Alternative Assessment of Weather Criterion for Ships With Large Breadth and Draught Ratios by a Model Experiment: A Case Study on an Indonesian Ro-Ro Ferry

Abstract

The weather criterion is one of stability criteria to verify ability of a ships to withstand the combined effects of severe wind and rolling criteria in dead ship condition. An overestimated roll angle is obtained when the weather criterion is applied to ships with breadth and draught ratios larger than 3.50 and ratios between vertical centre of gravity and draught larger than 1.50. This paper discusses the assessment of weather criterion for an Indonesian ro-ro ferry by model experiments. The drift test is performed in four wave steepnesses with wave frequencies near the roll natural frequency. The maximum roll amplitude is used to calculate the effective wave slope coefficient correponding to the wave steepness, with Bertin’s coefficient obtained by the roll decay test. The damping factors correspond to the breadth and draught ratio as well as the bilge keel contribution are determined using the formula of weather criterion with the roll angle obtained by the Japanese formula with a correction factor of 0.70 due to the irregularity of waves. The obtained effective wave slope coefficient and the damping factors due to breadth and draught ratio and the bilge keel are smaller than those used in the weather criterion.

Vulnerability of Ship with a Large Breadth to Draught Ratio Against Excessive Acceleration Criteria

“…The weather criterion was statistically formulated by using sample ships with a maximum ratio between breadth and draught of 3.5 and the vertical centre of gravity (VCG) divided by draught ranges from 0.7 to 1.5. An overestimate stability can be found if the weather criterion is used to assess stability of a ship with a larger ratio between breadth and draught as well as ratio between VCG and draught [2]. On the other hand, some accidents due to stability occurs to ships comply with the criteria.…”

Roll dynamic coefficients approach of decay test using the Generalized Reduced Gradient method (GRG)

“…This IMO stability criterion adopts Rahola's study using the energy balance moment of the ship righting arm principle and comparing this principle with statistical data of ships that survive and capsize so that the standard criteria are obtainable [11] [12]. The IMO criteria were developed not only for calm weather conditions but expanded for terrible weather conditions with the introduction of the weather criteria [13]. Subsequent developments issued particular stability criteria based on the types of new ships that are increasingly varied [14] [15].…”