Bilge blowers
- Thread starter Roy
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As I said, we had this discussion a few years ago and the guys who favor sucking the fumes, won over the Blowers.
I remain unconvinced they are correct. As well as logically I'm correct.
The Positive Pressure in the bilge should move the fuel laden air (Gasoline) toward the exit.
It's impossible for an electric Bilge Blower, meant to move volume and not pressure, to provided enough pressure to cause combustion of fumes.
Moving HAZ through an Electric motor, especially if the motor is protected by Thermal or some other switching device is a good way to ignite the fumes.
There is NO electric motor than can be started without some kind of Arc. To start a Bilge blower immersed in fuel laden air is inviting the Big Bang.
So the Blower should be up above the Fumes, and force the Fumes out a vent hole to the atmosphere.
If you have a logical argument for my theory, let's hear it right now!
I remain unconvinced they are correct. As well as logically I'm correct.
The Positive Pressure in the bilge should move the fuel laden air (Gasoline) toward the exit.
It's impossible for an electric Bilge Blower, meant to move volume and not pressure, to provided enough pressure to cause combustion of fumes.
Moving HAZ through an Electric motor, especially if the motor is protected by Thermal or some other switching device is a good way to ignite the fumes.
There is NO electric motor than can be started without some kind of Arc. To start a Bilge blower immersed in fuel laden air is inviting the Big Bang.
So the Blower should be up above the Fumes, and force the Fumes out a vent hole to the atmosphere.
If you have a logical argument for my theory, let's hear it right now!
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As previously stated, gas fumes are heavier than air so no matter how much fresh air you blow into an unsealed area the gas fumes will remain at the lowest level. This said, the only way to get them out is to evacuate the lowest level by sucking out the contaminated air. As the heavier fumes are evacuated lighter air fills the void until all the fumes are gone and the explosive condition is remedied. As for there being no motor being started without a spark, if the spark occurs outside the exploxive atmosphere it won't ignite. The same is true if the spark is in an atmosphere sealed from the explosve vapors as in starter motors, a/c circ pumps,alternators, bilge pumps,you get the idea. So there.s your logical explanation.
Mr. Keith, the issue of diesel spaces is totally different from gasoline bilge venting. The issue with gas bilge venting is to evacuate,with a negative pressure, combustible vapors to prevent the big bang. With diesels the ideais to create a positive atmospheric pressure to provide the engines with the copious quantity of air needed to support the big bang in a confined space, ie cylinder. Diesels require huge quantities of air due to the fact that their combustion is generated by pressure in the cylinder rather than by an ignition spark. They also generate more heat and require more air for cooling machinery spaces.
Mr. Keith, the issue of diesel spaces is totally different from gasoline bilge venting. The issue with gas bilge venting is to evacuate,with a negative pressure, combustible vapors to prevent the big bang. With diesels the ideais to create a positive atmospheric pressure to provide the engines with the copious quantity of air needed to support the big bang in a confined space, ie cylinder. Diesels require huge quantities of air due to the fact that their combustion is generated by pressure in the cylinder rather than by an ignition spark. They also generate more heat and require more air for cooling machinery spaces.
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The other thing to look for is what kind of louvers you have on the bilge vents. Some are directional, in that you can face one rearward and one forward. On my boat the one that the blower hose is connected to is facing rearward and the one that lets in the fresh air is facing forward. That way the forward motion of the boat helps suck fumes out the blower side and force in fresh air on the other side.
Now if you had these reversed, with the forward facing one on the blower side, it would be fighting the blowers effort to suck fumes out, because the forward movement of the boat would tend to force air into the vent.
Now if you had these reversed, with the forward facing one on the blower side, it would be fighting the blowers effort to suck fumes out, because the forward movement of the boat would tend to force air into the vent.
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the ventillation system for closed engine spaces should have a minimum of two vents - one inlet, one outlet. As mentioned earlier, the inlet has louvers (or physically) facing forward, so the natural breeze when underway will force air in. The outlet louvers face rearward, again so the natural breeze while underway will entrain outgoing air - the system allows for a natural circulation and continual venting while the boat goes forward (which is why most mfgrs say you don't need to run your blowers above 5 mph!)
Outlet vents are ducted to draw air from the lowest points in the bildge, thus fresh air in 'on top' of the ER, and bad air out 'from the bottom' of the ER - where gas fumes could collect.
My blowers are physically on the the ER fwd bulkhead, and are 'piped' in the ducting between the bildge and the outlet vent. With the blowers energized, there is a slight negative pressure in the ER - the premise being that a positive pressure could potentially blow stray gasoline fumes thru the cracks in ER hatch, bulklhead penetrations, etc, so as to create an explosive environment outside the ER!!
hope this helps.
Outlet vents are ducted to draw air from the lowest points in the bildge, thus fresh air in 'on top' of the ER, and bad air out 'from the bottom' of the ER - where gas fumes could collect.
My blowers are physically on the the ER fwd bulkhead, and are 'piped' in the ducting between the bildge and the outlet vent. With the blowers energized, there is a slight negative pressure in the ER - the premise being that a positive pressure could potentially blow stray gasoline fumes thru the cracks in ER hatch, bulklhead penetrations, etc, so as to create an explosive environment outside the ER!!
hope this helps.
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This is not quite correct. Just about all manufacturers of planing hulls recommend that the blower be run anytime the boat is operated "below cruising speed."
I guess the answer to my original question is that both blowers should be in the exhaust direction so they are both sucking air out of the ER. I only asked the question because someone suggested that if one blows and the other sucked it would create circulation that would be better at getting rid of explosive fumes.
Roy
Roy
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On the negative side of this discussion,while I was in the bilge a few days ago I happened to glance at my blowers mounted under the gunwale and discovered that they are both blowing in! Da**,'nother project. Think I'll replace 'em while I'm at it. They're kinda old and noisy.
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"if one blows and the other sucked it would create circulation that would be better at getting rid of explosive fumes."
That might occur. However if the pressures/volumes of the two blowers are not perfectly matched you may have a positive pressure in the bilge, which in gasoline powered vessels, is undesirable. The better solution is to a) Have the passive ventilation sufficient that blowers are not required when the vessel is in motion and b) all blowers suck ( exhaust ) air. Which is why the rules are written such that all blowers should exhaust air from the engine spaces.
Ref.: Boat Builder's Handbook
That might occur. However if the pressures/volumes of the two blowers are not perfectly matched you may have a positive pressure in the bilge, which in gasoline powered vessels, is undesirable. The better solution is to a) Have the passive ventilation sufficient that blowers are not required when the vessel is in motion and b) all blowers suck ( exhaust ) air. Which is why the rules are written such that all blowers should exhaust air from the engine spaces.
Ref.: Boat Builder's Handbook