This might be kinda stupid so, go easy on me here.

My engine bay gets pretty hot, so, I'm thinking that some sort of ducted air supply to the carbs would helpful. I'm thinking of a duct system that might connect to the underside of the hatch, directed to a vent with an in-line blower like a bildge blower. Now, I'm not assuming boost here, just a supply of fresh, cooler air to the intakes.

Well, dumb or maybe worthwhile?

That's not a dumb idea at all. Hot intake air, even in a normally aspirated motor is a definite power killer. Getting fresh, cool air in there will be good for everything in there as well. Be careful when you make any modifications that you don't set up something that will funnel heavy spray or a wave taken over into your motor....

P*V=n*R*T where T is in absolute.

Assuming you can get the engine bay from say, 110 deg F to 80 deg F, you will actually be putting in (459+110)/(459+80), or 5% more air molecules for the same volumetric flowrate.

The thing to think about, though, is that a BBC, even a mild one, is probably sucking 800 cubic feet per minute at full power. That's a massive amount of air, and it has to be coming from somewhere. That means that the little bilge blower pushing 100 cfm or less might not really do anything different. A hood scoop would move enough air to make a difference.

Hope that helps.

*snicker*

I was just talking to a friend about this last week. He built a box under the bench seat to feed air directly to the carbs through hoses and kind of a ram air box around the flame arrestors. All this was to feed cooler air to the motors that wasn't heated up in the engine compartment. Then we got to the point of the twin 800 CFM carbs and say a 160 cubic foot engine compartment. The air in the engine compartment would be replaced every 6 seconds, so how much does it really heat up? It does seem hot in there when you idle around, but I'd be interested to see what a real time engine compartment temp gauge would read at WOT.

a quick easy test crack your engine hatch while you are running at a steady rpm if you pick up rpm you do not have enough air to the motors if no change in rpm you are probably ok.

what lotti said
When I first brought my current boat home 5 year ago I was able to gain over 300rpm by just opening the hatch a 1/4inch.
I then put on a pair of scoops. BIG difference
Once thing you could do is put in a set of side vents. I thought about that, but did not want to take away from my graphics and like the looks of the scoops much better.
First pic shows without hte scoops
second show the day after I picked the boat up with them

MnFastBoat, those scoops look great.
Oh, by the way, where was that last foto taken? Sure doesn’t look like Minnesota!

Interesting answers, thanks. Never really considered what Bill272 said, if its turning air over that quick its probably getting cool enough air. I'll try cracking the hatch a tad at speed.

I think there's 2 different issues here. One is restricted airflow which will affect performance and it would seem that cracking the hatch open a bit would add performance if the engines were starved for air. The other is the engine compartment temp affecting performance. The idea that the engine doesn't really heat up the intake air at WOT is just an untested idea. I still would be interested putting a temperature sensor under the hatch while the boat is running. Maybe use one of those thermometers that you put in the house with a wired sender for outside temp?

Check this out. Great product that doesn't look bad! www.ramairbox.com