Fuel tank breathers?

[David_J]

I guess there has to be one or the fuel wont come out!
On my 9.80 each tank has an overflow outlet so when you fill up with fuel eventually a bit may dribble out the overflow (or rather a lot as someone found when they kept on pumping hard even when they could hear the tank backing up!!) Anyhow do you suppose this overflow has a non return valve, surely its not just an open connection into the internal filler pipe. Or is this overflow also the breather so needs to be an open connection?
Reason for the question is that last summer we had the bug so bad I had to get a company in to centrifuge 600l of fuel. Not cheap but it worked. I know becuase I got paranoid and kept on draining both primary filters and as we used fuel, with addative, the remnants of bug gradually went away.
Filled up for the winter with addative but this Spring the bug back again, not nearly as bad but nonetheless back again.
If this overflow is an open way into the internal filler pipe its just letting damp into the tanks all through winter which may not be the root cause of bug but not helpfull.
Anyone know the design idea on the fuel fill overflow?

 

[msimms]

On mine (sessa 35) it's effectively an open pipe terminating in a thru hull fitting.

From new the breather pipe was poorly run and dipped down below the top of the tank. This meant that after any run out there was always a quantity of diesel in the breather that would spew out when re-fuelling! I re-routed the pipe so it was always going to drain back in to the tank. The pipe has a 'U bend' about 8 inches above the breather so if any water did get in to the breather it would drain out the way it came in rather than in to the tank. So far - touch wood - never had the bug. I do use an additive every fill up though.

No non-return valve fitted as it needs to expel air when filling and draw air in when running.

 

[oldsaltoz]

Most have the breather located along the centreline of the vessel and much higher than the tank, some also have a none return valve to prevent an outflow.

Transom if tanks are very low or up one side of the companionway. Others have a simple non return valve on a stalk in the centre of the tank, again with a non return valve of the floating type.

 

[PaulGooch]

For the purposes of the OP's question, it is irrelevant whether or not the overflow doubles up as the breather, or not. The point is, there will be a breather and that breather will allow air into the tank, it's the purpose of the breather.

To minimise the air, hence moisture, that gets into the tank, keep the tank full. At the end of any trip out, fill it right to the top. Same thing obviously applies to any layup periods.

I'd also suggest that the OP uses a suitable additive on each fillup.

 

[oldsaltoz]

For the purposes of the OP's question, it is irrelevant whether or not the overflow doubles up as the breather, or not. The point is, there will be a breather and that breather will allow air into the tank, it's the purpose of the breather.

To minimise the air, hence moisture, that gets into the tank, keep the tank full. At the end of any trip out, fill it right to the top. Same thing obviously applies to any layup periods.

I'd also suggest that the OP uses a suitable additive on each fillup.

I beg to differ, the OP has fuel running out creating a mess inside or pollution outside. By relocating the breather/overflow the problem of an open ended pipe direct to the fuel supply storage is avoided.

 

[DAKA]

The breather/overflow will have a goose neck or loop in the pipe that is higher than the tank.

Diesel will sit in the pipe and act the same as your water trap to stop smells under your sink at home.

As such there will not be any free flow of winter air (nominal amount of temperature change expansion will bubble through the goose neck).

The most likely cause of water ingress is rain water/wash down water through the deck filler.
Replace the O ring and place carpet tape over the filler during the layup period, petroleum jelly on the O ring will also help seal and make it easy to unscrew in spring.

If you havent got a loop it is very easy to add one.

 

[PaulGooch]

I beg to differ, the OP has fuel running out creating a mess inside or pollution outside. By relocating the breather/overflow the problem of an open ended pipe direct to the fuel supply storage is avoided.

The OP's question related to air/moisture being allowed access to the tank and promoting bug growth. There has to be a vent/breather to allow air into the tank, so it doesn't matter where he move/re-routes/re-designs the breather system, his perceived problem of air in the tank will be unchanged.

The issue of fuel spillage is a different matter and was not his question. I'd agree with you though, if the vent is allowing fuel to spill somewhere before the tank is full, it is badly designed. It really should be designed so that the tanks can be fully filled without any fuel escaping. It may be that his system is suitably designed, but just needs some care when filling. I know with my own boat that it's possible to force a small amount of fuel out through the breather if i'm using a high volume marina pump and not paying attention. The tank fills so quickly that a small spurt of fuel can escape before the fuel shoots up the filler neck. That's a different question to the one the OP asked though.

 

[PaulGooch]

The breather/overflow will have a goose neck or loop in the pipe that is higher than the tank.

Diesel will sit in the pipe and act the same as your water trap to stop smells under your sink at home.

As such there will not be any free flow of winter air .

The most likely cause of water ingress is rain water/wash down water through the deck filler.
Replace the O ring and place carpet tape over the filler during the layup period, petroleum jelly on the O ring will also help seal and make it easy to unscrew in spring.

Sorry, but that's not all correct. If the boat is fitted with an overflow that works as you describe, there must also be a separate breather that is open to the air. I'm obviously not familiar with every tank setup on every boat. There may be systems that use an overflow how you describe, stopping air getting back in. There may be systems where the breather has a one way valve to stop fuel escaping.

But, in order for the fuel system to work, air must be allowed back into the tank as the fuel is used.

 

[David_J]

The breather/overflow will have a goose neck or loop in the pipe that is higher than the tank.

Diesel will sit in the pipe and act the same as your water trap to stop smells under your sink at home.

As such there will not be any free flow of winter air (nominal amount of temperature change expansion will bubble through the goose neck).

The most likely cause of water ingress is rain water/wash down water through the deck filler.
Replace the O ring and place carpet tape over the filler during the layup period, petroleum jelly on the O ring will also help seal and make it easy to unscrew in spring.

If you havent got a loop it is very easy to add one.

Thanks to all for comments.
My reference to the overflow was simply to demonstrate there is what may be a clear connection straight into the internal filler pipe. This overflow is through the hull at a level higher than the top of the tank. Careless re-fueling can result in a gush of fuel into the marina but its obvious from the sounds when the tank is close to full so easing off on fueling rate prevents any spill at all.
The idea of a goose neck in the breather seems totally logical. Difficult to check in my case as access is restricted but a camera on a stick may help!!
Similarly, new O ring and sealing the filler for the Winter is a great idea although both fillers are effectivel inside the canopy and under a flap so dont see direct rainfall or runoff. Still definitely worth doing though.
I may just seal the overflow on one tank and leave the other tank that feeds the heating. See if there is any discernable difference in the Spring.
I always keep the tanks full over Winter but as an experiment this time am going dose them heavily with additive.
We are doing nothing different on this boat than we did for five years on the last one and that had no bug problems.
I suspect this bug issue is a bit of an unknown from one case to the next. It may be water in the fuel (although last fill at home port we used a water seperator to check the fill up for contamination, took for ever, no water!) it could be water getting in later, it could be remnants of the last contamination hiding in a corner somewhere or it could be the spawn of the extra terestrial.
Thanks again to all.

 

[DAKA]

Sorry, but that's not all correct. If the boat is fitted with an overflow that works as you describe, there must also be a separate breather that is open to the air. I'm obviously not familiar with every tank setup on every boat. There may be systems that use an overflow how you describe, stopping air getting back in. There may be systems where the breather has a one way valve to stop fuel escaping.

But, in order for the fuel system to work, air must be allowed back into the tank as the fuel is used.

I am sure it is not an overflow.
It is a breather that will overflow occasionaly as the air is forced through the goose neck/loop diesel trap too fast.

I didnt mean to suggest the air flow is not allowed, I meant the free flow of air is stopped.

Air can still pass either way but has to bubble through the diesel trap, this will reduce any condensation issues.