PRINCIPLES OF NAVAL ENGINEERING 



16 



14 

 12 

 10 



8 9 10 II 12 13 14 15 

 DISPLACEMENT (TONS - IN THOUSfiNDS) 



16 



17 18 



19 20 



8.46 



Figure 3-4.— Displacement curve of a cruiser. 



RESERVE BUOYANCY 



The volume of the watertight portion of the 

 ship above the waterline is known as the ship's 

 reserve buoyancy. Freeboard , a rough measure 

 of the reserve buoyancy, is the distance in feet 

 from the waterline to the main deck. Freeboard 

 is calculated at the midship section. As indicated 

 in figure 3-7, freeboard plus draft is equal to 

 the depth of the hull in feet. 



When weight is added to a ship, draft and 

 displacement increase in the same amount that 

 freeboard and reserve buoyancy decrease. Re- 

 serve buoyancy is an important factor in a 

 ship's ability to survive flooding due to damage. 

 It also contributes to the seaworthinesss of the 

 ship in very rough weather. 



INCLINING MOMENTS 



The moment of a force is the tendency of the 

 force to produce rotation or to move an object 

 about an axis. The distance between the point 

 at which the force is acting and the axis of ro- 

 tation is called the moment arm or the lever 

 arm of moment,^ To find the value of a moment, 

 we multiply the magnitude of the force by the 



The significance of the distance between the force 

 and the axis of rotation may be seen if we consider 

 a simple see-waw. If two persons of equal weight sit 

 on opposite ends, equally distant from the center 



distance between the force and the axis of ro- 

 tation. The magnitude of the force is expressed 

 in some unit of weight (pounds, tons, etc.) and 

 the distance is expressed in some unit of length 

 (inches, feet, etc.); hence the unit of the moment 

 is the foot-pound, the foot-ton, or some similar 

 unit. 



When two forces of equal magnitude act in 

 opposite and parallel directions and are sepa- 

 rated by a perpendicular distance, they form a 

 couple. The moment of a couple is found by 

 multiplying the magnitude of one of the forces 

 by the perpendicular distance between the lines 

 of action of the two forces. 



When a disturbing force exerts an inclining 

 moment on a ship, causing the ship to heel over 

 to some angle, there is a change in the shape 

 of the ship's underwater body and a consequent 

 relocation of the center of buoyancy. Because 

 of this shift in the location of B, B and G no 

 longer act in the same vertical line. Instead 

 of acting as separate equal and opposite forces, 

 B and G now form a couple. 



The newly formed couple produces either a 

 righting moment or an upsetting moment , de- 

 pending upon the relative locations of B and Go 

 The ship illustrated in figure 3-8 develops a 



support, the see-saw balances. But if one person 

 moves closer to or farther away from the center, 

 the person farthest away from the support moves 

 downward because the effect of his weight is greater. 



36 



