A power transmission device for an outboard motor, comprising a drive shaft, an endless loop flexible drive coupling and a propeller shaft, wherein the endless loop flexible drive coupling operatively connects said drive shaft to said propeller shaft for transferring output power from the drive shaft to the propeller shaft. The power transmission device comprises a first drive shaft, a second drive shaft, a first endless loop flexible drive coupling, a second endless loop flexible drive coupling, a first propeller shaft and a second propeller shaft, wherein the first propeller shaft is connected to the first drive shaft through the first endless loop flexible drive coupling to rotate the first propeller shaft in a first direction, and wherein the second propeller shaft is connected to the second drive shaft through the second endless loop flexible drive coupling to rotate the second propeller shaft in a second direction opposite to the first direction.

A propulsion device for a ship capable of preventing idling of a propeller without any limitation on the type of clutch. The propulsion device includes: a forward and reverse switching clutch for transmitting the driving power from an input shaft to a forward gear or a reverse gear; a forward one-way clutch, disposed between the input shaft and the forward gear, for connecting the forward gear and the input shaft when the forward gear is rotated at a higher speed than that of the input shaft; and a reverse one-way clutch, disposed between the input shaft and the reverse gear, for connecting the reverse gear and the input shaft when the reverse gear is rotated at a higher speed than that of the input shaft.

A propulsion device for a ship capable of preventing idling of a propeller without any limitation on the type of clutch. The propulsion device includes: a forward and reverse switching clutch (17) for transmitting the driving power from an input shaft (11) to a forward gear (13A) or a reverse gear (13B); a forward one-way clutch (18A), disposed between the input shaft (11) and the forward gear (13A), for connecting the forward gear (13A) and the input shaft (11) when the forward gear (13A) is rotated at a higher speed than that of the input shaft (11); and a reverse one-way clutch (18B), disposed between the input shaft (11) and the reverse gear (13B), for connecting the reverse gear (13B) and the input shaft (11) when the reverse gear (13B) is rotated at a higher speed than that of the input shaft (11).