J/46 main fuel tank venting issue

[David Jade]

On our J/46, it’s taken a while, but I now understand that our main fuel tank has an issue. When filling the tank, air gets trapped because the vent line is in the aft end of the tank and that gets submerged before the tank is completely full. It seem that on our J/46, that end of the tank is lower than the forward end, leading to trapped air. Adding more fuel that then compresses any trapped air.

 

This a concern to me because that trapped air gets compressed when filling the tank – enough that the tank gets pressurize. I now know this because I can see a bit of a bulge after topping up and if I loosen an inspect plate screw compressed air comes out – quite a lot. Air also flows out if I open the gravity feed second tank valve which is at the forward end of the tank – you can hear the air flowing back into the secondary tank. This seems like not a good thing long term. Last thing I want to happen is to break a tank weld or something from over pressurizing it.

 

Has anyone else ever experienced this and if so, how did you solve it?

 

I can’t figure out why the vent end of the tank is lower than the forward end but I suspect it’s maybe always been this way. Perhaps we are sitting low in the stern (but not due to loading/weight – we’ve tried shifting weight as a solution – this happens even with empty stern lockers). Our waterline/boot stripe looks pretty even to me as well. So other than putting in another vent line at the forward end of the main tank, I’m not sure how else we could “fix” this issue.

 

David             _/)

~~~~~~~~~~~~~~~~~~~~~

 

 

[Todd Stevens]

I'm looking at a similar situation right now, but not sure it will be a problem. I'm installing a new auxiliary tank in my J/42. It looks like the aft end is going to sit higher than the forward end, unless I wedge it up, leaving a larger volume of the sail locker unusable, beneath the tank.  The original 1995 design had both the fill and the vent near the forward end, and I can see how that would trap air in the aft end.  According to the plans from the OEM, the design was revised in 2002 to move the vent all the way forward (low end) and the fill all the way to the aft end.  So it shouldn't trap any air, but a few inches of fuel will rise up into the vent tube.  

So apparently people experienced problems with the original design and they changed it toward the end of the run.  

Otherwise, it seems like the options are to A. wedge up the end of the tank that is too low or B. install a new vent at the high point, which seems like it might take a lot of effort for small gain.  

FWIW, Original design, photo by Galen Todd

New design as received (at last) from OEM last week.

Well, I guess there will still be a small wedge of trapped air between the fill neck and the end of the tank.  

Todd

[Dennis Boyd]

All, 

We uncovered our second tank yesterday, it is under a bench/shelf in large port locker.   Tank has support glued on.     It looks almost idential to the top photo, since it's fresh on my phone, here is what it looks like.  I am not aware of an issue filling, but I've only topped it off once.   But I am reading Bill's books (3) , and I seen no mention of issues with tanks.   Only the price of fuel in some ports.    Fuel gauge (s) were disconnected by Bill.    I do not know why.   On my list of things to figure out. 

Dennis

Jaywalker, J 42 #6

[Al Goethe]

I’m putting my chemical Engineering hat on for a second with this observation: the only pressure that you can put on the tank is hydraulic pressure stemming from the fact that you filled the tank above ‘full’ such that there’s diesel in the fill line. The liquid level in the vent line will by physical necessity be identical to that level in the fill line. Whether or not you have trapped air in the tank is actually immaterial to this problem. 

Is that a problem? I want to claim: no. I would certainly recommend not filling the tank to that high a level, but even if you fill the tank all the way up to the deck fitting (by that time you’ll already have diesel coming out of the (lower situated) vent) the maximum pressure you can generate is the liquid ‘head’ generated by the overfill. That’s not much at all and I cannot imagine that it would do any harm to the metal tank. 

Even if you let the tank sit over-full and it heats up, the pressure will not increase, because diesel will be expelled from the vent (messy!) to maintain the same liquid head on the tank and therefore the same tank pressure. That diesel leak to me is the biggest downside of overfilling the tank. Of course, all bets are off if the tank vent is plugged or such!

Also consider, that the bottom seams of the tank routinely have hydraulic pressure on them from the weight of the diesel in the tank. Let’s say that the over-fill doubles the pressure on the bottom seams. I’m claiming - without evidence - that the manufacturer of the tank will not have made this pressure the design limiting aspect of those low welds. Instead, it will be the structural integrity of the tank in an -at times wild - offshore environment. 

Of course, as soon as you’ve run the engine for a short while this entire issue disappears, because the diesel in the fill and vent lines is the first to go, and then there’s no more hydraulic pressure. 

Al Goethe

On Nov 26, 2025, at 13:54, 'Todd Stevens' via J/4X Owner's Group <j4x-owne...@googlegroups.com> wrote:

I'm looking at a similar situation right now, but not sure it will be a problem. I'm installing a new auxiliary tank in my J/42. It looks like the aft end is going to sit higher than the forward end, unless I wedge it up, leaving a larger volume of the sail locker unusable, beneath the tank.  The original 1995 design had both the fill and the vent near the forward end, and I can see how that would trap air in the aft end.  According to the plans from the OEM, the design was revised in 2002 to move the vent all the way forward (low end) and the fill all the way to the aft end.  So it shouldn't trap any air, but a few inches of fuel will rise up into the vent tube.  

So apparently people experienced problems with the original design and they changed it toward the end of the run.  

Otherwise, it seems like the options are to A. wedge up the end of the tank that is too low or B. install a new vent at the high point, which seems like it might take a lot of effort for small gain.  

FWIW, Original design, photo by Galen Todd

<aux fuel tank.jpg>

New design as received (at last) from OEM last week.

<IMG_0056.jpeg>

Well, I guess there will still be a small wedge of trapped air between the fill neck and the end of the tank.  

Todd

On Tuesday, November 25, 2025 at 12:19:43 PM UTC-7 David Jade wrote:

On our J/46, it’s taken a while, but I now understand that our main fuel tank has an issue. When filling the tank, air gets trapped because the vent line is in the aft end of the tank and that gets submerged before the tank is completely full. It seem that on our J/46, that end of the tank is lower than the forward end, leading to trapped air. Adding more fuel that then compresses any trapped air.

 

This a concern to me because that trapped air gets compressed when filling the tank – enough that the tank gets pressurize. I now know this because I can see a bit of a bulge after topping up and if I loosen an inspect plate screw compressed air comes out – quite a lot. Air also flows out if I open the gravity feed second tank valve which is at the forward end of the tank – you can hear the air flowing back into the secondary tank. This seems like not a good thing long term. Last thing I want to happen is to break a tank weld or something from over pressurizing it.

 

Has anyone else ever experienced this and if so, how did you solve it?

 

I can’t figure out why the vent end of the tank is lower than the forward end but I suspect it’s maybe always been this way. Perhaps we are sitting low in the stern (but not due to loading/weight – we’ve tried shifting weight as a solution – this happens even with empty stern lockers). Our waterline/boot stripe looks pretty even to me as well. So other than putting in another vent line at the forward end of the main tank, I’m not sure how else we could “fix” this issue.

 

David             _/)

~~~~~~~~~~~~~~~~~~~~~

 

 

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<IMG_0056.jpeg>

<aux fuel tank.jpg>

[David Jade]

Thanks Al. Your analysis gives me another way to think about it and perhaps alleviate some of the concern.

 

I should add though that the tank sometimes suddenly bulges during fueling – not always just a gentle expansion. We’ve actually heard it “pop” like when a piece of sheet metal flexes and snaps into a new position. It can be loud when this happens – loud enough to hear outside. It’s that sudden flexing stress that worries me the most long-term.

 

The hard part in preventing that seems to be knowing what is “full” while refilling. While refueling, the gauge will read full well before that tank is at capacity. It can usually hold anywhere from 4-6 more gallons more at that point. That’s a significant amount of fuel to leave behind if we stopped when the gauge says full - especially since we seem to already hold well below rated capacity as well. So we’ve typically filled until we can hear the fuel start to come up the fill tube (there is a pronounced change in pitch). So far this has not made it leak out the vent on the stern.

 

That reduced capacity I mention above is also why I’d like to find a way to let that trapped air out. Our tank reaches full capacity a good 8 or so gallons before the tested capacity label says it should. I know this for sure because as part of my fuel gauge replacement project, I completely pumped out all the fuel and carefully measured it going back in until the tank was as full as I could get it. It only hold 49 gallons, despite the tested capacity label claiming it holds 58 gallons. That’s fuel capacity I would really like to get back if possible (if it is this trapped air pocket that is taking up the space instead of fuel).

 

Looking at our main tank, we do have an unused fitting/plug for adding more lines at the forward end so teeing in a second vent line there may be an option.

 

David             _/)

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[David Jade]

Interesting. I wonder if they ever revised the design of the J/46 as well to have them at opposite ends like this new design. 

Since our main tank seem to have an unused fitting with a plug in it at the higher end, I may try and see if using that as a secondary vent to let that trapped air out makes any difference.

[Todd Stevens]

In case anyone was wondering, yes, the TANK will fit through the sail locker hatch, but the welded-on fill and vent tubes will not.  It’s going to require cutting away the lip of the hatch and  epoxying it back on.  AFAIK this shouldn’t have any structural consequences, just needs to make a watertight seal. 

But not today.  Spent all morning emptying the locker and carrying everything down the ladder (boat is in the yard.) And all afternoon extracting the vacu-flush system.  Boat Yoga with Nasty.  

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[Todd Stevens]

Also, the actual REASON for the design change was not given. But SOMETHING must have been troublesome enough to bother changing it.  

[Albrecht Goethe]

David,

If the worrisome ‘popping’ happens when you’re getting close to being full, ie the diesel is splashing against the top of the tank, that wouldn’t require much pressure at all, so seems consistent with my earlier thoughts. If it happens much before that, I’m thinking that might indicate that there’s some flexing of the tank happening, maybe from insufficient support?

If you’re missing 9 out of 49 gallons capacity that’s almost 10%! The tank would have to be very noticeably slanted in the boat to trap that much volume! Is it?

Al Goethe

On Nov 26, 2025, at 15:52, David Jade <da...@mutable.net> wrote:



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[David Jade]

Prior to this week, I had assumed that the bulge expansion of the tank was due to the fuel level rising and causing the tank to bulge from the head pressure of fuel in the filling line alone. If I look at the tank in it’s relaxed state, the top sags a little. What I didn’t realize until this week is that it actually bulges up a bit beyond just level and that’s the pop I hear – the sag inverts to a bulge. There is really nothing to keep it from expanding upwards because there is nothing touching the top of the tank.

 

But after loosening an inspection plate screw while the tank was in a bulged state and getting a fairly large volume of pressurized air released, I now think it is because that trapped air is getting compressed and then exerting a force on the tank as fuel starts to come up the filling line while the air vent line in the low end is submerged. Maybe that’s no more force than if fuel was pressing the top instead of air though.

 

From what I can tell, using a slope gauge, the fore-aft rise on the top of the tank is about 1-2 degrees. Over the tank’s 6ft length that could be as much as 1.5-2.5” higher at one end, I think.

 

A gallon fills about a 2” x 11” x 11” cube. With such a wide and long tank, it seems plausible that my 8 or gallons of missing capacity might be a large but 1-2” shallow pocket of trapped air.

 

At least that’s the only explanation I can think of for the discrepancy between the tested capacity (listed as 58.216 gallons on the tank label) and my measuring of how much fuel fits in the tank from an empty state.

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[Todd Stevens]

Well, FWIW I think I’ve discovered why the fill and vent tubes were welded to the inspection plate in the first place.  If you remove that, the tank will fit through the cockpit locker hatch. At least on the J 42.  With the tubes welded solidly to the tank, the only way to get it in or out is to cut something.

Todd

s/v Wild

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[Dick York]

David-

I have not had such a problem on my J/46 (hull #9).  If I fill the main tank, and if the vent were low, I would have expected to see diesel flowing out of the vent on the port transom.  I do not see vent overflow on my main tank, even though I can see diesel in the fill fitting when I pull the nozzle out. (N.b. that is not the case with the second tank, where I spurt fuel out of the vent before I hear fuel moving up in the filling hose.)  

In any case, the tank should not be pressurized, as fuel should move up the vent line and out the transom.

Perhaps your vent is clogged, or the vent hose is crimped????  

Also: Make sure you do not have the drain valve from the secondary tank open when filling tanks.  Also, make sure it is closed while running, at least until the top 55% of the main tank has been used up.  Otherwise you will be trying to force fuel returning from the engine into a potentially full main tank. But, even in that case, the fuel should spurt out the vent on the transom.....   

Dick York, J/46 #9, ARAGORN

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[David Jade]

Dick-

 

As far as I can tell, the vent line isn’t clogged – it seems to be functioning as we can sometimes hear gurgling during refueling if you put your ear close to it.

 

But somehow we still end up with air trapped which is then pressurize by the weight of adding more fuel. It is pretty evident when I remove a screw from inspection plate or open the secondary drain and hear compressed air flowing into the second tank (which may be a work-around if I know our second tank is empty enough to do that). I’m not exaggerating when I say you can hear the compressed air that is trapped when it gets released – it’s quite a lot of compressed air that makes a long whooshing sound until it is released.

 

In any case, I’ll get the vent line off again and check that air really flows Ok.

 

And thanks for the tip on the second tank. I did know this but at the same time I was really surprised to learn just how much fuel gets recirculated back to the tank when running the engine. It’s something like 10:1.

 

Thanks,

David             _/)

~~~~~~~~~~~~~~~~~~~~~

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[Dick York]

David-

Early on I did have the drain from the secondary tank open, starting with about 3/4 tank in the main tank.  The fuel return filled the main tank very fast!

I forget how I avoided a mess of diesel all over, but it did not happen.  Got lucky again, I guess.

...Dick 

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[Albrecht Goethe]

I would think that the return is ALWAYS less than the amount drawn to the engine. I grant that there’s sometimes only a small amount actually bring ‘used’, ie combusted, but I don’t see how you can increase the tank level this way. There’s always a net reduction in fuel. 

Al Goethe

On Nov 28, 2025, at 14:40, Dick York <york.r...@gmail.com> wrote:



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[smruff01]

Hello everyone,

For what it's worth, I dug up a photo of the auxiliary fuel tank on Cayenne (J42 #65, 2002).  It is located in the port cockpit lazarette, all the way at the FWD end of the compartment, up against the aft head bulkhead.  There is a plywood cover, which has been removed in this picture. This was an option when she was ordered, and it was installed at the factory.  

Cheers,

Scott

[Bill Bowers]

Al

Actually no reason the rate of recycling flow can not be greater than a smaller amount normally used (tapped off) for combustion. That way no fuel starvation when the pedal is to the bulkhead. 

Cheers

Bill Bowers

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[Albrecht Goethe]

Right. What I meant is that it’s always less than what is drawn out of the tank. By material balance that means that as long as the engine is running what goes into the tank - as return - has to be less than what’s drawn out of the tank. Therefore, I don’t see how you can overfill the tank that way. 

Al Goethe

On Nov 29, 2025, at 17:41, Bill Bowers <wfb0...@gmail.com> wrote:



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[Chris Campbell]

I think some boats may have the return set to go to the main tank regardless of which tank is being drawn from - so if you're pulling from the reserve the main tank will get fuller from the return. Personally I believe a setup where the return switches at the same time as the feed is better - that way there's no risk of overfilling anything, and you don't wind up running out of the reserve as quickly.

[douglas wilder]

Hull #2 is set up this way.   Very poor design.   When you move from the main to aux, leaving fuel behind because priming the yanmar is not fun, you end up leaving a bit behind in both tanks.  Also I have a 45 gallon main and a 15 gallon aux.   which does not add up to the 90 gallons written into the specifications.  Just another reason why J should hire a math major to help them with ensuring accuracy. 

Douglas Wilder

SV Loretta

262-844-5932

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[Albrecht Goethe]

On our J/46 (#031) the back-up tank is ONLY connected to flow into the main tank (by gravity). Fuel to the engine is always drawn from the main tank.  As such, there’s no chance to return to the wrong tank. 

Al Goethe

On Nov 30, 2025, at 06:46, Chris Campbell <camp...@ramoak.com> wrote:

I think some boats may have the return set to go to the main tank regardless of which tank is being drawn from - so if you're pulling from the reserve the main tank will get fuller from the return. Personally I believe a setup where the return switches at the same time as the feed is better - that way there's no risk of overfilling anything, and you don't wind up running out of the reserve as quickly.

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[douglas wilder]

On Loretta the aux. tank does not drain into the main, which I was told was the original design.  That would be the best arrangement, and reduce the amount left unused if one is skidish of drying out the engine.  

Douglas Wilder

SV Loretta

262-844-5932

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[David Jade]

Ours (#19) is a gravity drain but I can’t tell if it is original or changed in some way. Oddly enough, the aux drain valves are a different type than the main shutoff valves on the main tank and I can’t see a reason why. The aux tank has black plastic levers while the others are all brass levers.

 

For those that have the aux drain setup on the J/46, could someone tell me what size hose the drain line is on yours? On ours the line is labeled and is only 5/16” ID and leads to very slow draining (overnight slow – I suspected it was clogged or restricted but it is not as I just disassembled it all to check that). Maybe others were this slow too and that’s why yours got changed to a direct engine feed?

 

Btw, our aux tank leaves about 4 gallons of fuel behind when it stops draining due to the side location of the drain valve (I know because I had to empty it through the inspection port to service the line and valve). Our aux tank is labeled with a tested capacity of 34 gallons, btw. So I guess that is about 30 gallons of useable aux fuel capacity.

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[John Burnett]

It takes about 1/2 hour for our aux to drain into the main tank.

re: return flow

Mack Boring claimed up to 90% of the fuel can be returned from the engine.  They said the fuel is used to help cool parts of the engine!

cheers,

John

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[David Jade]

That’s about what I though it should take which is why I am wondering if someone replaced our drain line or valves with ones that are smaller. 5/16” ID seems pretty small for a transfer line. But maybe that is stock?

 

The last time I drained/transferred about 20 gallons from our aux tank it took many, many hours – maybe a ½ a day, at what seemed like a trickle to me. I even removed the aux fill cap in case it was a venting issue and that did not speed things up. It was so slow I suspected it must have been clogged somewhere but after taking it all apart I can’t find any restriction or clog in any part of the aux drain line or valves. No real gunk anywhere either.

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[Dick York]

Al-

Yes, the return will be less than the draw to the engine. 

The critical situation is if the reserve tank is draining down and the main tank is also being fed the return fuel.  In that case the main tank can be filled up hard. As the return fuel is being pushed by the pumps, it will have substantial pressure behind it into a full tank with more fuel waiting to come down the hose from the secondary tank.  Is this enough to damage the tank, the hoses and gaskets, the fittings?  Most likely not, but early on I did "overfill" the main tank this way and it made me tense.  Am I being a paranoid skipper? Yes.  Is being a paranoid skipper a good thing? Probably also yes.

..........Dick

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[Albrecht Goethe]

Hamburg II takes about 30 minutes to drain the aux tank. If it takes much longer there might be a plugging issue?

Al Goethe

On Dec 1, 2025, at 13:25, David Jade <da...@mutable.net> wrote:



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[David Jade]

Al-

 

What size hose is the aux tank drain on Hamberg II? Do we all have a tiny 5/16” ID hose?

 

Over the last couple of days, I:

 

• Drained the aux tank and removed the inspection plate
• Pumped out the remaining fuel from the tank
• Used an inspection camera to check the inside of the tank
• Disconnected the hose (a big pain to reach on the aux tank end) and checked for obstructions
• Used pipe cleaners to clean each valve and elbow fitting

 

Nothing pointed to any plugging issue that I could find. Nothing seemed particularly dirty or fouled either. The hose seem completely unobstructed. I know we had some bad junked-up fuel at one point (that we had cleaned out), so maybe there was once a blockage and now it’s either sucked through or dissolved.

 

Now that I have it all back together, I’ll transfer fuel again soon and see if it’s gotten any better.

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[Albrecht Goethe]

I think we have the same size hose and fittings as you, David.

Al Goethe

On Dec 1, 2025, at 20:29, David Jade <da...@mutable.net> wrote:



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