The influence of wave profile on ship righting arm is among the possible stability failure issues addressed by the recent activity at IMO. The present paper is mainly focused on the so called "pure loss of stability", a dangerous phenomenon that can occur for specific conditions in following seas. In the paper, the development of a computational tool is described, able to evaluate the influence of wave profile on ship metacentric height and righting arms, giving the possibility to investigate different ship geometries and loading conditions in relation with wave profiles in terms of different lengths and steepness. The tool is applied to several ship typologies and a special attention in the investigations is given to transversal metacentric height GM and to the maximum righting arm GZmax since, in some proposals, these are the parameters assumed as indicative while performing first and second level vulnerability analysis for pure loss of stability.

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Although the appraisal of economical and commercial parameters relating to ship business is never trivial, it is rather intuitive that goods transportation by larger ships is more profitable, given of course the appropriate logistic boundary conditions. As far as oil tankers are concerned, it may be of common perception that a bigger ship represents a greater hazard. In order to gain a possible practical insight about the specific aspect of ship size influence on the relevant environmental impact, a Product, a Panamax, an Aframax and a VLCC have been investigated and compared in terms of oil outflow performance. As measurement tools, the three parameters defined in the IMO probabilistic methodology for the approval of alternative oil tanker designs have been exploited. As it can be expected, not only the size but also the internal subdivision has a significant influence on the ship outflow performance.@en