Over last decades, due attention was paid to development of new stability criteria, especially probabilistic rules for damage stability, strongly influenced by the loss of many ro-ro ships in last decades of past century. In the last years, also a revision of intact stability code started and proposals have been implemented introducing stability in waves. These proposals deal with the equilibrium of a ship in regular waves to evaluate initial stability (GM) as well as righting arm curve in waves (GZ). This paper is not intended to critically review the present and proposed initial stability code, but is limited to assess how expected behavior of intact ships in waves is affected by accuracy in computer programs for assessing actual hydrostatic properties. An updated computer code, designed for onboard application and based on 3D pressure integral, has been developed and tested for a ro-ro ship. Then a comparison between hydrostatic and Airy effective waves has been carried out to analyze the relevance of differences between the two correspondent pressure fields affecting the equilibrium position and hydrostatic properties of the ship. It is demonstrated that these differences appear relevant beginning from sea state 4.
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