復原余力と復原性曲線

抄録

Wind moment M_w' (cf. the previous paper in vol. 108 of this Journal) acting on a ship will much decrease when she moderately lists. Ships, therefore, with critical stability or, for example, the critical work ratio C=1 of the official “Stability Standard” in which a constant wind moment D_wW is used, will have the work ratio C_o' of about 2 if D_wW is replaced by M_w'. It may be supposed that the difference between these work ratios shows a margin for the secondary actions of steering, shifting of goods, shipping seas and so on which are not explicitly taken in the Standard and will be covered by adapting the constant moment. We shall now name the margin a residual stability because a ship still has some residue of stability in the above meaning although her reserve of stability in usual sense has been already lost.
The present work deals with the residual stability in two aspects, how much reduction in it will be caused by the secondary actions and what form of a stability curve will prove toughness to them. 21 ships were examined, however, under the simplified situations where (i) they were steered by helm 30° when their engines had been in cruising revolutions (denoted by the symbol r in the figures), (ii) weights on board were shifted (denoted by s), (iii) seas were shipped on exposed upper decks (by wa), (iv) sea water stayed in the deck houses (by wb) or, (v) sea water stayed in the under-deck spaces (by wc and wd), and, at the same time, under the actions of wind and waves provided in the Standard.
From the results of examinations, saying particularly about the characteristics of stability curves, ships with GM larger than about 7.5% of the beam and the range of stability less than about 50° are very week for the secondary actions.