Engine room air intakes

Toolate

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Any general guidelines on this? I was under the assumption that an engine consumed about 1.5-2X the hp it was producing (400-500 cfm air for 250 hp or so).

Lots of talk lately of gasketing/sealing engine rooms to reduce sound but there has to be a way to get the air in for combustion.

My boat has 2 3" flex hoses mounted on scoops plus a number of holes in bulkheads etc that will allow air into the space. Should I be doing some math on this and maybe adding some louvers/ducts? Would love a simple solution or an idea on what builders are doing today to let air in without letting sound into the cabin.
 

John 40

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I think 1/2 a sq. inch per horsepower is adequate. Make sure you subtract for louvers.
 

dthompson004

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Any general guidelines on this? I was under the assumption that an engine consumed about 1.5-2X the hp it was producing (400-500 cfm air for 250 hp or so).

Lots of talk lately of gasketing/sealing engine rooms to reduce sound but there has to be a way to get the air in for combustion.

My boat has 2 3" flex hoses mounted on scoops plus a number of holes in bulkheads etc that will allow air into the space. Should I be doing some math on this and maybe adding some louvers/ducts? Would love a simple solution or an idea on what builders are doing today to let air in without letting sound into the cabin.
Toolate, I'm going to at least double mine, as you know we have the same boat and same engine and from what I heard and just general feel from last season the engine is starved for air even with a clean arrestor. The last rule of thumb formula I saw was 2.5 cfm per HP. The original set up was for a 5.7 litre with substantially less HP. I guess you can't have too much but can have too little.
 

Badlatitude

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Somewhere there is a great photo of under gunnel air boxes built to let er breath plus quiet things down. Im sure someone will remember where its posted lol
 

chortle

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Somewhere there is a great photo of under gunnel air boxes built to let er breath plus quiet things down. Im sure someone will remember where its posted lol

This may not be exactly what you were referring to but this is what Soundown sent to me as a guide for building an air intake baffle to reduce the noise coming from my air inlet pipes on either side of the pilothouse.

air inlet sound attenuation.jpg
 

CEShawn

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Why not do what carolina boats do and just put it outside!!! Never understood that... Still wnat to put mine outside, luckily with high sheer isnt a problem, but hell with my GM 26 still wasnt a problem on the outside...
 

plowin

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The size of these intakes were specified by the engine manufacturer. We placed them outside to avoid hearing any additional engine noise in the wheelhouse and it worked great IMG_0397.jpg

IMG_0397.jpg
 

Toolate

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So basedon my 375 hp, I am looking at about 180 square inches of space (1/2 in square per hp).

That is really prety big- I bet most boats are under this number if you really look.

My boat, like Dthompsons, has 2 flex intakes at 3" is all. That is 14 inches square so we need to add another 150 or so to be up to this spec.

Simplest way out for me would be to add a louvered opening in the rear wall of the house and then cut the same hole in the cabin floor and box out a duct I think.

Thought I might consider (add to the list) some vents near my transom to facilitate bilge ventilation fore and aft as well as let in some air for the engine. Maybe glass some tubes into the cockpit corners and leave them about 2" short of the bottom of the gunwales.

I have anumber of other openings here and there for hoses, wires, etc. plus the bilge blower hoses run air in reverse when they are off so this is not an absolute but I am way below what you all are saying so have some thinking to do.

Raise your hand if you are drastically below this spec!
 

Bill

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Thanks for asking this question. I also have the shitty 3 or 4" flex hose that comes from holes in the trunk cabin and run under the deck.. I guess the reason is that there was no holes in the stringers for the air to circulate in the bilge/ and at the same time give the engine breathing air.. I'm not there yet but I would really like to cover up those holes in the trunk and go with just air boxes .,

Is this doable ? Not something I have really looked at or thought about .. Are the air boxes sufficient that are under the gunnels and back aft in the corners ( if outfitted)
 

Badlatitude

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That sort of makes no sense. Especially on a naturally aspirated gas engine. Lets say you have a 650 cfm carb. Whats the total square inch of the four butterfly openings .... DEFINATLY no where near180 square inches so how could that be? So if you put a piece of 6in PVC over the top of your carb like a stack through your floor..... the engine wouldn't get enough air?
 

captainlarry84

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air

Here is a good test;
Run your boat at WOT then open the engine hatch and see if the tach moves at all. If is does you need more air. Another good test is to hang a dollar bill in front of any of the air inlets in the cockpit, or the forward engine assess hatch if the dollar bill pulls in then you need more air.
 

Raider Ronnie

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Here is a good test;
Run your boat at WOT then open the engine hatch and see if the tach moves at all. If is does you need more air. Another good test is to hang a dollar bill in front of any of the air inlets in the cockpit, or the forward engine assess hatch if the dollar bill pulls in then you need more air.

A $100 work better than a $50.
 

CEShawn

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First problem I saw I think was with an Albamarle 27, can happen anywhere though. I remember doing it and went, WOW these engines get so much louder LOL... Ahh to be young again and naive.

I really think ventillation is key to so many things, including rot, etc... I was thinking about putting one of those solar ventilation mushrooms up near the bow to draw air a little more from the front of the boat... I think I can do it and not have water issues... right only only have air openings in the back... going to do mid hull ones when I get home...
 

Keelboater

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The idea is to allow enough air for not only the motor to breathe well, but to keep the bilge at a reasonable temperature as well with natural ventilation. I think a dry stack boat may require more ventilation than a wet exhaust boat for that reason. The drystack may need twice the air flow if I recall. I have used a minimum vent area in sq. inches of .3 x HP for wet exhaust, but .5 x HP would be even better as Team 1 mentioned.
 

Toolate

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That sort of makes no sense. Especially on a naturally aspirated gas engine. Lets say you have a 650 cfm carb. Whats the total square inch of the four butterfly openings .... DEFINATLY no where near180 square inches so how could that be? So if you put a piece of 6in PVC over the top of your carb like a stack through your floor..... the engine wouldn't get enough air?

I think the idea is to make the air readily available rather than make the engine work for it. Like Larrys post points out, if you dont have enough air flow into the bilge or engine room, the whole space is effectively in a vaccum of negative pressure and you are hurting your performance (rpms low).

Bill you posted about bilge ventilation a couple months ago I think in your deck wrecking phase. I would think that ventilation is ventilation no matter where it is.

I personally put two of those solar fans on my boat and, after this, am going to be looking into letting some air in as far aft as I can for the engines but also to facilitate cross ventilating the bilge.

For some reason I dont like the idea of taking combustion air from my cabin. I can imagine a fried belt or something smoking the whole place out in seconds.
 

captainlarry84

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The idea is to allow enough air for not only the motor to breathe well, but to keep the bilge at a reasonable temperature as well with natural ventilation. I think a dry stack boat may require more ventilation than a wet exhaust boat for that reason. The drystack may need twice the air flow if I recall. I have used a minimum vent area in sq. inches of .3 x HP for wet exhaust, but .5 x HP would be even better as Team 1 mentioned.
Bilge temps are very important. Here is a neat little trick on monitoring bilge temps. I use an outside inside temperature gauge. The outside probe goes into the bilge the unit itself monitors the cabin temperature.

Picture 4899.jpg
 

Toolate

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I wouldnt want to rely on it but does anyone ever use any kind of fan to ventilate the bilge while running? Like a bilge blower in reverse?

I have heard of people running with bilge blowers (blowing out) on all the time but never thought about it starving the engine of air until now.
 


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