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.