Issue with fuel tank/s

[Todd j]

I didn?t consider every possibility outcome of my fuel tank design.   Who really can, our conditions vary so much.  I have been fighting fuel expansion in my 120 gallon tank.  The vents are doing their job, but that is puking fuel on my sheer deck.  Even with a short fill and the cool temps.  I now, keep the boat outside in the driveway.  I have the fill and vent at the aft end of the tank, near the vent and fill of my 50 gallon auxiliary tank.  I keep the bow up a bunch to aid in deck drainage.  I have decided that this pushes the air forward in the tank.  I?m guessing as the fuel in the tank expands this cushion of air is pushing the fuel up and out of the tank.  Not only does this make an expensive mess it has begun to compromise the paint around the vent thru hull.  today I lowered the tongue jack about 8?.  Another gush of fuel from the vent and a wash job and the drive is clean again.  I?m hoping with the jack low the length of the tank will now have an air space on top of the tank?s length. 
   Just some food for thought.  Do consider your vent location where and how you plan to store your boat.

[Brian.Dixon]

Is it too late to add another vent line on the opposite end of the tank?  Depressurize the trapped air?

[Todd j]

That ship has sailed.  You like what I did there 

[Lindy]

I had a similar problem, my vent was fwd and fill aft, when fuel expanded fuel came out of my vent. I was able to put a T in my fill line and place another vent,  thus allow it to vent from the Aft end if needed. So far so good. Wonder if all future builders should consider vents at both end of the belly tanks. Wondering if loosing your fill would reduce the tank pressure or just make a bigger tube for fuel to spew out?

[Brian.Dixon]

Yeah ... belly tank venting/filling can be a pain, but it's the only way to get a LOT of fuel into the boat without encroaching on the human space ...

[Todd j]

I had a similar problem, my vent was fwd and fill aft, when fuel expanded fuel came out of my vent. I was able to put a T in my fill line and place another vent,  thus allow it to vent from the Aft end if needed. So far so good. Wonder if all future builders should consider vents at both end of the belly tanks. Wondering if loosing your fill would reduce the tank pressure or just make a bigger tube for fuel to spew out?

I think vents at both ends of belly tanks is the way to go.  Wish I would have thought of that.  Loosening the fill cap this time of year for me is a no go. I could possibly put a catch tank with a tee from the existing vent line, but that would take constant monitoring. to empty
  Brian, perhaps the dual vent for belly tanks could make it into the next rev. 

[cj8mule]

Was just at the machine shop placing an order for fuel tanks.  You're recommending putting a 5/8" vent tube on both ends?  Then connecting them with a T fitting to run a single line to vent.  I've had good luck with the vent/gas fill type and more problems with the separated vent.

 

[Todd j]

I?m thinking 2 independent vents.  One at each end.  My fills are threaded with o-Ring seals.   With bow up in my case fuel in the vent and the fill are pressurized.   I don?t think a combo vent fill would work any better in my case. 
  My .02 for what it?s worth

[Rbob]

Todd,

Another 2c for what its worth.  My previous boat had fill and vent at the aft of the tank, it would spit out when filling, it was a combo fuel fill / vent.

It did not leak fuel once full, I would purposely make sure the boat was bow up when filling so I could get more fuel in.

My GA has a moeller plastic fuel tank (not that it matters) fuel fill is 2/3 to the front of the tank and vent is 3" from the front.  I installed a fuel fill/vent combo.  It works flawlessly.

It may be an easy swap of the fuel fill, you could even "tee" into your old vent.  Tank vent hose to fuel fill, put Tee inline and connect to your installed vent (or not) just make sure the vent line does not have a belly. 

[Brian.Dixon]

I think vents at both ends of belly tanks is the way to go.  Wish I would have thought of that.  Loosening the fill cap this time of year for me is a no go. I could possibly put a catch tank with a tee from the existing vent line, but that would take constant monitoring. to empty
  Brian, perhaps the dual vent for belly tanks could make it into the next rev.

I agree, Todd.  I've noted it ... go with a vent on each end of the belly tanks.  You might get away with only a forward vent and fill on the aft belly tank, being shorter and deeper, but definitely a vent on each end of the forward belly tank.  If you fill the stern tank first, then you can use a forward fill for the forward tank?

As for separate vents or tying them together with T's, I think I'd vote for separate vent lines for each vent (there might be an ABYC or USCG rule on that, FYI).  Why? I'd rather have no connections in the line between the tank and the vent, and using a T adds a lot of length to the vent lines (tank to T ... twice .. then T to vent on hull).  Just my 2-bits.  I'd look into the ABYC in particular since the ABYC rules (they're guidelines, not law) were ALL written to address safety issues that actually did and do occur in boats.

[Djeffrey]

Fortunately my tank builder was smarter then I am, he put a vent on both ends. Never had an issue.

[Todd j]

we have had a very minute warm weather event relative to the weather we have  been having.  I was able to revisit this fuel expansion problem once again.  This time I put a level in the house and leveled the boat relative to Earth.  With my aft sloping cockpit deck, I'm hoping maybe I will never have this problem again.  Sure wish I had vents at each end of the tank!

[Brian.Dixon]

I kind of like the idea of having the fill at 2/3rds f'w'd and a vent on the f'w'd end ... Moeller's built a LOT of tanks and should know what they're doing.  Another idea that helps if filling while the boat's on the trailer is to adjust the bunks so the boat's f'w'd end is higher than the stern end ... kind of hedge your bets against a truck that sits a little low in the stern when hauling the boat.

I like independent vents on the ends too.