A- Every vessel has a range of stability, this range is considered the range in degrees over which a vessel is considered to have positive stability. The vessel can roll between this range and the various dynamic forces allow for a positive moment, bring the vessel back to an upright position. Once the vessel has gone passed the critical angle it is considered past the vanishing point and the vessel will have a negative righting arm causing the vessel to capsize. The range of positive stability should be shown in a vessel’s stability curve. The value of the righting arm is affected by many factors but can be increased operationally by having fuel and water tanks pressed up, minimizing a vessels free surface and keeping weights low in the vessel (ie deck cargo).
An additional factor creating a negative affect on stability is deck edge immersion. Once the edge of a solid bulwark has been reached water will pour over the edge adding weight and compounding the negative effects on the stability of the vessel. In many cases this may be the point of no return from a stability standpoint. In effect the increasing force of the water is pulling the deck further under. Based on the information at hand in the case of the Bourbon supply boat which was recently capsized while working with an anchor, we can infer that the downward force of the anchor overcame the vessels natural righting arm and caused the vessel to capsize. This is very similar to a large barge over-running a tug and “tripping” the tug. The transverse force of the barge over powers the positive stability forces of the tug.
