there is an out drive depth thread that was revived in the last couple of days, and like all threads they zig zag on and off the main topic, any way the engine room ducting topic came up, and

not only is it interesting but it's some thing on my mind as i'm getting ready to put a firewall in this race boat i bought, and i know the engine room has to breath.

right now it has a blower scoop and 1 vent that the bilge blower is connected to.

the last thing i will do is cut a hole in the boat, the engine hatch no problem, but not the hull or deck.

problem i see is position on the hatch, in -out -front -back -hi -lo etc.

what do you guys think?

A place many manufacturers use for venting to the engines is to the sides of the rear bench.

Some manufacturers just basically close off the venting an engine needs to breathe. Doh.

I was taught that yes you need fresh air obviously to feed the motor, but you dont't want postive pressure by having too much air fed into the engine room, or not enought air, hence the intake, and engine room exhaust ventilation for neutral pressure, but I am sure there are more learned members on here who can provide detail, and what works bets for your application.

Here is an example of a local Jersey boat, I love this boat. On my father's boat he had the engine scoops, then vents on the side to relieve any positive air pressure. The side air vent(s) face aft, one on each side of the Phantom, the Apache has two holes at the back of the engine cover as you see in the picture.

I think the engine(s) just need fresh air for the intakes. After all, the engines are water cooled, not air cooled. Cool fresh air for the intake is preferable to warm/hot recirculated engine room air. Run your blowers if things get a little toasty in there. Side vents are just a byproduct of not wanting to cut holes in rear hatches and sun pads. Air scoops usually are required for clearance issues on tall motors with blowers. Most scoops are reversed on boats, I assume to keep water out.

just my 2 cents.....

Dan. Is that a forward or rear facing scoop. If it's forward facing you might need a relief vent at speed if it's a rear vent I think you'll be just fine

And as for drive height what I have been doing for the last few weeks. I've taken off the 2 inch Shorty. And gained speed. Ran the stock bravo dual water pick up lower and started to get better handling. Today I am running a three-quarter inch spacer to lower it down even more. Rough water handling is getting better and prop slip has gone way down. I was running a 28 B1 and now down to a 26B1. More top end and 200 RPMs less. I'm at 5 3/4 prop shaft depth now.

To the best of my knowledge you cannot develop over atmospheric pressure (14.7 psi) without a blower or compressor..

Randy

I had always heard (grain of salt time) that if you run the engine up to max rpm, then crack the hatch an inch or two. If you get a few more rpms, you likely need better venting. Don't know how true that is but it sounded reasonable at the time.

Sure can. It's called forced induction/ram air. Don't know if it's possible on a boat. But motorcycles have been doing it since the 90s

Ever try to breathe with your head in a box