The successes of Naval Architects in predicting ship motions in waves, have been mainly confined to motions other than roll. For roll motions it has long been recognized that roll damping for wave encounter frequencies near the natural roll frequency is not only an extremely significant parameter but also very difficult to predict accurately. Because of the need to consider viscous flow (or even 'slightly viscous flow') to correctly model roll damping phenomena, there is still some way to go before an adequate numerical model of the hydrodynamics of roll is forthcoming. For this reason empirical and semi-empirical methods have played and will continue to play an important role in the prediction of roll motions. For small fishing vessels with deep skegs, hard chines and nonstandard hull shapes, the prediction of roll damping is particularly difficult due to lack of available data bases, semi- empirical formulae, and of course adequate theoretical models. It was for this reason that the present study was undertaken. In all, six small fishing vessels will be studied, each of which lies in the 'less-than 25 meter' class. They are all of similar dimensions but have varying hull forms ranging from the angular (e.g. model '363') to the rounded hull form of '366' (see figure 1).
Apart from providing a data base for the estimation of damping for different hull forms, the study will be used in the analysis of mathematical and/or numerical methods for the prediction of roll damping that the authors hope to develop (or hope will be developed) in the future. The present paper describes preliminary investigations of roll damping characteristics for just 3 of the boats. The methodology employed was that of the classical roll decay test with an innovative feature-namely the use of a newly developed method of analysis which enabled the authors to obtain the non- dimensional damping coefficient from the complete roll decay curve taken over just one full cycle. This method of analysis is based on an energy approach and is explained in [l]. Using this approach, the roll damping moment dependence on the initial roll angle is easy to obtain. The emphasis in this paper is a 'frequency domain' analysis of the results with equivalent linear damping as the primary target. The advantage of the simple decay test is that it allows for analysis in both the frequency and time domains. A study of the results in the time domain will be presented in a later paper. The simplicity of the roll decay experiment also means that many experiments can be perfomed and regression analysis carried out on the results. Over one thousand such tests were performed for the three models in this study. The body plans for the three models are shown in figures 1 (a), (b), (c), and their particulars are given in Table l.
In the experiments the models were attached to a dynamometer with just 2 degrees of freedom; the model was free to roll and heave, but restrained in all other modes.