Engine space ventilation

[Graycruiser]

On our 1994 Catalina 320, the blower is on the output side of the air flow. Originally, it was wired to only run when key on. I figured that it should run for a while after shutdown so I moved the pos wire to the hot side of the key switch.
The incoming hose ended in the shaft alley immediately across from the outgoing hose. This seemed dumb and ineffective so I extended the incoming hose to just behind the alternator thinking that fresh air would help the alt run cooler. With the engine hot, the air coming from the transom discharge was warm never hot, and the volume of air was not overly impressive. Then, it occurred to me that the engine itself is an air breathing monster and by itself would pull copious amounts of outside air into the space and at the same time exhaust great amounts of hot air through the normal aspiration process.
Not matter how much I run the ventilation, the aft cabin bunk over the shaft alley always seems rather warm which isn't so nice on hot summer evenings.
So, my question is, what is the best way to setup and use the engine space ventilation?

 

[JK_Boston_Catalina310]

That unit is not for blowing air in. It's to remove any explosive vapors prior to starting the motor. On gasoline powered boats its a big deal. Run the blower for 10-20 minutes before starting. The actual standard is designed based on the number of air changes in the area. But most achieve this by running the blower for 10 minutes or more. Although I have seen boats not run the blower, especially after filling up. That is dangerous and stupid.

On diesel boats the main concern would be gasoline stored on board for the dingy or a small generator or propane from the cooking system. You can get some diesel fumes down there but diesel fumes need to be heated to over 140 degrees to be ignitable.

If yours is blowing air in then something is wrong. Reverse polarity maybe or installed backwards.

For supply air there is another duct on the other side of the boat. This one just is open to the exterior of the hull through the cowl on the stern. The engine creates enough suction to make it work.