Ventilated Waterjets
Air ingestion of a ship propulsor is a common problem for fast ships operating in waves. If air ingestion is significant and frequent, it leads to the drop in thrust and efficiency of traditional propulsors, as well as to increased vibration and engine overload. On marine vehicles employing aerostatic lift or drag-reducing air lubrication, air can appear at propulsors even in calm-water operations. Special air-protection parts or deepening the propulsor may prevent air ingestion, but such measures also increase structural complexity, draft, and appendage resistance.
Better solution to the air ingestion problem is the Ventilated Waterjet (VWJ), originally developed at Krylov Shipbuilding Research Institute. An example arrangement of this propulsor is shown in the first picture below. In contract to conventional waterjets, VWJ has shorter intake channel and has neither a stator nor a nozzle. In the design regimes, the VWJ thrust is generated by the face sides of the impeller blades. The cavities ventilated by the atmospheric air appear on the back sides of the blades, playing a role of the nozzle that reduces the effective hydraulic section area (second figure).
Reported experimental efficiencies of VWJ at operational advance ratios are within 0.55-0.75. VWJ units are characterized by significantly smaller associated appendage drag when compared to other propulsors (except for surface-piercing propellers). Hence, the overall propulsive performance of fast VWJ-driven ships can be highest in some cases. Due to the absence of vaporous cavitation on the VWJ impeller blades, these propulsors do not impose restrictions on the ship top speed and power. Reverse thrust is provided by simply reversing the shaft rotation. Significant thrust augmentation of fixed-diameter VWJ units can be achieved by using interceptors at the trailing edges of the face sides of the impeller blades. Stable air-ventilated cavities on the back sides of VWJ blades eliminate erosion problems of the propulsor impellers and reduce vibration and noise. Additional VWJ advantages include shallow draft, minimal unsteady loads, and high reliability.
VWJ units are currently applied on the series of Russian-built air cavity ship Serna and are also installed on several experimental boats.
I am conducting R&D on VWJ propulsors and their applications for Air Cavity Ships. An example of calculations of one VWJ model for a 75-t patrol-type boat is given in the figure below (Resistance and thrust in kilonewtons are given along the vertical axis, and speed in knots along the horizontal axis).
