Chapter 3-STABILITY AND BUOYANCY 



aNGLE OF HEEL 

 EXflGGERATED 

 FOR CLARITY 



FORCE OF BUOrflNCY 



FORCE OF GRAVITY 



147.44 

 Figure 3-28.— Diagram showing virtual 

 rise in G. 



A PARTIALLY FULL 



B 



SHALLOW 



C ALMOST FULL 



8.61 



Figure 3-29. — Pocketing of free surface. 



in the compartment from wl to Wj^lj, the breadth 

 of the free surface is reduced and the free sur- 

 face effect is thereby reduced. A similar re- 

 duction in free surface effect occurs in the al- 

 most full compartment shown in part C, again 

 because of the reduction in the breadth of the 

 free surface. As figure 3-29 shows, the bene- 

 ficial effect of poclceting is greater at larger 

 angles of heel. 



The effect of pocketing in reducing the over 

 all free surface effect is extremely variable 

 and not easily determined. In practice, there- 

 fore, it is usually ignored and tends to provide 

 a margin of safety when computing stability. 

 Most compartments of a ship contain some 

 solid objects, such as machinery and stores 

 which would project through and above the sur- 

 face of any loose water. If these objects are 

 secured so that they do not float or move about, 

 and if they are not permeable, then the free 

 surface area and the free surface effect is re- 

 duced by their presence. The actual value of 

 the reduction (surface permeability effect) is 

 difficult to calculate and, like the value of 

 pocketing, if ignored when calculating stability 

 will provide a further margin of safety. 



Swash bulkheads (nontight bulkheads pierced 

 by drain holes) are fitted in deep tanks and 

 double bottoms to hinder the flow of liquid in 

 its attempt to remain continuously parallel to 

 the waterline as the ship rolls. They diminish 

 the free surface effect if the roll is quick, but 

 they have no effect when the roll is slow. A 

 ship taking on a permanent list will incline 

 just as far as if the swash plate were not there. 

 When a fore- and -aft bulkhead separating two 

 adjacent compartments is holed (ruptured) so 

 that any flooding water present in one is free 

 to flow athwartship from one compartment to 

 the other, a casualty duplicating the effect of 

 a swash bulkhead has occurred. In this case, 

 it is incorrect to add the free surface effects 

 of the two compartments together; an entirely 

 new figure for the flooding effect must be com- 

 puted, regarding the two as one large compart- 

 ment. 



In summary, the addition of a liquid weight 

 with a free surface has two effects on the me- 

 tacentric height of a ship. First, there is the 

 effect on GM and GZ of the weight addition 

 (considered as a solid) which influences the 

 vertical position of the ship's center of gravity, 

 and the location of the transverse metacenter, 

 M. Secondly, there is a reduction in GM and GZ 

 due to the free surface effect. 



55 



