Mich. Admin. Code R. 281.1237
Rule 7. All motorboats, except open boats, the construction or decking over of which is commenced after April 25, 1940, and which use fuel having a flash point of 110 degrees Fahrenheit or less, shall have at least 2 ventilator ducts, fitted with cowls or their equivalent, for the efficient removal of explosive or flammable gases from the bilges of every engine and fuel tank compartment. There shall be at least 1 exhaust duct installed to extend from the open atmosphere to the lower portion of the bilge and at least 1 intake duct installed to extend to a point at least midway to the bilge or at least below the level of the carburetor intake. The cowls shall be located and trimmed for maximum effectiveness to prevent displaced fumes from being recirculated.
Mich. Admin. Code R. 281.1237