An outboard motor for a marine vessel application, and related methods of making and operating same, are disclosed herein. In at least one embodiment, the outboard motor includes a horizontal-crankshaft engine in an upper portion of the outboard motor, positioned substantially positioned above a trimming axis of the outboard motor. In at least another embodiment, first, second and third transmission devices are employed to transmit rotational power from the engine to one or more propellers at a lower portion of the outboard motor. In at least a further embodiment, the outboard motor is made to include a rigid interior assembly formed by the engine, multiple transmission devices, and a further structural component. In further embodiments, the outboard motor includes numerous cooling, exhaust, and/or oil system components, as well as other transmission features.

A boat propulsion machine includes a power source, a drive shaft, a propeller, a gear mechanism, a gear case, a tank configured to store a cooling liquid for cooling the power source, a heat sink configured to cool the cooling liquid, a cooling liquid passage connecting the power source and the heat sink, and a pump a pump configured to pump the cooling liquid to circulate through the cooling liquid passage. The power source, the tank, and the pump are provided at an upper portion of the boat propulsion machine. The gear case is provided at a lower portion of the boat propulsion machine. The drive shaft extends in an upper-lower direction between the power source and the gear mechanism. The heat sink is provided at a position lower than the power source and higher than the gear case in the boat propulsion machine.

A boat propulsion machine includes a power source, a drive shaft, a propeller, a gear mechanism, a gear case, a tank configured to store a cooling liquid for cooling the power source, a heat sink configured to cool the cooling liquid, a cooling liquid passage connecting the power source and the heat sink, and a pump a pump configured to pump the cooling liquid to circulate through the cooling liquid passage. The power source, the tank, and the pump are provided at an upper portion of the boat propulsion machine. The gear case is provided at a lower portion of the boat propulsion machine. The drive shaft extends in an upper-lower direction between the power source and the gear mechanism. The heat sink is provided at a position lower than the power source and higher than the gear case in the boat propulsion machine.

An outboard motor includes a gear housing configured to rotatably house a propeller shaft that transmits a rotative power output from an engine to a propeller device. The gear housing includes a torpedo shape portion and a strut portion. The torpedo shape portion has a shape tapered toward a front side, and a shape biased upward toward the front side. The strut portion is disposed on an upper side of the torpedo shape portion. An outer peripheral surface of the torpedo shape portion is smoothly coupled to an outer peripheral surface of the strut portion via first to third curved surfaces. The first to third curved surfaces are each a curved surface inclined rearward and downward, a curved surface parallel to a front-rear direction, or a curved surface constituted of a part inclined rearward and downward and a part parallel to the front-rear direction.

An outboard motor includes a gear housing configured to rotatably house a propeller shaft that transmits a rotative power output from an engine to a propeller device. The gear housing includes a torpedo shape portion and a strut portion. The torpedo shape portion has a shape tapered toward a front side, and a shape biased upward toward the front side. The strut portion is disposed on an upper side of the torpedo shape portion. An outer peripheral surface of the torpedo shape portion is smoothly coupled to an outer peripheral surface of the strut portion via first to third curved surfaces. The first to third curved surfaces are each a curved surface inclined rearward and downward, a curved surface parallel to a front-rear direction, or a curved surface constituted of a part inclined rearward and downward and a part parallel to the front-rear direction.

An outboard motor includes an engine, a drive shaft, and a shift shaft disposed forward of the drive shaft. The shift shaft includes a first shift shaft that extends in an upward-downward direction, a second shift shaft disposed below the first shift shaft and spaced apart from and rearward of the first shift shaft, the second shift shaft extending in the upward-downward direction, and a shift force transmission to transmit, to the second shift shaft, a shift force applied to the first shift shaft.

An outboard motor includes an engine, a drive shaft, and a shift shaft disposed forward of the drive shaft. The shift shaft includes a first shift shaft that extends in an upward-downward direction, a second shift shaft disposed below the first shift shaft and spaced apart from and rearward of the first shift shaft, the second shift shaft extending in the upward-downward direction, and a shift force transmission to transmit, to the second shift shaft, a shift force applied to the first shift shaft.

An outboard motor includes an exhaust passage that discharges exhaust gases generated in a V-type engine into water from an exhaust opening. The exhaust passage includes first and second branch passages connected to cylinders and disposed inside a V-shaped line, a first upstream collecting passage connected to each of the first branch passages, a second upstream collecting passage connected to each of the second branch passages, and a downstream collecting passage connected to the first and second upstream collecting passages. A portion from an upstream end of the downstream collecting passage to a downstream end of the downstream collecting passage is disposed outside cylinder heads of the V-type engine.

An outboard motor includes an exhaust passage that discharges exhaust gases generated in a V-type engine into water from an exhaust opening. The exhaust passage includes first and second branch passages connected to cylinders and disposed inside a V-shaped line, a first upstream collecting passage connected to each of the first branch passages, a second upstream collecting passage connected to each of the second branch passages, and a downstream collecting passage connected to the first and second upstream collecting passages. A portion from an upstream end of the downstream collecting passage to a downstream end of the downstream collecting passage is disposed outside cylinder heads of the V-type engine.

An outboard motor is provided with an upper separator disposed on the lower side of an oil case for storing a lubricating oil of an engine, and an extension case which is separably connected on the lower side of the upper separator. The upper separator has a central exhaust passage through which exhaust gas flows, and a cooling water flow part through which cooling water flows on the outside of the central exhaust passage, the central exhaust passage and the cooling water flow part forming an integrated structure. The exhaust gas and the cooling water are caused to mix in the extension case.