The effective wave slope is a fundamental concept in simplified 1-DOF roll models, and predicted roll motion strongly depends on the assumed effective wave slope coefficient. This is relevant to regulatory frameworks based on 1-DOF roll models, such as IMO Weather Criterion or the under development Second Generation Intact Stability Criteria. The paper examines different methods for effective wave slope coefficient prediction, which could potentially be used for intact stability assessment of river-sea ships. In fact, there is a lack of studies specifically addressing this type of vessels. A database of existing inland and river-sea vessels is used for determining the effective wave slope coefficient by the considered methods, and results are compared and discussed. The used approaches comprise a direct calculation method based on linear seakeeping hydrodynamics, methods based on Froude-Krylov approximation, and semi-empirical methods. Obtained results suggest that the considered methods based on Froude-Krylov approximation provide similar predictions for river-sea ships, and tend to be conservative in the vast majority of cases. Results also suggest that the considered semi-empirical methods for effective wave slope coefficient prediction at natural roll frequency are unsuitable for river-sea ships, with a general tendency towards the overestimation.
