Why must you operate the exhaust blower before starting boats inboard gasoline engine?

Running the exhaust blower will expel fuel or exhaust vapors trapped in the engine compartment. This is a precautionary step. A visual inspection the engine compartment is also a good habit to get into if you have not started the boat for some time.

Things to look (and smell) for in the engine compartment that might tell you something is wrong with your boat before you start it include: fuel or exhaust, water, and broken or loose motor parts.