Stepped hulls: model experimental tests and sea trial data

In last years advance materials and techniques, allowed building lighter boat and most powerful, smaller and lighter engine. Not only racer boat have an weight/power ratio (Δ/P) of 3,5 : 1 or 4,5 : 1, depending on the category, more series R.I.B. (Rigid Inflatable Boats) or boats have Δ/P ratio about 5 : 1. This has been possible because boat builders are using infusion techniques laminating reducing average 30 % of R.I.B. or boat weight, at the same time engine builders are developed 410 kW (557 Hp) standard engine with Δ/P ratio of 1 : 0,85. The reduction of weight/power ratio involves an increasing of maximum speed, as result naval architects are slowly drawing stepped hull, to decrease resistance and increase stability. We think that the development and diffusion of these hull forms, even in widespread use, may have significant advantages from towing tank tests. For this reasons it has been carried out towing tank tests with scale model of standard R.I.B., Mito 31 built by Motonautica Vesuviana S.r.l., with different testing methodologies. The small model dimension make difficult the experimentation and towing tank-sea correlation, cause of scale effects problems and high ship speed (50 Knots, Fn 2,964, F 6,762). However, it has been setup a particular test system which reproduces in towing tank the same dynamic condition at sea. This new setup has been developed to reproduce in towing tank test the same angles measured in sea trial. The speed and resistance measurements have been transferred with Froude methodology, up to speed of 50 Knots.