AFT Air Venting

You really can't have too much air into an engine room provided it is not forced in. When engines are running hard they are pulling an enormous amount of air into them. Those engines are going to be pulling over 500 CFM each. Cram 2 or three of them into a tiny engine room and you can see why having the hatch open or closed won't make that big a difference. Many engine rooms are engineered with too little air intake area or poor separation of salt water spray. Also they often do not have the proper volume to keep the temperature down. Insufficient air flow will cut down on your power and shorten the life of your engines.

Now if you have gas engines you NEED to ventilate properly. Gasoline vapors are heavier than air and your hoses should extend down into the bilge area to pull any fumes out. Too many times I have seen blowers with no hose or only 1' of hose on them. You also need matching air intake area for your exhaust fan. Since you are redesigning the exhaust blowers please make sure that you have at least as much air flow as you had before. Every bend, every hose etc. reduces air flow.

Not sure what you mean by blown out motors? Blower or supercharger? I guess it's local. Some guys around here would call a mechanical blower (671 type on top of the engine) a blown motor and the smaller accessory driven type supercharged. So a blown motor can be a good thing or a very bad thing depending on the context. Either way with the blowers you are pulling even more air through the engine room. Turbo's also but you don't see many turbo gassers on the water. But even a conventional 6L V8 will pull close to 500 cfm at cruise. Put two of those in an engine room that's maybe 200 cubic feet and your are exchanging the entire volume of air every 30 seconds. Put two supercharged 500 cu inch big blocks and you're doing it every 15 seconds.

The key phrase is "that you need" Any more than that can cause problems. If you create positive pressure in the engine room you are supercharging the engine. Even a few psi will cause increased pressures and a lean condition. If you are forcing in enough air for WOT then you will have too much air at idle. Unless of course you have enough exhaust vents to let all the extra air out. Which means of course you didn't need to shove it into the engine room in the first place. Plus any positive pressure at all will tend to force any fumes from the engine compartment into the living spaces.

There are high end systems on some larger yachts that basically maintain a certain static pressure with the use of intake and exhaust fans. This is to ensure proper ventilation at all speeds. As the engine rpm drops the fans change speed to maintain static pressure. These systems usually run even after shutdown to help get heat out of the engine room.

We're mixing a lot of types of boats and engines up here.

Gasoline inboards need sufficient exhaust fans to pull the fumes out.
When cruising they will move more air then the exhaust fans and keep the air temp down in the engine room.
When idling many run very hot to begin with and heat up the engine room. I haven't seen any that truly need the hatch open to run. The more you squeeze out of them the hotter they run. Some high output engines have trouble cooling at idle. Also many have very hot exhaust systems.

You are correct we don't measure engine room pressure in PSI. Just used that term for convenience and to show the extreme. Boost we measure in PSI. Ram effect on a boat engine is minimal unless it is a true drag racing boat. At best it reduces vacuum. Airflow just isn't that strong even on an 80 mph boat. To get the ram effect you need to be in clean air. I haven't seen any boats built with ram effect to the engine room.

Most boats have too little ventilation. Many have way too little ventilation so engine room vacuum (usually measured in inches of water) is what you are worried about.

All engines manufactures publish specs on minimum ventilation and maximum vacuum allowed. As for too much air you always want to avoid a situation where engine room fumes can get pushed into other areas of the boat.