PURPOSE: To form a compact flywheel magnet seal structure of an engine for ship propulsion.

CONSTITUTION: The end part of a crank shaft 23 is protruded from a mating surface between a crank case 20 and a cylinder block 21 externally of the crank case. A fly wheel magnet 40 is disposed at the end part thereof and on one side of an engine, an intake piping is connected to the lower part of the crank case and an exhaust expansion chamber is arranged at a flank on the other side. A flywheel magnet cover 43 is fixed to the opening end of a flywheel magnet case 42 and this constitution forms a containing space 50 for a flywheel magnet. A starter motor 54 is arranged at the flank on the other side of an engine and the exhaust expansion chamber is arranged in the vicinity of an engine above the starter motor. An exhaust gas passage extending to below is connected to the two ends in an axial direction of the crank shaft of the exhaust expansion chamber and the rear part of the starter motor is supported to the flank of the engine. Meanwhile, the front part thereof is extended through a through-hole 54A formed in the flywheel magnet case and fronts on a containing space, an intake system piping is positioned in the vicinity of the engine and extended in a vertical direction.