Bilge blower question

[Gpayne]

I have a 76 Thompson 18' open bow. When I got it, it didn't have a bilge blower, just the vent hoses. I bought a 3" in-line blower, and I'm wondering if I installed it properly. I mounted it to the transom so that it draws in fresh air from the vent and blows it under the engine cover to provide fresh air for the engine and to help make sure there's plenty of fresh air in the engine/bilge area. I took it out for the first time Sunday and everything worked great. I kept it on the whole time I was running the engine. At one point though, while I was just putting around, the I accidentally knocked the switch with my knee and turned it off without realizing. I'm not sure exactly how long it took, but eventually the engine starved for fresh air and shut down. I turned the blower back on, waited a couple minutes, and the engine fired back up and ran fine the rest of the day. Reading the archives, it looks like the blower should be mounted so that it draws air from the bilge, not the vents, and the intake hose should be routed into the lower part of the bilge. I'd appreciate any advice, insight, and/or opinions available. Thanks, Gary

 

[raven7]

Re: Bilge blower question

The blower is meant to draw fumes from the engine compartment and blow them out the vent. Fresh air will come in the other vent to replace what is drawn out. You should have two vents, one on each side. See if one has forward facing louvers. That would be where the fresh air comes in.

 

[Augoose]

Re: Bilge blower question

The purpose of the blower is to vent fumes and vapors out of the engine compartment, not necessarily to bring fresh air in, however you can certainly add a 2nd blower to do that if you wish. I would reverse the position of the blower so it draws air out from the bilge and pushes it out the vents.

As for the boat shutting down from lack of fresh air, are you certain that's what caused it? Without knowing the makeup of your boat, you'd have to have a pretty darn air-tight engine compartment in order to suffocate the engine to a point where it shuts down. I'd look for other causes...
Good luck!

 

[scoutabout]

Re: Bilge blower question

I'm impressed your engine cover is that airtight!

The way most work is to have the blower suck air up from as deep in the bilge as possible and blow it out one of the vents. This setup gets the heavier than air explosive fuel vapours out from where they collect at the bottom of the hull. The other vent is there to let fresh air in.

When I had an IO I ran the blower all the time at all cruising speeds (and before startup for a few minutes of course). Some will argue any speed on plane will create enough natural flow that you don't need the blower on. Depending on the configuration of the vents that may be true but I preferred to leave nothing to chance. I also installed a vapour detector for good measure.

 

[H20Rat]

Re: Bilge blower question

There is absolutely no way an engine could be starved for air... You would have had to have a fairly good vacuum going on, and the engine compartment would have collapsed with a couple thousand pounds of pressure. (~14.6 pounds per square inch)

 

[Gpayne]

Re: Bilge blower question

My engine cover is by no means airtight! It was just the most plausible answer I could come up with. When I tried to restart the engine, it cranked a bunch, then sputtered like it was flooded, then eventually fired up and ran fine the rest of the day. It may have been a fuel delivery problem, it may have been a loose wire somewhere, it could even have been tiny gremlins for all I know. I do know that it has a complete tune-up, all new fuel filters, a new water separator, and other than shutting down once, it ran fine all day.

 

[rallyart]

Re: Bilge blower question

Turning the vent fan off would not affect the airflow to the engine enough to make it run poorly so it might have been a coincidence. There is no possibility that any 12V 3" pump could come close to providing sufficient air to the engine.
The blower should blow out from the engine/bilge area and an air hose should run down to low in the bilge from the fan to extract the air that might contain fumes.

 

[H20Rat]

Re: Bilge blower question

When I had an IO I ran the blower all the time at all cruising speeds (and before startup for a few minutes of course). Some will argue any speed on plane will create enough natural flow that you don't need the blower on. Depending on the configuration of the vents that may be true but I preferred to leave nothing to chance. I also installed a vapour detector for good measure.

You don't need it at cruise speed... When are at a decent throttle, the engine is already processing far more air than a little fan could spit out, like 10x more. A typical 3" 'turbo' fan is rated at 120CFM with no load. Once you add a couple feet of tubing, you will cut that in half, so 60 cfm give or take. A chevy 350 will go through 650 CFM or more at WOT.

 

[Gpayne]

Re: Bilge blower question

Thanks guys, I think I'll go ahead and turn the blower around and reroute the hoses this weekend. And a Chevy 350 would be nice to have, but my little 2.5 Merc does run good, and it gets much better mileage.