System for reversing a high mass/low-pressure liquid propulsion device

A high mass/low pressure liquid propulsion device which includes:

• two counter-rotating impellers mounted inside a housing;
• an inlet which in use allows inflow of liquid into the housing on a first side of said impellers;
• an outlet which in use allows outflow of liquid from the housing on a second side of said impellers opposite to said first side;
• means for driving said impellers;
• wherein the improvement comprises the provision of means for reversing the drive to said impellers, such that the direction of flow of liquid through the device is reversed and liquid enters the device through said outlet, passes through said impellers, and leaves the device through said inlet;
• further including a bypass located on the underside of said housing adjacent the impellers, said bypass having a first position in which said bypass is closed and does not permit air or liquid to pass through the bypass into the housing, and a second position in which said bypass is open and allows liquid to pass through into the housing.