Wet exhaust

Unlike that of a motor car, the exhaust from a boat’s engine is rarely exposed to a flow of air, and is best cooled. The common way of doing that is to inject into the stream of gases some, or all, of the water which has already circulated through the cooling galleries of the cylinder block. The presence, or absence, of water at the exhaust outlet provides a primary check on the working of the whole engine-cooling system.