualty duplicating the effect ofa swash bulkhead has occurred. In this case,it is incorrect to add the free surface effectsof the two compartments together; an entirelynew figure for the flooding effect must be computed,regarding the two as one large compartment.In summary, the addition of a liquid weightwith a free surface has two effects on the metacentricheight of a ship. First, there is theeffect on GM and GZ of the weight addition(considered as a solid) which influences thevertical position of the ship's center of gravity,and the location of the transverse metacenter,M. Secondly, there is a reduction in GM and GZdue to the free surface effect.55