- The Cross Curves Of Stability are used to determine the length of the righting arm at any angle of inclination for a given displacement.
- To draw the curve of statical stability, we need GZ values for various angles of heel.
- For this we use the GZ cross curves of stability.
- These curves are provided for an assumed KG, tabulating GZ values for various displacements and angles of list.
- Called cross curves because the various curves actually ‘cross’ each other.
- Since the curves are plotted for an assumed KG, if the actual KG differs from this a correction (GG1Sineθ) needs to be applied.
- This correction is positive if the actual KG is less than the assumed KG and vice-versa.
- After obtaining the GZ values at various angles, the curve of statical stability is prepared
Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole is lesser in size than the corresponding circular hole. More over it is prime concern to have a smoothed generous radius at the corners to eliminate stress concentration. Hence other geometrical shapes like rectangle and square are ruled out. To compensate for the loss of material in the shell due to opening, a doubler ring has to be provided around the opening. The thickness of the ring depends on the axis length along the dirrection in which the stresses are maximum and the thickness of the shell. It is important to align the minor axis along the length of the vessel, as the stress in this direction is maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t= thickness of the shell plating Circumferential stress: Pd/4t More over a considerable material and weight saving is achieved as minor is along the direction of maximum stress.