Bilge Blower Placement

[swokrams]

I have to replace some of the hoses on my Sabre 28 including a bilge duct. As I understand the purpose of a bilge blower, it is to remove gas fumes from the A-4 which would suggest placement of the exhaust duct on the carburator side where the gas lines and exhaust mainfold is located.

As gas fumes are denser than air, it seems like the exhaust duct should be on the bilge floor (i.e. under the carburator).

If this is accurate, then one would think that the intake hose, which does not have a blower, would go over the engine somewhere (anywhere really).

However, I have read about folks punning the exhaust duct over the engine and the intake duct low in the bilge. The intent is to draw off heat in the engine compartment.

Where should I locate the ducts?

Steve

[baileyem]

Blower tube placement

Steve
I believe that the fumes you should be most worried about in your bilge are gasoline and propane, as they are both hevier than air. You may not have propane aboard, so gasoline fumes are the most likely to cause a problem in the area around your A4. The exhaust tube pickup should be under, or in very close proximity, to your carbureator and fuel pump. I am not sure of where the fresh air intake should be, but I don't think that its placement is as critical.
Mike

[jhwelch]

While the source of the fumes may be around the engine they do sink
and if you have any you would want to get them all sucked away, not
just from the point where they are originating from.

I have my hose down at the bottom of the bilge (which is pretty deep).

-jonathan

[MikeB.330]

If you place the hose in the lower sections of the bilge make sure it's high enough not to be covered up with water. Part of my SOP is to check the bilge before the engine is started for gas or oil fumes or standing fluids for that matter. If your fresh water tank sprung a leak the intake for the blower would be under water..

Mike

[swokrams]

Crp! - I posted a reply and it timed out on me.

Mike, Mike, and Jonathan,

Thanks for your tips. I decided to locate the exhaust vent under the carb sitting on the engine compartment floor. There is almost no air flow out of there to the keel bilge. The water pump side can vent profusely. Go figure.

I read some stuff alegedly taken from the USCG specs, but it all seems to reference "Engine compartment" bilges - which is what I have, but there is no mention of keel bilges. As if the engine compartments are relatively air tight? Who knows.

I like my A4, but I WANT TO GO SAILING and get my head out of the engine compartment. Agh!

Best,

Steve

[Chip Hindes]

How do you know there is almost no air flow out of the engine compartment to the keel bilge? In a way, it doesn't matter, because "almost none" is the same as "some", and even some is a problem.

It's just as much a problem that the water pump side can vent profusely. Where does it vent to?

Don't make the mistake of assuming that the only time you need to worry about fuel vapor buildup is when the motor is running. When the boat is sitting empty, if there is a fuel spill or leak from any part of the boat, including the lines or the tank (or even evaporation through the carburetor intake) the fuel vapor will flow downhill to the lowest point through even the smallest of paths. If, as you had feared, the carburetor or fuel pump were to leak, the vapor will flow to the bottom of the engine compartment, then over to the water pump side, then "vent profusely" to wherever the water pump side is vented to.

On my boat (Newport 30) everything under the cabin sole is level with or lower than the bottom of the engine compartment.

[swokrams]

Hi Chip,

On my boat the keel bilge is the lowest point, too. The downhill opening at the lowest point of the engine compartment (carb side) has about a 1 and 3/4 inch square opening through which a 1 and 1/2 inch bilge pump hose runs. If I were to route the bilge pump hose around to the other side of the engine, then maybe I could fork the exhaust duct and a smaller hose run to the bilge. I just can't get a 3" duct into the bilge anyway I look at it. The space is hard to describe. Here’s a picture.

The sole running across the front of the engine is just cropped off the picture. The whitish line on the sole is where the steps/engine cover sits on the sole. This is nearly airtight. The flywheel housing actually protrudes into the cabin. This boat feels bigger down below than it is and this is one of the reasons why, for better or worse.

The flywheel is slanted as the engine points downward in back, and there is cubbyhole in the bottom step of the cover. This cubby hole is accessible by lifting the bottom step. The cubby hole has a slanted back to be parallel with the flywheel cover. It is tight, tight, tight.

The bilge deepens just where the sole covers the bilge which is just where the cover sits. I can just barely get two bilge pump hoses down there – one, the 1.5 inch manual, and the other, a three quarter inch hose to a tinker-toy Rule bilge pump. I am adding a 3700 GPM bilge pump and I am running the hose forward to exit topside and avoid this area.

More than you wanted to know, I’m sure. But it helps to put this on paper. Right now I am thinking you are right and I do need to exhaust the lowest part of the bilge.

[jhwelch]

My troubles over the years with gas leaks have all been when the
carb. float fails to seal. Over time the boat slowly fills up with
fumes. If the engine is running the not quite seating right carb. float
still works fine; it's when I stop the engine that I realize I have
a problem (usually in the middle of the night).

So I'll still say that if you want to be safe the suck end of the
vent hose has to be at the lowest point of the bilge.

-jonathan

[swokrams]

All right Jonathan. . . Uncle! I'm convinced.

I think I may have found a soultion. If I use multiple smaller duct tubes, I think I can get them under the sole into the bilge. Here's a table I generated for number of vent ducts needed to make up the same as a single 3 inch duct. I am also thinking, if I have multiple vent ducts, I can have one or more terminate in the engine compartment. That way, if the bilge fills with water, fumes can still be drawn from the engine compartment.

[jhwelch]

Swokrams,

You bring up another interesting point -- my intake hose in my very
deep bilge is also used to tell me if I need to run the bilge pump -- my
pre-start process includes running the blower. If no air is coming out
then I have a leaky cutless bearing, etc. which has partially filled it up.
Since I run my engine a lot this is not an unusual event and this way
I catch it early-on.

Sometimes what looks like a bug may be a feature!

-jonathan

[swokrams]

Jonathan,

Interesting feature! I am thinking about sticking a telltail on the bilge blower exhaust port on the transome - like a streamer on a kid's bicycle. I run the blower whenever then engine is ruinning, but it can be hard to tell if it is really working when you are underway. The streamer would act as a simple flow meter.

Steve

[msauntry]

I have my blower discharging through a clamshell vent into the cockpit. I used to have it going through a lazarette hatch facing aft, which was quiet, but if there's ever an issue, I couldn't "smell" the problem. This way, I know if there's any fuel odor, belt's too tight, or melting electrical wires. Each gives off its own scent. For those of you that have fresh water cooling, you could also smell any coolant leaks.

You could motor for quite a while and not know there's a problem if you can't see or smell the vent.

[sonador]

Yikes! I only run my blower for a few minutes before I start the engine. Have I been just plain lucky all these years?

[High Hopes]

Yes, you should run the blower when the engine is running.

But, when the boat is underway, the forward and aft facing clamshells on your stern circulate the air with or without a blower. Usually the companionway and a hatch are open, so air circulates pretty well through them also. If you are in the cockpit and there is a gas leak, you will most likely smell it. Anyone below will certainly smell it. Gasoline has a potent odor.

Steve