Exhaust passage structure of outboard motor

An exhaust passage structure for an outboard motor equipped with a four-stroke engine, in which the four-stroke engine includes a cylinder block in which a cylinder is formed so as to extend in a horizontal direction;

• a cylinder head fixed to the cylinder block so as to cover the cylinder and to form a combustion chamber in conjunction with the cylinder, the cylinder head being provided with an exhaust port communicated with the combustion chamber to discharge exhaust gas;
• and a crankcase which houses a crankshaft extending in a vertical direction, the exhaust passage structure comprising:
• a plurality of cylinder assemblies each equipped with the cylinder, the combustion chamber, and the exhaust port are arranged side by side in the vertical direction in the cylinder block and under the cylinder head;
• an exhaust passage connected to the respective exhaust ports of the plurality of cylinders so as to lead the exhaust gas out of the engine includes a first exhaust passage portion and a second exhaust passage portion, in which the first exhaust passage portion is provided on at least one lateral side of the cylinder block in a width direction and configured to collect the exhaust gas discharged from the respective exhaust ports of the plurality of cylinder assemblies, while the second exhaust passage portion connects the first exhaust passage portion with an exhaust muffler chamber installed outside the engine;
• and
• an exhaust guiding portion is provided so as to oppose to a joint surface between the cylinder block and the cylinder head in a manner to be located in the first exhaust passage portion to guide the exhaust gas discharged from the exhaust ports to a connecting portion between the first exhaust passage portion and the second exhaust passage portion,
• wherein the exhaust guiding portion of the first exhaust passage portion is made up of a concave groove formed integrally with the cylinder block and configured to open to a side of the cylinder head so that the cylinder head closes the opening of the concave groove to thereby form the first exhaust passage portion.