PRINCIPLES OF NAVAL ENGINEERING 



FREE COMMUNICATION EFFECT 



If one or more of the boundaries of an off- 

 center compartment are ruptured so that the 

 sea may flow freely in and out with a minimum 

 of restriction as the ship rolls, a condition of 

 partial flooding with free communication with 

 the sea exists. The added weight of the flooding 

 water and the virtual rise in G due to the free 

 surface effect cause what is known as free 

 communication effect. With an off-center space 

 flooded, a ship will assume a list which will 

 be further aggravated by the free surface effect. 

 As the ship lists, more water will flow into 

 the compartment from the sea and will tend to 

 level off at the height of the external waterline. 

 The additional weight causes the ship to sink 

 further allowing more water to enter, causing 

 more list until some final list is reached. The 

 reduction of GM due to free communication 

 effect is approximately equal in magnitude to 



ay^ 



decrease in stability, 

 as sluicing. 



Such free flow is known 



where 



feet 



area of the free surface in square 



y = perpendicular distance from the geo- 

 metric center of the free surface area to the 

 fore-and-aft centerline of the intact waterline 

 plane in feet 



V = new volume of ship's displacement 

 after flooding, in cubic feet. Thus reduction in 

 GM is additional to and separate from the free 

 surface effect. 



The approximate reduction in GZ may be 

 computed from 



ay2 

 V 



GZ = 



sin e 



This may be considered as a virtual rise in 

 G, superimposed upon the virtual rise in G due 

 to the free surface effect. 



K two partially filled tanks on opposite sides 

 of an intact ship are connected by an open pipe 

 at or near their bottoms allowing a free flow of 

 liquid between them, the effect on GM is the 

 same as if both tanks were in free communica- 

 tion with the sea. Hence, valves in cross con- 

 nections between such tanks should never be 

 left open without anticipating the accompanying 



SUMMARY OF EFFECTS OF LOOSE WATE , 



The addition of loose water to a ship alters 

 the stability characteristics by means of three 

 effects that must be considered separately: 

 (1) the effect of added weight; (2) the effect of 

 free surface; and (3) the effect of free communi- 

 cation. 



Figure 3-30 shows the development of a 

 stability curve with corrections for added weight, 

 free surface, and free communication. Curve 

 A is the ship's original stability curve before 

 flooding. Curve B represents the situation after 

 flooding; this curve shows the effect of added 

 weight (increased stability) but it does not 

 show the effects of free surface or of free 

 communication. Curve C is curve B corrected 

 for free surface effect only. Curve D is curve 

 B corrected for both free surface effect and 

 free communication effect. Curve D, therefore, 

 is the final stability curve; it incorporates 

 corrections for all three effects of loose water. 



LONGITUDINAL STABILITY AND 

 EFFECTS OF TRIM 



The important phases of longitudinal in- 

 clination are changes in trim and longitudinal 

 stability. A ship pitches longitudinally in con- 

 trast to rolling transversely and it trims for- 

 and-aft, whereas it lists transversely. The 

 difference in forward and after draft is defined 

 as trim. 



CENTER OF FLOATATION 



When a ship trims, it inclines about an axis 

 through the geometric center of the waterline 

 plane. This point is known as the center of 

 flotation. The position for the center of flota- 

 tion aft of the mid- perpendicular for various 

 drafts may be found from a curve on the curves 

 of form (fig. 3-25). When a center of flotation 

 curve is not available, or when precise cal- 

 culations are not required, the mid-perpendicu- 

 lar may be used in lieu of the center of flotation. 



CHANGE OF TRIM 



Change of trim may be defined as the change 

 in the difference between the drafts forward 

 and aft. If in changing the trim, the draft forward 



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