An outboard motor equipped with a four-stroke engine and provided with a catalyst installation structure, in which an exhaust passage is connected to the exhaust ports of the plurality of the cylinders to lead the exhaust gas out of the engine is provided with a catalyst storage chamber configured to store a catalytic converter for cleaning up the exhaust gas, the exhaust passage being integrally formed to the cylinder block, and the catalytic converter is mounted to the catalyst storage chamber by being inserted from a lower side thereof in such a manner as that at least one side end portion thereof is fitted to the catalyst storage chamber to permit expansion of the catalytic converter in an axial direction thereof.

In an exhaust passage, an exhaust manifold connected to exhaust ports of each cylinder of an engine body and formed to extend vertically is arranged in a side portion of a cylinder head. A secondary air supply system has an intake silencer, a check valve configured to permit only a flow to the exhaust passage side, a secondary air supply control valve configured to control a flow rate of the secondary air, and a secondary air supply passage configured to connect those components to each other and the exhaust passage. The secondary air supply system is arranged in a side portion of a cylinder block placed between a crankcase of the engine body and an assembly of the exhaust manifold and the cylinder head.

A vessel propulsion apparatus includes an engine and a propulsion unit to be driven by the engine. The engine includes cylinders arranged in series, exhaust ports respectively connected to combustion chambers of the cylinders and curved in a predetermined downstream direction to allow exhaust gases to flow out from the combustion chambers, and a collecting exhaust pipe. The collecting exhaust pipe is integral with the exhaust ports, and extends in the downstream direction to allow exhaust gases in the exhaust ports to flow in the downstream direction. At least one of a plurality of connector portions that individually connect the exhaust ports to the collecting exhaust pipe includes a concave portion at an inner surface thereof.

An intake system is arranged in a side portion of one side of an engine body in a left-right direction, and the exhaust passage and the air pump are arranged in a side portion of the other side of the cylinder block and the cylinder head of the engine body in the left-right direction.

An engine for an outboard motor is provided with an engine body, an intake system configured to supply combustion air to the engine body, an exhaust passage formed by connecting the engine body and the middle and lower units thereunder, a catalyst provided in the exhaust passage, and an air pump configured to supply secondary air to the upstream side of the catalyst. The air pump is driven by an electric motor.

A marine propulsion device and a method of making a marine propulsion device including an internal combustion engine that discharges exhaust gases. The internal combustion engine has first and second castings. A first exhaust flow passage is in the first casting and a second exhaust flow passage is in the second casting. The second exhaust flow passage has a first and second leg that are transversely oriented to each other. A catalyst is disposed in the second exhaust flow passage and configured to treat the exhaust gases. A flow control element is between the first and second flow passages. The flow control element forms a smooth transition that is devoid of edges such that the exhaust gases flow across and are dispersed by the flow control element before being treated by the catalyst.

An elongated exhaust conduit comprises a first end receiving hot exhaust gas from a marine engine and a second end discharging the exhaust gas. An elongated cooling water jacket extends adjacent to the exhaust conduit. The cooling water jacket conveys the raw cooling water adjacent to the exhaust conduit to thereby cool the exhaust conduit and warm the raw cooling water therein and discharges the warmed cooling water to cool the marine engine. The cooling water jacket comprises first and second channels that are separate from each other. The cooling water is oriented in a helical flow around the exhaust conduit.

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.