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Bilge blower question.

Old 09-04-2020, 08:20 AM
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Default Bilge blower question.

I have two gas engines and two bilge blowers, one under each of the engines. One is sucking air in and the other is blowing air out. Is that the way it should be? There are stringers and batteries between the batteries so I can't imagine how it would work to circulate air better. I am thinking both should be blowing air out of the engine room.
Old 09-04-2020, 08:24 AM
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Yes in my opinion they should both be discharging the air/gas vapors out , and this will pull fresh outside air in.
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Old 09-04-2020, 08:29 AM
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The blowers should both suck air out of the engine compartment, drawing from the lowest point in the engine compartment, because gasoline fumes (if they exist) are heavier than air.
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Old 09-04-2020, 08:34 AM
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10-4. I will fix the one sucking air in today.
Old 09-04-2020, 08:48 AM
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Depending on how many vents you have on the gunnels you can set it up so you have the vents hooked up to the blower facing aft and you can hook up extra hoses to forward facing vents that will bring in fresh air to help displace the bilge air being sucked out by the blowers. Also an addition I feel should be standard is a gas vapor detector, they are easy to install and hook up...
ideally with a twin engine gas boat you’d have two vents on each gunnel one facing fwd and one facing aft. One is the blower exit (facing aft) for the engine on that side and the other (facing fwd) brings in fresh air....
Old 09-04-2020, 09:59 AM
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My boat was built with one blower taking in air (starboard side) and the port side discharging air. I prefer to run them whenever the main engines are running. I also have a bilge fume detector and feel as though they be mandated for any boat with an enclosed gas engine.
Old 09-04-2020, 01:41 PM
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Both of my i/o boats were rigged with a blower pushing air into the lowest part of the bilge and another blowing out of the bilge. Both connected to deck vents on opposite sides of the boat. If you set up two blowers pulling air out you are more likely to leave a dead space as the air exchanges now count on the blowers overcoming the resistance of flow from the vents and not the positive pressure applied by the blower that is pushing into the engine room.
Another reason to set up for air exchange is that you won't be pulling air out and creating negative pressure when the engines are running. Most boats recommend having the blowers running at no-wake speeds, it keeps engine intake temps and engine room temps down.

Old 09-04-2020, 03:27 PM
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My dual engine cruiser had one in and one out, and the ABCY rules seem to require that.
Old 09-04-2020, 03:37 PM
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2 blowers blowing out will do nothing if air can’t get in.
Old 09-04-2020, 04:01 PM
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Originally Posted by phillyg View Post
My dual engine cruiser had one in and one out, and the ABCY rules seem to require that.
I believe what you refer to is only for the passive part of the ventilation system which requires breeze-forced inlet and outlet. cross ventilation.
i believe electric blowers should only be pilling pushing potentially very dangerous bilge fumes ( and engine heat) out to discharge fixtures , never blowing air in to push heavy gas fumes further into the bottom of yet more boat compartments. including living spaces. Even with 1 blowing in and 1 pushing out, if the 1 "out " fails you end up with that bad situation.

All that said , check your passive and active ventilation system carefully at least once a year. I suspect ~~30-75% of powerboats much over 5 yrs old & requiring ventilation systems have a leaky duct with rot-holes or full breaks, and/or ducts fallen off the hard to inspect inside connection of hull vent boxes, and /or an inoperative blower.
These ventilation systems can only help prtect you if they are inspected and maintained.


Here are the ABYC rules :
https://law.resource.org/pub/us/cfr/....H-02.1989.pdf

They state ventilation blowers are to be used as EXHAUST blowers and must have their intake at the lower 1/3 of any compartment where fumes may collect. And any passive ventilation exhaust-outlet vent also have their inlets in the lower 1/3 of such compartments or bilge areas.
Old 09-04-2020, 04:04 PM
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Care must be taken that the outside air intake is free of CO and other fumes when using two
Old 09-04-2020, 04:07 PM
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Originally Posted by phillyg View Post
My dual engine cruiser had one in and one out, and the ABCY rules seem to require that.

Did it have a genny?
Old 09-06-2020, 07:48 AM
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Originally Posted by Thalasso View Post
Care must be taken that the outside air intake is free of CO and other fumes when using two
Don’t care about CO (in the context of venting the bilge)The whole concern when it comes to this topic is gasoline fumes.

or, I should say, the primary concern and purpose is clearing the bilge of gasoline vapors.

Last edited by NedLloyd; 09-06-2020 at 07:54 AM.
Old 09-06-2020, 07:51 AM
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Originally Posted by From the Helm View Post
Both of my i/o boats were rigged with a blower pushing air into the lowest part of the bilge and another blowing out of the bilge. Both connected to deck vents on opposite sides of the boat. If you set up two blowers pulling air out you are more likely to leave a dead space as the air exchanges now count on the blowers overcoming the resistance of flow from the vents and not the positive pressure applied by the blower that is pushing into the engine room.
Another reason to set up for air exchange is that you won't be pulling air out and creating negative pressure when the engines are running. Most boats recommend having the blowers running at no-wake speeds, it keeps engine intake temps and engine room temps down.
I hope that was not the case. Gasoline vapors are heavier than air. The blowers need to draw in (suck) air (and vapors if they exist) from the lowest part of the bilge and dump them over the side of the boat.
What you described will only blow the vapors and spread them all over the bilge.
Old 09-06-2020, 05:06 PM
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Originally Posted by wingless View Post
The blowers should both suck air out of the engine compartment, drawing from the lowest point in the engine compartment, because gasoline fumes (if they exist) are heavier than air.
My boat now has five of the 220 cfm Shurflo blowers.

Two were installed by the factory, sucking low, below each engine, wired to the dash blower switch, to exhaust the fuel fumes, if they ever exist. The factory wired these using a module to provide the low current circuit protection and to blink the rocker switch if one should ever fail.

Two were installed by me, sucking from directly above each engine. These are connected to an automatic temperature controller, sensing the engine compartment temperature using a 10K Ohm NTC thermistor. Each is protected by the correct rating fuse. This custom system has been 100% effective at eliminating heat soak vapor lock, after stopping the boat for an hour.

The last one dumps fresh exterior air onto my custom high volume high pressure air compressor. This is wired to operate whenever the compressor motor is running. It is protected by a correct rating fuse. This is so effective that I can continuously hand hold the compressor cylinder during operation.
Old 09-06-2020, 05:53 PM
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Originally Posted by Thalasso View Post
Did it have a genny?
No.

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