vent tube on permanent fuel tank question

[bob johnson]

how far INTO the fuel tank does the vent tube NEED to go or SHOULD go?

I have seen most , are just open tubes...on one tank I cut open it was welded into the tank on one end and ran the full length of the tank on the inside!!! up against the top of the tank. I have seen other tanks where is is little more than a hole in the top of the tank...with NO protrusion INTO the tank...

I have also see new vent tubes being sold that screw into a fitting on the top of the tank and protrude DOWN several inches..wonder how that can work.... I am THINKING the vent tub should be the highest part of the tank or the spot that fills last.....


bob.

 

[Chris1956]

Vent should not go into the tank at all. If it went into the tank, fuel would splash out.

 

[gm280]

Vent tubes are mere openings to allow air to come in as fuel is removed out. And the vent has to be at least the same volume flow type opening as the fuel volume leaving or differences in air flow to fuel flow can happen. That doesn't mean the vent needs to be the same diameter as the outlet. Only the the same amount of air flow has to be happening. If you look at most lawn mowers, you will see only a pin size hole in the caps to allow proper flow. And that is because, while the fuel outlet is much larger in physical size, the actual fuel flow isn't. So it flows without hindrance. So a vent opening in a fuel tank really needs to be at the top of the tank and not under the fuel. level. JMHO

 

[bob johnson]

well I have seen commercially made vents that stick INTO the tank..thats whay I asked...I guess I was thinking the vent was so when you FILLED the tank the air had a way to get pushed out!!!...wasn't thinking of the vacuum effect of fuel leaving.....my vent hose will travel all the way up to the fill opening as it is....so there is little chance fuel will splash out...plus now days they want a canister filter for carbon, I think...on the vent....

on another note: I was looking at one of those reed valve fuel senders....but I have a tank that is only 5.75" deep.... and they make 4", 4.5" , 5" , 5.5" and 6" and larger senders. I asked one seller what is that dimension?. his reply was that is from the underside of the flange to the bottom of the float...but the manufacturer recommends 1" of space UNDER the unit???.... I thought : that would mean the guage would read empty, when in fact my tank has 1" of fuel in it...and at 96" long and about 2-3" wide down the the bottom V....that would be significant amount of fuel still left

bob

 

[GA_Boater]

The vent should have little protrusion in the tank in a short tank. The fill tube needs to be lower than the top of the tank and vent tube to create expansion space.

The long vent tube is for long tanks like you're building so only air/vapors are vented when the fuel expands from heat and to have a clear air space so the fuel isn't aerated by the vent air drawn into the tank as the fuel sloshes. As with the short tank. the fill tube needs to be long enough inside for creating the expansion space.

 

[GA_Boater]

When the vent sticks into the tank, the fill tube is still longer.

Do you plan on running to E before adding fuel? Call the remaining fuel on empty the reserve.

 

[bob johnson]

The vent should have little protrusion in the tank in a short tank. The fill tube needs to be lower than the top of the tank and vent tube to create expansion space.

The long vent tube is for long tanks like you're building so only air/vapors are vented when the fuel expands from heat and to have a clear air space so the fuel isn't aerated by the vent air drawn into the tank as the fuel sloshes. As with the short tank. the fill tube needs to be long enough inside for creating the expansion space.

so I am a little confused.......the tank that I cut open , that had the LONG vent tube inside running almost the whole length of the tank..... was on a 52 gallon tank that was only 23 " wide and about 9" deep.....it might have been 48".....but if the tank was full....the whole tube is in gas....how does that add expansion space?? and if the fuel is below the opening of the vent.... then the long interior vent adds zero extra expansion that the airspace in the tank doesn't already offer!!

besides that...I have looked at 100's of tanks the last few weeks online...any that were open to see the inside, didn't have long expansion tubes..

the fact I have a vent that runs to the atmosphere....isn't that enough expansion capacity??? seems the worst scenario is a full tank that warms up and the fuel expands, and fuel starts to travel up the vent hose ....I guess is would do the same up the fill tube.... with a foot or more drop from the opening of both those ...to the tank.....that's a lot of expansion room.......

truly I am trying to learn the whys..... but it seems all the people who build tanks and post pictures...do it several different ways

bob

 

[gm280]

bob j, yes fuel expansion does go up the vent tubes, and if there is a way for air to escape, even up the fuel inlet tube as well. Some boaters have real problem with their tank vent lines having gas coming out as well when full. So it is better to stop short of a totally full tank for that reason. The fact that your vent tube runs the entire length of the tank allows for fuel expansion before coming out. But it does the exact same thing as any vent tube.

 

[Fed]

In a perfect world they would all be vented from the top at both the front & back of the tank with no protrusions.
You need to consider the attitude of the boat & the effect it has on the fuel level when thinking about this.

 

[GA_Boater]

A long vent tube will be perforated along it's length or it will be open on both ends. Time for a pitcher or two - Don't laugh.

No tank is filled completely. The one below shows the red filler is deeper than the dark blue vent. As the tank is filled with gas, when the level reaches the bottom of the fill tube, the gas nozzle will click off because the tube will fill faster than the vent can relieve the pressure. The long tank example is the same when the tank is level.



But what happens as gas sloshes back and forth or the boat isn't level when refueling? The long vent tube relieves pressure during filling, when the boat is parked when gas expands and during operation giving a path for vent air.

A short vent tube like the top drawing above will spew gas if the tube is covered and it is more susceptible in a long shallow tank like you need. A long perforated or open on both ends vent tube eliminates spewing and ensures good venting by always having a vent into the free air space. With a long shallow tank like you need it becomes really important.





Hope this helps some.

 

[bob johnson]

I am trying to consider the attitude of the boat.....for the most part when under power, and especially so at sower speeds...the front of the tank will be much higher than the back of the tank. there for the pick up has to be at the stern.... and I think the sender needs to be towards the stern...at least in the back 1/4. Hose is cheap, and I can weld anything onto the tank...so two vents can easily be done..i can even do the long internal vent tube...but I wasn't sure it purpose.

I plan to set the boat up on the trailer so that when on level ground the bow is slightly higher than the stern....so water drains...and I don't end up with a 800 lb tounge when I go to hook up the boat.

so should I do twp vents or the long vent inside... and which direction should I run it inside...front to back..or back to front?


I ran into another issue..i just found out I don't have room between the top of the tank and the bottom of the floor for a welded on fitting for a 3/8th NPT fitting port( a 3/4" block of aluminum barely accepts the 3/8th npt female thread.. I don't have 3/4"!!!!

I am thinking of having the pickup tube exit the tank at the very top of the back of the tank!! since the bottom of the pick up tube is at the bottom of the tank...sloshing should put undo force on the gas to run out the gas line towards the motor...

the filler can be up front as well as a vent because the front of the tank is under the front deck of the boat!!!


bob

 

[GA_Boater]

Level the boat on the trailer for better towing. Use the jack to raise the bow when parked.

If you go with two vents, tie them into a single external vent. It gives you the same as a long, internal vent tube. If you do go with the internal tube, run it front to back, about as long as the tank.

 

[bob johnson]

ok so I am looking at venting the hose...from 2 sources, and am looking at hardware....but what I have found is that every vent I have found that goes through the hull ..has a OD for 5/8' ID or larger hose!!!..... I cant seem to find a fitting for a barb with a 5/8th OD, with a 1/4 npt or even a 3/8th npt on the other end....

I have machined a recess block that I will weld onto the tank that will take my pick up and one vent.... I don't think the 1" or larger OD of standard 5/8th is hose will fit easily into the recess I have made.....I was going to go with 3/8th ID hose, just like my fuel line......thinking its just a vent......but I cant find a hull vent fitting with a bard that small


bob