Ventilation effects on Cavitating Wedges and Struts

Reduction of ship hull skin friction can represent significant drag reduction for high-speed ships, especially those designed for low wave drag or using catamaran-type hull designs. A method is proposed to reduce this friction by shielding the side portions of the hull from the water using ventilated cavities created with protruding wedges. These wedges create low-pressure regions in their wake that naturally ingest air at the surface, reducing the drag of the wedge while lengthening their trailing cavities. The proposed scheme takes advantage of the natural combination of high speeds and low pressures that exist at the waterline of a high-speed ship without requiring auxiliary pumps or machinery. As a first step for engineering estimates of drag reduction, preliminary experiments are presented that use two-dimensional wedges and similarly shaped struts to validate theoretical ventilated-cavity predictions. Tests are conducted on parabolic and wedge-shaped sections and struts in a closed-jet water tunnel at 20 and 30 knots, using a pipe to ventilate behind the strut using natural suction. The wedge drag and back pressure and resulting cavity lengths are measured over a range of ventilation flows. The two-dimensional wedge drag shows fair agreement with linearized theory predictions at low cavitation numbers, and this drag can be reduced over 50% from its fully wetted value using natural ventilation. The change in drag is attributed mostly to the change in the pressure on the back of the wedge caused by the ventilation. The measured ventilated cavity lengths are close to linear theory models using a double spiral vortex for cavity closure conditions. For the range of ventilated conditions tested, the air flow rates required for ventilation are only a few percent of the equivalent flow rate swept out by the passage of the strut through the water. These flowrates could become much greater at lower pressures representative of shallower depths.