Abstract: The invention relates to a hydraulic actuator for the control of valves such as the kind found in ships. The actuator has a piston which is actuated in a valve opening direction with a hydraulic pressure medium and may be actuated in the return direction by a gas-compression type spring. Certain emergency situations require an immediate closing of a valve or group of valves controlled by the piston acting through a spindle. Emergency closing apparatus includes an abutment on the piston which forms an annular chamber when the piston abuts the end wall of the piston cylinder. The housing has an emergency passage means in fluid communication with the annular chamber when the piston abuts the end wall to effect an emergency moving of the piston in a valve closing direction when a pressure medium is admitted into the emergency passage.