A full-scale investigation into the hydrodynamic behaviour of a highly faired flying-boat hull

Abstract

This report describes an investigation into the hydrodynamic qualities of a Sunderland flying-boat hull, weight 50,000 lb, fitted with a main-step fairing of fairing ratio 17 : 1. The fairing was equipped with ventilating ducts, drawing air at atmospheric pressure through ports on the hull side, and discharging it through exit vents on the afterbody planing bottom. No pumping apparatus was fitted, the airflow being induced by the sub-atmospheric pressures on the fairing. The main conclusions of the investigation may be summarised in this manner: (a) A highly faired hull of this kind is hydrodynamically satisfactory, with a ventilating area equivalent to 0.042 (beam) 2 placed immediately behind the main-step line. (b) For satisfactory hydrodynamic behaviour, the step line, i.e., the junction between forebody and afterbody must be kept sharp, but only an angular discontinuity in the vertical plane is necessary. (c) Without ventilation, the highly faired hull exhibits severe hydrodynamic instability during take-off and alighting, and the resistance is about 30 per cent higher than the ventilated hull. (d) Pressure measuremenfs on the afterbody indicate that skipping instability is caused by the presence of a region of sub-atmospheric pressure, covering almost the whole afterbody during skipping, and having maximum suctions of up to 4 lb/sq in. occurring at about 0.4 beam lengths aft of the step line. The general conclusion of the investigation is that successful hulls may be designed without conventional steps, provided that sufficient internal ventilation is provided.