Marine propulsion device

A marine propulsion device having an internal combustion engine, the internal combustion engine comprising:

• a cooling water passage provided in an engine main body of the internal combustion engine;
• a thermostat valve provided in the cooling water passage for opening and closing the cooling water passage according to a temperature of cooling water in the cooling water passage;
• and
• a cylindrical cooling water introduction member separately formed from the engine main body and provided in the cooling water passage so as to surround the thermostat and separate the thermostat valve from a surrounding wall defining the cooling water passage,
• wherein a part of the cooling water passage downstream of the thermostat valve is jointly defined by an outer surface of the engine main body having a first opening directly communicating with a part of the cooling water passage receiving the thermostat valve therein, and a second opening communicating with a part of the cooling water passage further downstream of the thermostat valve, a lid plate attached to the outer surface of the engine main body and having a first hole and a second hole aligning with the first opening and the second opening, respectively, and a cover member attached to the lid plate so as to define a communicating passage communicating the first opening with the second opening via the first hole and the second hole.