Study of a Novel Air-augmented Waterjet Boost Concept

Abstract

This research presents analysis, experiment, and prediction of the performance of a unique marine propulsion concept, the air-augmented waterjet, having a revolutionary potential for significant thrust augmentation and boost capability of waterjet systems. So far, this concept has not been realized in any operational vessel. The air-augmented waterjet propulsion concept is similar to an after-burner in the aeronautical turbojet engine. The thrust augmentation results from the injection of air bubbles into the water flow, converting their expansion work òpdV into additional kinetic energy of the exhaust jet without affecting the pump operation. It can enable substantially augmenting boost capability, overcoming hump resistance, and increasing maximum attainable vessel speed while avoiding cavitation problems. Static tests at the Technion's water tank, using a jetski waterjet engine of a nominal power of 50 kW, have been conducted with and without air injection over a range of motor revolutions per minute (rpm). The addition of air increased the thrust by 20% to 50% depending on the airflow rate and engine rpm. Air expansion work increased the exhaust jet kinetic energy at an efficiency of 70% approximately. Based on the static experiments, prediction of the system behavior for different vessel speeds and engine power levels has been made, revealing a higher relative thrust augmentation for the same pump power and airflow rate at higher vessel speeds. The air-augmented waterjet concept may add a new dimension to marine propulsion as well as upgrade existing vessels.