When is Loww wash Low Wash? - An Investigation Using a Wave Wake Database

The past decade has seen a dramatic increase in concern from both the public and regulatory authorities regarding alleged negative effects from the waves generated by marine vessels. Subsequently, in order for a vessel to be accepted it has become commonplace for designers, builders and operators to show that their vessel will meet set criteria, or display "low wash" characteristics. Unfortunately, there is considerable misunderstanding about how to measure a vessel's wash characteristics. When this is coupled with each designer's wish to illustrate that his vessel has "low wash" characteristics and the fact that the regulatory authorities have different approaches to criteria, the result is a considerable muddying of the waters. This makes it very difficult to independently assess what level of wave wake should be expected from a particular vessel and hence whether it could be considered to have low wash characteristics or not. As a first step towards solving this problem the authors have developed a clear and precise definition for a standard numerical measure to be utilised whenever dealing with measurements of vessel generated waves. The establishment of such a measure has ensured that a consistent and accurate analysis technique was utilised when analysing the experimental data for over 80 different hull form configurations (involving over 6000 individual wave cuts). All experiments were conducted within a deep, wide model testing basin so as to obtain multiple longitudinal wave cuts in both the near and medium field in order to confirm the wave decay rate. As a result, each experimental data set has been analysed and collated within a database from which each model can easily be scaled to either a constant waterline length or displacement. The results provide an accurate, direct and fair comparison of the performance of each vessel at a specified vessel speed and distance from the vessel's sailing line. Thus, this comprehensive database can be utilised to undertake the following activities with confidence: • determine an achievable and rational criteria for any specific location for a proposed vessel and vessel speed; • make direct and fair comparisons between competing designs or against specific limits; • assist in determining whether a multihull is preferable to a monohull for a specific purpose; • investigate the effect that a particular design variable, such as waterline length or displacement, has on the waves generated; and, • determine whether a vessel can truly be described as displaying "low wash" characteristics. Specific examples of results are presented and discussed.