engine room air circulation
 

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

Yeah, but I'm not sure how cunnilingus relates to this whole engine venting issue.....

yes you can Yamaha my r1 had what they called ram air picked up hp over 140mph i think.

LOL !!!!!!!!!

Same rules apply, I learned along time ago to use scuba gear when going diving ! Hah !

Pretty much as air is drawn or forced into the intake, the vents into the bilge will allow the air to be drawn into the engine compartment. You just need to make sure that the vents into the bilge are large enough to feed the engines. The more HP, the more air that is needed. How do you know the right size?....that is above my pay grade 😳

I agree very hard to go positive

Took the air filter off my old Nordic with a 496HO and raised the hatch...no difference.

having enough air to run is nice. having enough airflow through the bilge to keep gas fumes from accumulating to an explosive mix is much nicer. they put those cute little fins on vents one side facing forward, one side aft. I don't know if that works or not. if they are where airflow causes low pressure not much will get in. bigger sportfish sometimes have engine air intakes going straight to the side vents so they get air that hasn't been pre-heated by the engines and exhausts. even the hottest day the ambient air is cooler than your engine compartment.

For visuals to make it easier

502cid x 5000rpm = 1,255,000 cubic inches (726 cubic feet) of air per minute
2 x 502cid's x 5,000rpm = 2,510,000 cubic inches (1452 cubic feet) of air per minute

The following is the size of 1000cuft

http://indulgd.com/wp-content/upload...cube-home2.jpg

Post 11 is killing me. he must have watched Dr. Ruth way back in the day for all that proper terminology lingo

Wouldn't that be assuming 100% volume metric efficiency?

I do know when we were testing the engine room was about 20-25 degrees warmer then outside.

engine compartments must provide sufficient volume of air for the engines(s) to breath and must vent enough hot plus fumes. So its double duty. The bigger cubes of an engine will always require more incoming air and vent more hot air / fumes.

Pressure differential of outside engine compartment versus inside engine compartment should not exceed 2 in (51mm) of water measured with a manometer at max air flow rate.

2" of negative PSI, correct BUP?

i totally lost track of this thread, and started searching for it after i saw this thread

http://www.offshoreonly.com/forums/g...rving-air.html

you guys are awesome i appreciate the input

6 yeah, i'm talking about the harwood scoop on the hatch that faces forward, there is no relief vent as you call it, that's exactly what i'm talking about
the only vent is on the transom that the bilge blower is connected to. and i'm looking for a way to get air out, or (traveling thru) the engine compartment. enough to feed the engine and keep the engine room cool

post 21 bup this is exactly what im talking about but i don't understand the 2 number or how to achieve it

i like those naca ducts and think 2 of them on the engine hatch facing backward would let air flow thru the engine room, problem is almost every video i see turbulent air/ water mixture on the transom so the issue of placement is another factor.