3. A HYDRAULIC CONTROLLABLE PITCH MARINE PROPELLER SYSTEM COMPRISING IN COMBINATION: A TUBULAR PROPELLER SHAFT; A HOLLOW HUB, HAVING AN AXIAL CYLINDRICAL BORE, FIXED TIGHTLY TO SAID SHAFT AND MOUNTING RADIAL JOURNALED BLADES; MECHANISM WITH SAID HUB FOR TURNING SAID BLADES AXIALLY IN UNISON, INCLUDING A DOUBLE-ACTING SERVO-PISTON DISPLACEABLE IN SAID BORE ACCORDING TO THE QUANTITY OF PRESSURE LIQUID THEREIN; A POWER SOURCE OF PRESSURE LIQUID; A TRAVELING AUTOMATIC HYDRAULIC AMPLIFIER VALVE, HAVING AN INTEGRAL TRAVELING DOUBLE-ACTING DUPLICATING-CYLINDER HYDRAULICALLY CONTROLLED BY SAID VALVE, FOR CONTROLLING QUANTITATIVELY THE FLOW OF PRESSURE LIQUID IN PRECISE PROPORTION TO THE TRAVEL OF SAID VALVE; AND PROPER HYDRAULIC CONNECTIONS BETWEEN SAID SOURCE, SAID VALVE, SAID DUPLICATING-CYLINDER AND SAID HUB BORE, WHEREBY SAID VALVE CONTROLS THE QUANTITY OF PRESSURE LIQUID DISPLACING SAID SERVO-PISTON; AND AUTOMATIC VALVE MEANS FOR SYNCHRONIZING THE DISPLACEMENT OF SAID SERVO-PISTON IN ACCORDANCE WITH THE TRAVEL OF SAID VALVE.