a collapsible bilge pump includes outer and inner cylinders telescopable between retracted and extended posiions . a piston is movable up and down within the inner cylinder . check valves in the bottom of the outer cylinder and in the piston permit the pump to take in water at the bottom end of the pump and discharge it from the top end . the collapsible pump is easily stowed , and its short stroke , limited to the length of the inner cylinder , is substantaially easier to perform .