Engine room air

[Jaguar2728]

Greetings all .. looking for your opinion on aeration of the engine room. I have a Cooper 37, mid cockpit, with the engine room underneath. The original design only has ventilation holes at the port and starboard gunwales of the mid cockpit. I want to install electric fan forced ventilation, as the air quality is really bad down there. ( sets off detectors ).
I'm going to install two fans, one on each side. Would you set both to draw air out (suck) both to pump fresh air in (blow), or one suck and one blow?

[a64pilot]

I'd do one sucker with the air exhausted away from living spaces. Think like a powerboat bilge blower is done, difference is you want the hose up high to exhaust heat, not down low to exhaust gasoline fumes.
Suck as you don't want the heat and fumes getting into the cabin

[Jaguar2728]

Thanks, I had not thought of where the exhaust gases were heading. But isn't the idea of ventilation to rid the bilge of dangerous gases(fumes) that have collected down low in the bilge? That's where my detector is installed (low) that occasionally goes off. My ski boat (gas) has electric fans installed in tubes that suck air from the bottom of the bilge. I should also mention that I have a propane system for heat and cooking. There is also a bulkhead for and aft that enclose the engine room area.

[a64pilot]

You can of course install a bilge blower with the idea of removing dangerous fumes, like your ski boat, but usually engine room ventilation on a Diesel powered boat is for heat removal, and to remove heat, you need the intakes up high, where the heat is.
It depends on if your after removal of heat or explosive gases.
With the engine running and certainly if you have an exhaust fan too, there is a very slight negative pressure in the engine room, meaning of course the airflow is into the engine room and any noxious gases and heat ought to go out the exhaust fan or out the engine exhaust.

My boat came from the factory with a bilge blower mounted up high in the engine room and the duct exhausts out of the top of the transom so no heat is blown into the cockpit.

If your wanting to exhaust explosive gases like on your ski boat, make sure the fan is "ignition protected" so it can't be an ignition source

[Jaguar2728]

Ahh. good idea, on a long trip motoring, the enclosed cockpit in foul weather does get a little warm. What about one high and one low fan. Would that be to much negative pressure?

[a64pilot]

I doubt your engine room is nearly sealed well enough to have any measurable negative pressure, depending on size of the engine it's pulling more air than you may think out of the room now, an engine is nothing but a big compressor right?
I may not see the need, but don't see the harm either.