Stability with Draught of Water in Ships. 



223 



stability, and thus considering the variation of stability with draught 

 of water, the curves of righting moments require to be constructed, 

 and not merely curves of GZ, or lengths of righting arm. The 

 ordinary curve of stability usually has for its ordinates the lengths 

 of GZ at the various angles of inclination. This is right enough for 

 the condition under which such curves are constructed ; because the 

 displacement is then constant, and the curves represent either 

 lengths of righting arm or righting moments, according to the scale 

 upon which the ordinates are measured. In the cross-curves of 

 stability, however, draught is one of the variable elements, and the 

 displacement changes with it. A cross-curve whose ordinates repre- 

 sent the lengths of righting arm at various draughts of water is 

 therefore quite different in character from a cross-curve of righting 

 moments, whose ordinates are lengths of righting arm x displacement. 

 It is necessary, in order to judge accurately of the variation of 

 stability with draught of water, to use curves of righting moments, 

 and not merely curves of GZ, such as are usually considered sufficient 

 when the draught of water is fixed. 



FIG. 11. 



Fig. 11 shows cross-curves of stability, at angles of inclination of 

 30, for two homogeneous floating bodies of prismatic form, and of the 

 same breadth and depth ; one being rectangular and the other ellip- 



