PURPOSE: To eliminate the need for fuel so that the cost of navigation is greatly reduced and to prevent pollution of air, etc., by installing inside the ship a device for generating the force to propel the ship by gravity and hydraulic power.

CONSTITUTION: The hull of a ship is fabricated into double hulls consisting of an outer hull 1 and an inner hull 2, and the cargo room of the inner hull 2 is formed into a plurality of sections, each of these cargo rooms capable of retaining water. When the amount of cargo is small, sea water is retained in place of cargo so that the draft can be held at the same height as that with a maximum amount of cargo. During navigation, water is drawn into the outer hull 1 by driving hydraulic turbines 8, 9 installed at the lowermost part of the inner hull 2, and the ship is floated up by means of the buoyancy of the inner hull 2. The sea water in the outer hull 11 is pressurized with the strong pressure of the total weight of the inner hull when floating up, and is ejected from a plurality of jet nozzles provided under the surface of sea at the stern of the outer hull 1, and the hull is advanced by the reaction of these jets.