THIS invention relates to a propeller and more particularly, but not exclusively, to a propeller for use with inboard and outboard boat engines. The propeller includes a hollow hub and a plurality of primary blades extending substantially radially outwardly from the hub, with each primary blade including a blade face, a blade back and a root section. The propeller is characterized in that a set of secondary blades are provided inside the hub, and that an inner volume of the hub is in flow communication with a volume radially outwardly of the propeller hub.

A method is for making a marine drive for propelling a marine vessel in water. The method includes providing a gearcase; installing a propeller shaft assembly that extends forwardly from the gearcase; coupling front and rear propellers to the propeller shaft assembly, forwardly of the gearcase, such that rotation of the propeller shaft assembly causes rotation of the front and rear propellers, respectively, which thereby propels the marine vessel in the water; and reducing deleterious effects of cavitation on the gearcase by the combination of forming the gearcase with a wide trailing end portion, in particular to maintain pressure alongside the gearcase, and configuring the front and rear propellers so that the front propeller absorbs more torque/thrust load than the rear propeller during said rotation.

A method is for making a marine drive for propelling a marine vessel in water. The method includes providing a gearcase; installing a propeller shaft assembly that extends forwardly from the gearcase; coupling front and rear propellers to the propeller shaft assembly, forwardly of the gearcase, such that rotation of the propeller shaft assembly causes rotation of the front and rear propellers, respectively, which thereby propels the marine vessel in the water; and reducing deleterious effects of cavitation on the gearcase by the combination of forming the gearcase with a wide trailing end portion, in particular to maintain pressure alongside the gearcase, and configuring the front and rear propellers so that the front propeller absorbs more torque/thrust load than the rear propeller during said rotation.

An outboard motor includes a first case accommodating an engine and an upper housing below the first case. At least a portion of an exhaust passage is in the upper housing and extends downward from the engine. A second case is below the upper housing. The second case includes a torpedo portion, an exhaust inlet, an exhaust outlet, and an exhaust chamber. The torpedo portion includes an internal space and a rear end. A propeller shaft is in the internal space of the torpedo portion. The rear end of the torpedo portion is forward of at least a portion of the exhaust passage as seen in a side view. The exhaust passage is connected to the exhaust inlet. The exhaust outlet communicates with the internal space of the torpedo section. The exhaust chamber communicates the exhaust inlet with the exhaust outlet.

An outboard motor includes a first case accommodating an engine and an upper housing below the first case. At least a portion of an exhaust passage is in the upper housing and extends downward from the engine. A second case is below the upper housing. The second case includes a torpedo portion, an exhaust inlet, an exhaust outlet, and an exhaust chamber. The torpedo portion includes an internal space and a rear end. A propeller shaft is in the internal space of the torpedo portion. The rear end of the torpedo portion is forward of at least a portion of the exhaust passage as seen in a side view. The exhaust passage is connected to the exhaust inlet. The exhaust outlet communicates with the internal space of the torpedo section. The exhaust chamber communicates the exhaust inlet with the exhaust outlet.

An end cap for guiding an exhaust gas flow of a boat engine. The end cap is configured to be attached onto a propeller nut of the boat engine. The end cap including: a base body having a bottom surface, a lateral surface and a tip, the tip and the bottom surface being spaced apart from each other in a longitudinal direction of the base body. The lateral surface extends from the bottom surface and converges in the tip. The bottom surface having a recess for receiving the propeller nut; and at least one exhaust gas-guiding structure extends helically along the lateral surface.

An end cap for guiding an exhaust gas flow of a boat engine. The end cap is configured to be attached onto a propeller nut of the boat engine. The end cap including: a base body having a bottom surface, a lateral surface and a tip, the tip and the bottom surface being spaced apart from each other in a longitudinal direction of the base body. The lateral surface extends from the bottom surface and converges in the tip. The bottom surface having a recess for receiving the propeller nut; and at least one exhaust gas-guiding structure extends helically along the lateral surface.