Contra-rotating propeller marine propulsion device

A contra-rotating propeller marine propulsion device comprising:

• (a) a hollow outer shaft having a front propeller attached to a rear end thereof and rotatably supported by a ship body;
• (b) an inner shaft having a rear propeller attached to a rear end thereof and rotatably supported in the outer shaft;
• (c) a driving unit that is a rotational driving power of the outer shaft and the inner shaft;
• (d) a power transmission unit that transmits the rotational driving force of the driving unit to the outer shaft and the inner shaft, and making the front propeller and the rear propeller rotate in directions opposite to each other;
• (e) a contra-rotating thrust bearing connected to receive a thrust load from the outer shaft and transmits the thrust load to the inner shaft;
• and
• (f) an inner thrust bearing connected to receive the thrust load from the inner shaft and transmits the thrust load to the ship body,
• wherein the power transmission unit has an outer output shaft that is arranged coaxially with the outer shaft to transmit a rotational driving force to the outer shaft,
• wherein the outer output shaft and the outer shaft are connected together via a hollow flexible joint,
• wherein the outer output shaft and the outer shaft rotate in the same direction,
• wherein the flexible joint is a gear coupling,
• wherein the flexible joint absorbs axial displacement and angular displacement of a shaft center of the outer shaft to transmit a rotational driving force of the outer output shaft to the outer shaft, and
• wherein the gear coupling includes an first inner cylinder provided on the stern side, and an outer cylinder surrounding the inner cylinder and connected to transmit the rotational driving force of the outer output shaft to the inner cylinder.