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 corresponding 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.

Estimation of Effective Wave Slope Coefficient of Ships with Large Breadth and Draught Ratio

“…If the obtained results of the Eq. 1 are larger than 1.0 then the effective wave slope coefficient used to calculate the roll-back angle in the weather criterion is taken to be 1.0 [5], Nevertheless, results of some researches show that for certain types of ships, especially large passenger ships and ro-pax, the effective wave slope coefficient obtained by using the formulae of weather criterion is larger than that obtained by model experiment [2], [3], [14], This means that the roll-back angle obtained by using the effective wave slope coefficient based on the formulae of weather criterion will be larger than that obtained by using the effective wave slope coefficient based on model experiment. Therefore, IMO recommends using the model experiment for evaluation of the weather criterion for ships with geometry characteristics different with those are used to develop the criterion.…”

“…In order to calculate the effective wave slope coefficient by model experiment, two model experiments should be conducted, namely the roll decay test and the drift test in beam seas [8], The roll decay test is performed to determine the linear and nonlinear damping coefficients as well as the natural roll period. Those damping coefficients are obtained following the method recommended by International Towing Tank Conference (ITTC) [12], The Bertin s coefficient is then calculated by using the following equation: (14) where and are the linear and the nonlinear coefficients of polynomial equation of decay amplitude of roll as function of average two consecutive roll amplitude of roll decay test results. The effective wave slope coefficient is calculated by using the equation as follows [8]: (15) is the maximum roll angle obtained by drift test in regular beam seas with wave steepness, .…”

“…Effective Wave Slope Coefficient Obtained by Simplified Strip Theory in The Natural Frequency Of RollFigure 9. Effective wave slope coefficient obtained by model experiment[14] …”

Effect of Bilge Keels Position On Roll Motion Performance Of Traditional Wooden Boat

“…The roll natural period of the ship without bilge keels was 4.14 seconds, and that for the ship with bilge keels position with 15 degrees of the slope was 4.18 seconds. For merchant ships such as an Indonesian ro-ro ferry, the natural roll period increases due to bilge keels [9]. The different results could occur due to dimension of bilge keels.…”

“…2020. 190-197 (pISSN: 2541-5972, eISSN: 2548-1479 191 [9]. This means that the available damping factor in the weather criterion is valid only for ships with geometry characteristics the same as those used to develop the weather criterion.…”