Tham khảo tài liệu 'ship hydrostatics and stability 2010 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Statical stability at large angles of heel 117 in Chapter 2 shows that the longitudinal position of the centre of buoyancy changes if the heel angle is large. It happens so because at large heel angles the waterplane area ceases to be symmetric about the centreline. If the centre of buoyancy moves along the ship while the position of the centre of gravity is constant the trim changes too. Therefore cross-curves calculated at constant trim may not represent actual stability condition. Jakic 1980 has shown that trim can greatly influence the values of cross-curves and therefore that influence should be taken into account. The stability regulations BV 1033 of the German Navy require indeed the calculation of the cross-curves at the trim induced by heel. Modern computer programmes for Naval Architecture include this option. As we shall show in Chapter 9 waves perpendicular or oblique to the ship velocity influence the values of cross-curves and can cause a very dangerous effect called parametric resonance. This effect too must be taken into account and modern computer programmes can calculate cross-curves on waves. The stability regulations of the German Navy take into account the variation of the righting arm in waves see Arndt 1965 Arndt Brandl and Vogt 1982 . Summary In this chapter we dealt with the righting moment at large angles of heel MR The quantity GZ called righting arm is the length of the perpendicular drawn from the centre of gravity G to the line of action ofthe buoyancy force. We assume that the ship heels at constant displacement. This is the desired situation in which the ship neither loses loads nor takes water aboard. Then the factor A is constant and the variation of the righting moment with heel is described by the variation of the righting arm GZ. The value of the righting arm is calculated from GZ 4 -YG sin where 4 called value of stability cross-curve is the distance from the reference point K to the line of action of the buoyancy force G .
