Impeller drive for a water jet propulsion unit

A water propulsion unit including

• an upstream impeller and a downstream impeller,
• a pump housing,
• a water inlet to communicate with the upstream impeller and
• an outlet to directly communicate with the downstream impeller,
• the said impellers being mounted on and, in use, driven by shafts so as to be co-axial with each other, within the pump housing;
• wherein said impellers are spaced apart and are adapted to be rotated within the pump housing in opposite directions, and wherein each impeller includes a series of impeller blades extending radially from a central boss, and the blades of the upstream impeller are of opposite pitch to the blades of the downstream impeller,
• characterised in that:—
• the water jet propulsion unit is a low pressure high mass device, designed to operate efficiently in the range of 0-40 psi;
• one of the impellers is arranged to impart less energy to the water than the other impeller;
• and
• the cross-sectional area of the outlet is such that in use it presents minimal impedance to the flow of water therethrough.