Q. What other common tugboat maneuvers cause significant vertical and horizontal transverse force/tension?

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.

Ship’s officers need to understand the basic concepts of vessel stability and the consequences when a vessel reaches high angles of heel. In these specialized operations, a ship’s officer needs to be able to think ahead and plan for the consequences of unexpected events that could possibly put the vessel beyond its stability limits as was the case with the Bourbon vessel. Officers should be familiar with their vessel and not only read the stability curves but be able to understand what they mean and the consequences of significant amounts of weight (forces) on the vessel. In practice we know exactly how far we can over the moment we have passed it and the vessel is floating on its side or upside down. The term situational awareness is used in all kinds of training situations but it is very important when working with oversized cargoes or heavy weights such as a drilling platform anchor. As in any marine incident, the trick is to not let the vessel get into a position of extremis and to know when to say this is not safe. Experience and caution go hand-in-hand.
By Professional Mariner Staff