If your engine is gasoline powered, (note: gasoline fumes are lighter than air) you'll want to run the blower for a few minutes before attempting to start the engine as a matter of safety. Unless your boat has some serious defects, there will be no way that Propane fumes can reach the engine space. If they can, however, you have much more to be concerned about than where the exhaust vent is positioned.
This is absolutely untrue !!!!!!!!!!
Gasoline fumes are heavier than air and this is why ABYC requires that blower exhaust ducts terminate as near under the engine as possible while remaining above normal bilge water levels.
Strongly suggest you read Safe Gasoline Engine Compartment Ventilation