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.

Go-Fasts are usually gas boats with very blown out motors.

Hmmmm, so when the motors "blow out" they open the hatches and have to go slow until the motors get fixed?

Blown out; not blown up. But I think a wise man like you understood that.

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.

Bored & stroked plus the rest. Something like this:
Chevy Big Block Crate 454 Engine - Import Killer, Part 2
925 hp from 454 ci
From the June, 2009 issue of Car Craft
By Matthew King
Photography by Steve Dulcich, Matthew King
As crate engines go, GM Performance Parts' 454 H.O. crate engine is not only a bargain at its street price of around $4,600, its forged crank, pistons, and rods also make it capable of handling nearly as much power as you're capable of throwing at it. We proved that in our Aug. '03 issue ("Import Killer: 780 hp From 7.4L") by taking a bone-stock 454 H.O., swapping its stock hydraulic-roller cam for a bigger Crane Cams hydraulic roller, upgrading the GM dual-plane intake manifold to an Edelbrock Victor Jr. single-plane, and bolting on a Vortech carbureted supercharger kit for a total gain of more than 300 hp over the 470hp baseline.

No wonder they get blown up.

Right after they get blown out.

Must be a New York thing.

I've been around turbocharged, supercharged, blown, turbo-compounded, normalized and naturally apirated engines all my working life but the only things I've ever seen "blown out" are windows, explosion doors, ballast tanks, pipes, boiler fires, an oil well, pressure relief discs, candles, tires, a friend's knee, and a few tennis shoes.

Must be or an age thing. It's the bee's knees.

Hi,

Do you mean you wouldn't recommend the use of fans to shove all the air in that you need?

Partially yes.

Ram air effect will increase static pressure inside the engine room (note how typically small the vents are outside the boat) - this is because the cockpit (interior) of the boat will have most of the air inlet openings under the gunwales where (at speed) there is an area of high pressure air built up. Once at a speed sufficient that there is some pressure in the engine room, there is also enough flow both into the engines and out the vents to exhaust the heat.

When you run slowly, however, the heat starts backing out those cockpit air inlets and cooking the passengers in the boat.

Far easier and more effective to let the heat, which tends to rise, vent out the top, no?

Or you can continue to be snarky.

If you start spraying bilge water up the walls and knocking paint off the engines, you might have more air than you need.

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.

... require using fans that consume just about as much power as the engines produce. And unless those mirrored engine hatches are some kind of skookum, that "few" (3) psi equates to around 432 pounds per square foot trying to open them. When the hinges let go you won't have to worry about running lean anymore, you'll just have to wonder where that cute little deck ornament who was sunbathing there landed.

In which case the extra air carries away the heat and fumes which is one of the reasons why fans are installed to begin with ... so the engine hatches don't have to be open when running and Trixie has a place to sunbathe.

Snarky ?? Ooooh wow.

I was trying to understand how these engines need to have the hatches opened at low speeds but are fine when running at high speeds. I find it interesting that the ram air effect is so effective, and that these areas of high pressure build up. Even more interesting that this pressure is being measured in psi, when every acceptance trial I have been on measured engine room depression with a manometer that gave units of inches of water column.

Well, I guess your engines weren't blown out enough then were they?

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.

I guess not. Makes me feel so inadequate

Anyone with enough money can get blown.