Running bilge blowers?

[RollerCoastr]

That being said, I don't "think" the standard issue blower is officially rated for continuous duty.

Mine are not. Somehow whenever I transit the Welland Canal between Lake Erie and Lake Ontario, temps are at least mid 90's. It can be an entire day at idle, so I want keep the blowers going. They don't make it the whole day if I don't let them rest.

 

[mobocracy]

I'm repeating this not because I follow it as advice, but because it was interesting logic.

A guy told me that the bilge blower isn't really necessary once the engine is running because the engine itself acts as a bilge blower because it draws bilge air for combustion and this draws fresh air into the bilge through the vents.

I did the math for the hell of it. At 1000 rpm, each piston has about 8 intake strokes per second which draw air. A 350 cubic inch engine has .2 cubic feet of displacement, so you draw 1.6 cubic feet of air per second or 96 cubic feet per minute. On a twin engine like mine, that's 192 cubic feet of air displacement per minute. At the extreme case of idle which is roughly half the RPMs, it's back down to 96 cubic feet per minute.

A rough measure of bilge displacement is about 10 * 10 * 6, or 600 cf. If a bilge blower is 250 cfm, the 4 minute recommendation is about 1.6 bilge air changes.

So the break even point on my twin engine would be about 1300 RPM of engine speed required to draw the same air volume as the bilge blower, or 250 cfm.

One question I have is, do the bilge vents have any kind of forced induction effect? Sort of like a hood scoop for the bilge, so if you're moving you improve air flow?

 

[almostenuff]

I'm repeating this not because I follow it as advice, but because it was interesting logic.

A guy told me that the bilge blower isn't really necessary once the engine is running because the engine itself acts as a bilge blower because it draws bilge air for combustion and this draws fresh air into the bilge through the vents.

I did the math for the hell of it. At 1000 rpm, each piston has about 8 intake strokes per second which draw air. A 350 cubic inch engine has .2 cubic feet of displacement, so you draw 1.6 cubic feet of air per second or 96 cubic feet per minute. On a twin engine like mine, that's 192 cubic feet of air displacement per minute. At the extreme case of idle which is roughly half the RPMs, it's back down to 96 cubic feet per minute.

A rough measure of bilge displacement is about 10 * 10 * 6, or 600 cf. If a bilge blower is 250 cfm, the 4 minute recommendation is about 1.6 bilge air changes.

So the break even point on my twin engine would be about 1300 RPM of engine speed required to draw the same air volume as the bilge blower, or 250 cfm.

One question I have is, do the bilge vents have any kind of forced induction effect? Sort of like a hood scoop for the bilge, so if you're moving you improve air flow?

Are you by chance an engineer?


Sent from my iPhone using Tapatalk

 

[mobocracy]

Are you by chance an engineer?

No, which means I may be totally wrong about all of this, and all my calculations are kind of back of the envelope estimates.

Engine compartment free air space is obviously wrong, tanks, mechanicals, engines all take up volume, so I'm over estimating bilge air volume by something like a factor of 2, I'd guess.

But I thought the concept of engine air pumping was interesting enough to do the math on it and see how much air the engines themselves empty out of the bilge just by running.

I'd love it if an actual engineer were to look at it and tell me what's wrong. Could be my assumptions on intake strokes per RPM are off (I googled that).

And of course, I'm not advocating for not using bilge blowers, either. Without question if an engine has been off, you need to run them for 4 minutes to vent the engine compartment and at low speeds and still winds, even if engine pumping equals bilge blower volume, there's still arguments for forced venting of the engine compartment.

 

[SmilingMonkey]

I usually lift open hatch for a few minutes once I get to the boat ... Then once I fire up I have them running for a few minutes and then turn them off .. unless creep for a little bit too long

If you're using your electrical hatch lift, that could cause a spark and ignite any fumes in the bilge. You should run the blower BEFORE you open your hatch.

 

[Lazy Daze]

Interesting side topic there... the engine pumping/bilge space conversation. Here's some more talking points to add to the mix: So, yes, an engine is essentially an air pump, and moves (ingests) a lot of air. However, gas fumes are heavier than air so the gas fumes would collect under the engine. The engine's air intake is on top of the engine and somewhat close to the bilge vent openings. So, when the engine is running, does it basically only pull fresh air in? How much "bilge" air does it pull in? How much circulation of bilge air does it cause? Does it ever really suck in any bilge air that is at the bottom of the bilge?

On the flip side, the blower's hose is (should be) routed under the engine to suck out the air that is at the bottom of the bilge.

 

[romanservices]

No, which means I may be totally wrong about all of this, and all my calculations are kind of back of the envelope estimates.

Engine compartment free air space is obviously wrong, tanks, mechanicals, engines all take up volume, so I'm over estimating bilge air volume by something like a factor of 2, I'd guess.

But I thought the concept of engine air pumping was interesting enough to do the math on it and see how much air the engines themselves empty out of the bilge just by running.

I'd love it if an actual engineer were to look at it and tell me what's wrong. Could be my assumptions on intake strokes per RPM are off (I googled that).

And of course, I'm not advocating for not using bilge blowers, either. Without question if an engine has been off, you need to run them for 4 minutes to vent the engine compartment and at low speeds and still winds, even if engine pumping equals bilge blower volume, there's still arguments for forced venting of the engine compartment.

I always leave my blowers running, no harm in it at all. if it wears out I'll replace it. That being said when I had my twins replaced and we went for a shakedown ride the engine hatches were open. The noise coming from the air intake was amazing, they need lots of air.

not quite as loud in the video but definitely loud

https://www.youtube.com/watch?v=ez-oiDq5goo&t=265s

 

[CaptRonn]

Bilge blowers are a cheap item to replace.

With that said, this is one subject that people seem to like to beat long after the horse has died.

 

[mobocracy]

Interesting side topic there... the engine pumping/bilge space conversation. Here's some more talking points to add to the mix: So, yes, an engine is essentially an air pump, and moves (ingests) a lot of air. However, gas fumes are heavier than air so the gas fumes would collect under the engine. The engine's air intake is on top of the engine and somewhat close to the bilge vent openings. So, when the engine is running, does it basically only pull fresh air in? How much "bilge" air does it pull in? How much circulation of bilge air does it cause? Does it ever really suck in any bilge air that is at the bottom of the bilge?

On the flip side, the blower's hose is (should be) routed under the engine to suck out the air that is at the bottom of the bilge.

Reasonable comments. From a pure devil's advocate perspective, I would guess that airflow is bilge free air before the vent air comes in, as vent air is drawn by the vacuum created by loss of bilge air. To draw vent air without drawing bilge air would essentially be pressurizing the bilge as you would be increasing the mass of air in the bilge (draw more in without consuming or expelling existing air). The bilge wants to stay at atmospheric pressure.

The movement of external engine components creates some air circulation through movement, although its debatable how much airflow this actually generates.

But I think you're right, the bilge blower makes more sense at low engine speeds (or where engine intake CFM equals free bilge air volume) due to poor internal circulation. Once RPMs are high enough that engine intake exceeds free bilge air volume by...1.5? 2? More? You may have enough air intake into the system to effectively circulate air.

 

[mnm99]

As I mentioned, anyone ever hear stories of explosions while at idle cruise? I'm not saying I won't turn them on, just looking for some facts.

 

[Stee6043]

I'm repeating this not because I follow it as advice, but because it was interesting logic.

A guy told me that the bilge blower isn't really necessary once the engine is running because the engine itself acts as a bilge blower because it draws bilge air for combustion and this draws fresh air into the bilge through the vents.

I did the math for the hell of it. At 1000 rpm, each piston has about 8 intake strokes per second which draw air. A 350 cubic inch engine has .2 cubic feet of displacement, so you draw 1.6 cubic feet of air per second or 96 cubic feet per minute. On a twin engine like mine, that's 192 cubic feet of air displacement per minute. At the extreme case of idle which is roughly half the RPMs, it's back down to 96 cubic feet per minute.

A rough measure of bilge displacement is about 10 * 10 * 6, or 600 cf. If a bilge blower is 250 cfm, the 4 minute recommendation is about 1.6 bilge air changes.

So the break even point on my twin engine would be about 1300 RPM of engine speed required to draw the same air volume as the bilge blower, or 250 cfm.

One question I have is, do the bilge vents have any kind of forced induction effect? Sort of like a hood scoop for the bilge, so if you're moving you improve air flow?

This is awesome. A great bit of math.

 

[Stee6043]

Bilge blowers are a cheap item to replace.

With that said, this is one subject that people seem to like to beat long after the horse has died.

They may be cheap but it takes three priests, two rabbis and one small child to reach them...at least on my boat. It's almost as bad as trying to add fluid to my trim tab reservoir...

 

[Lazy Daze]

Reasonable comments. From a pure devil's advocate perspective, I would guess that airflow is bilge free air before the vent air comes in, as vent air is drawn by the vacuum created by loss of bilge air. To draw vent air without drawing bilge air would essentially be pressurizing the bilge as you would be increasing the mass of air in the bilge (draw more in without consuming or expelling existing air). The bilge wants to stay at atmospheric pressure.

The movement of external engine components creates some air circulation through movement, although its debatable how much airflow this actually generates.

But I think you're right, the bilge blower makes more sense at low engine speeds (or where engine intake CFM equals free bilge air volume) due to poor internal circulation. Once RPMs are high enough that engine intake exceeds free bilge air volume by...1.5? 2? More? You may have enough air intake into the system to effectively circulate air.

Agreed... BUT (more devil's advocate stuff here)...... There wouldn't necessarily need to be a COMPLETE evacuation (used up) of bilge air before vent air is drawn in. Actually, 100% of the bilge air CAN'T be used up before more air comes in via the vent. Vent air MUST be entering all the time and constantly. Meaning (and excuse my lack of scientific terms here :lol: ) as soon as 1% of the bilge air is used up, it will need to be replaced. That replacement of course would come from vent air.

So the real question then is (relying solely on the engine for air movement and keeping the blower out of the convesation), does the bilge air actually get completely used up? Or does the bilge air sort of become stagnant as there is more than enough vent air being drawn in? I suppose this comes down to a "path of least resistance" thing? I dunno the real answer, but it's fun to talk about it.

 

[CliffA]

IMO the air drawn into the ER by the engines is used by the engines for combustion......none of that air is circulated throughout the ER....the engines only draw in as much air as they need...no more....the only fresh air circulation created in the ER is when the boat is moving fast enough to force the air through the ER vents which creates a positive pressure in the ER......

the air that is consumed by the engines is most likely only the lighter fresh air being drawn in through the ER vents since the engine air intake is on top of the engine and close to the ER vents....if any gas fumes were present in the ER I would imagine they would stay in the bilge of the ER since they are heavier than air....I doubt the engines would pull these fumes from the bilge into their air intake.....if the boat is moving fast enough the fumes could be forced to circulate in the ER and then the engines could pull them in....

as far as removing heat from the ER, I have found running the bilge blowers make very little difference in the ambient temp inside the ER...I have a temp monitor in the ER and have observed the ER temp with the blower running and not running under the same conditions....the difference in the ER temp is only a few degrees if any at all.....heat rises to the top of the ER where the ER vents are located and will naturally find its way out of the vents even if the boat is at idle or moving slowly....the size of the ER would have an affect on this also.....

my theory only...... .....

cliff

 

[mobocracy]

My sense is that internal airflow in the engine compartment is probably pretty complex. You have convection due to heat, you're drawing in colder, denser air than what exists in the engine room, etc.

It would be interesting to "dose" the bilge with some kind of marker smoke and watch on a camera how it dissipates under various situations of engine speed, speed over ground, wind conditions, bilge blower operation, etc.

 

[Lazy Daze]

Those are all good points, Cliff.

It would be interesting to "dose" the bilge with some kind of marker smoke and watch on a camera how it dissipates under various situations of engine speed, speed over ground, wind conditions, bilge blower operation, etc.

I was thinking the same thing! Like adding dye to engine oil to find a leak.

 

[Skuza]

Life practice has been before start, during idle speed and while running genny.

 

[mobocracy]

Those are all good points, Cliff.



I was thinking the same thing! Like adding dye to engine oil to find a leak.

Hold my beer while I grab a go pro and a fog machine.

 

[CliffA]

Hold my beer while I grab a go pro and a fog machine.

might want to hold on to that beer.....it gets hot in the ER.....especially with the hatch closed.....

cliff

 

[sbw1]

No, which means I may be totally wrong about all of this, and all my calculations are kind of back of the envelope estimates.

Engine compartment free air space is obviously wrong, tanks, mechanicals, engines all take up volume, so I'm over estimating bilge air volume by something like a factor of 2, I'd guess.

But I thought the concept of engine air pumping was interesting enough to do the math on it and see how much air the engines themselves empty out of the bilge just by running.

I'd love it if an actual engineer were to look at it and tell me what's wrong. Could be my assumptions on intake strokes per RPM are off (I googled that).

And of course, I'm not advocating for not using bilge blowers, either. Without question if an engine has been off, you need to run them for 4 minutes to vent the engine compartment and at low speeds and still winds, even if engine pumping equals bilge blower volume, there's still arguments for forced venting of the engine compartment.

I'm not an engineer either but wonder if engines draw air from nearer the top of the bige you would be safe thinking the fumes are evacuated with engine draw. My understanding is gasoline fumes, if present, accumulate in the lowest areas of the bilge which could be stagnant absent a bilge blower.