Fuel Tank Vent Solutions

So there I was at the fuel dock with the temperature at minus 7 celsius, a 20 knot north wind, and the vent lines to the fuel tanks were plugged.

Brrrrr went to Grrrrr pretty fast.

Tried to clear the vents with some wire but it must have been fine particles of snow that got blown into the vent holes, then settled at the bottom of a U shape in the lines.

Any tips or gadgets for fueling up when the vent lines are plugged?

For a short term MacGyver solution I was thinking of snaking a small diameter hose down the fill pipe with something at the end which hooks onto the edge at the top of the tank (bottom of the fill pipe). Would have a string attached so as not to lose it and to maintain its position at the top of the tank. Plausible?

There must be a gizmo/solution out there...

Shop vac on the vents (outside against the hull) after you open the fuel fills?

Larry M said:

Shop vac on the vents (outside against the hull) after you open the fuel fills?

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Good one. Your answer sucks, but in a good way

I'm not sure if I can describe this so it will make sense but you can take some soft copper tube of say 3/16 ID (or bigger if you have a large fill opening) and 18" - 24" long (or longer if your fuel fill hose goes straight to the tank with no bends) and then bend one end of it a coil that's a bit bigger in diameter than the hole in your fuel fill and over lap the coil about half a coil or so. Then bend the coil 90 deg to the unbent length of the tube. After that bend the over lap portion 90 deg to the coil so it is in basically the same orientation as the unbent length of tubing but pointing the opposite direction.

When you go to fill your fuel tank stick the long end of the breather tube you just made into the fill. The coil will let it sit around the opening. Then stick the fuel nozzle down the fill thru the coil and start filling. Air should vent out of the copper tube.

The short version of this is normally used to eliminate the fuel burping problem some boats have doing fueling by letting air get passed the nozzle. So I think could cure your blocked vent issue of you don't fill to fast.

Murray do you have a photo of the outboard end of the vent line? Maybe something like a clamshell cover would help stop the snow from getting in......

Fuel tank vents can be a PITA. I dont have any good ideas about how to fix your dilemma but I do sympathize. My nemesis was always "mud dobbers" spiders a close second. Screens helped with that but wouldnt do anything in your circumstance. However, on my trawler I decided to do something about it. The fuel fills are in the cockpit, in a recessed, covered "box" of sorts. The vents both come to that "box" and puke fuel there when full. It drains overboard thru a hose to a thru hull fitting if needed. Its very easy to know when the tanks are getting full as the sound changes, and you know exactly when there full because your looking at the vent. Thats not so easy to do when they're on the hull side. I REALLY hate pukin diesel in the water at the fuel dock. That and hull side vents are just a huge PITA.

Capt.Bill11 said:

I'm not sure if I can describe this so it will make sense...

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Made perfect sense, thanks!

Larry M said:

Shop vac on the vents (outside against the hull) after you open the fuel fills?

Click to expand...

Just make sure it's diesel fuel before you try this one!

I vent my diesel tanks on the inside, I'm sure many of you purists will be horrified, but it works well for me.
I run the line as high as practical under the gunnel, then back down a few inches, and mount a catch can under it with an oil absorber pad.
Remember, a "visible spill" can cost you $10,000, and it doesn't take half a teaspoon to create quite a rainbow.

Most displaced air exits the fill pipe as the tank is filled, not a tiny vent.

The vents come into play when the fuel fill is sealed air tight and fuel is removed from the tank.

Why does a tiny blocked vent not allow the fuel tank to be filled on your vessel?

Larry M said:

Shop vac on the vents (outside against the hull) after you open the fuel fills?

Click to expand...

I am not sure I would advocate sucking fuel fumes into a vacuum since they often have brushed motors and those would be a ready source of ignition...

rochepoint said:

Murray do you have a photo of the outboard end of the vent line? Maybe something like a clamshell cover would help stop the snow from getting in......

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No, I don't have a photo. What shows on the aft transom are small metal domes with a hole in the middle, about 1/2" in diameter and raised 1/4" from the transom surface...the holes aren't protected from horizontally driven snow, so I'll look into the clamshell idea...thanks.

FF said:

Most displaced air exits the fill pipe as the tank is filled, not a tiny vent.

The vents come into play when the fuel fill is sealed air tight and fuel is removed from the tank.

Why does a tiny blocked vent not allow the fuel tank to be filled on your vessel?

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Maybe it's because the pump hose and pump nozzle are big (guesstimate would be close to 2") and even at its slowest setting the volume of fuel is enough to "fill" the boats fill pipe?

This issue is an ongoing concern because on our summer holidays during a routine check I saw that one tank wasn't contributing and remained full while the others were lowering. After some head first upside down and twisted sideways lazarette contortions I managed to clear it.

kapnd might be onto something as an interim solution...

FF said:

Most displaced air exits the fill pipe as the tank is filled, not a tiny vent.

The vents come into play when the fuel fill is sealed air tight and fuel is removed from the tank.

Why does a tiny blocked vent not allow the fuel tank to be filled on your vessel?

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Most likely because like a lot of boats, between the the fill nozzle taking up most of the space in the fill hole and the fact that the in rushing of fuel stops or significantly slows any air that is trying to come up the fill tube. And/or air is being sucked into the tank with the in rushing fuel. So,you end up with blow by and burping without an open vent. And if the nozzle is of the type that will shut off when it senses back pressure you can end up having to fill very slowly.

In the spirit of PITA problems the logical solution...15* latitude further South. The copper tube idea sounds the best and could stay in place if necessary.

FF said:

Most displaced air exits the fill pipe as the tank is filled, not a tiny vent.

The vents come into play when the fuel fill is sealed air tight and fuel is removed from the tank.

Why does a tiny blocked vent not allow the fuel tank to be filled on your vessel?

Click to expand...

In my case, the fuel filler and hose are a little smaller than ideal and the fuel nozzle fills most of the diameter. I have trouble with all but slow speed pumps in any conditions. I can't just set the nozzle into the fill, I have to hold it up a bit and try to control the speed of the fuel with the handle. Not fun or easy.

For the OP, if your vent lines have a dip and water freezes in the dip (water and fuel is a bad situation for other reasons), the solution is to get rid of the dip. Dress the vent hoses s0o there's dip. They should be uphill all the way.

IF the FILL line has a dip it would cause the slow fill big burp hassle.

WesK said:

For the OP, if your vent lines have a dip and water freezes in the dip (water and fuel is a bad situation for other reasons), the solution is to get rid of the dip. Dress the vent hoses s0o there's dip. They should be uphill all the way.

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There is a dip (U shape) in each vent line and will take a close look at them next time at the boat...just the thought of fine particles of snow blowing in, melting, then freezing a plug in the vent lines which could stop fuel from reaching the engine give me the heebie jeebie's

MurrayM said:

There is a dip (U shape) in each vent line and will take a close look at them next time at the boat...just the thought of fine particles of snow blowing in, melting, then freezing a plug in the vent lines which could stop fuel from reaching the engine give me the heebie jeebie's

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It should be very difficult for snow to blow into your fuel vent line. There should be a cover makes the opening face down and aft and snow would have a hard time making a change in direction to go up and then into the vent line.

My guess is that it is condensation that has blocked your fuel line. Water vapor that tried to escape through the vent but condensed when it couldn't get past the loop.

If you configure the vent line so it's as short as possible and uphill all the way, I'll bet that will take care of any blocked vent problems.


You might be able to take the vent line above the fitting and back down to the fitting (an upside down "U"). This would make it much harder for water or snow to actually get into the tank.

FF said:

IF the FILL line has a dip it would cause the slow fill big burp hassle.

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Yes, that's possible. Mine are short with an "S" curve but no dip so that's not my problem. I think it's a 1 1/4" nozzle in a 1 1/2" hole that's making it difficult to fuel up without blow back.

WesK said:

It should be very difficult for snow to blow into your fuel vent line. There should be a cover makes the opening face down and aft and snow would have a hard time making a change in direction to go up and then into the vent line.

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Should be, but there isn't...see photo below...will cover with clamshell.

WesK said:

My guess is that it is condensation that has blocked your fuel line.

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That's it, I think. The Bonmar hatch has a small leak which allowed some rain to get into the lazarette. The temperature is low enough that there's a layer of frost on the inside of the hull (only on the hull, not on any tanks, steering gear, etc) between the waterline and the bottom of the deck.

Temperatures are supposed to get above freezing in a week or so...will dig into things then.

Thanks for your thoughts everyone

Change the covers. Look at other boats, then head for West Marine.

WesK said:

Change the covers. Look at other boats, then head for West Marine.

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Good plan, except it's a 16 hour drive.

The dip is just wrong, they need to have an inverted U above the vent so that they can never fill whether it is with fuel or condensation or splash, snow, etc.
Most fill problems are result of poor planning that creates long flat runs and sharp bends, and as previously mentioned, too small ID to acommodate flow.
It is difficult or impossible to moderate the flow from modern pump nozzles.