Fuel tank vent line size

[model14]

I am currently building a boat and I have installed a below deck fuel tank. The tank was originally used for diesel, so it has a standard 5/8" vent fitting and a 3/8" return line fitting. My problem is that the 5/8" fitting is vertical and wont clear under the deck with a hose installed. The 3/8" fitting clears easily. I know the 5/8" vent is standard, but if I can safely go with 3/8" it will solve a major issue. The tank is 36 gallon and the motor is a Johnson 90 HP outboard.
​Advice would be appreciated.

 

[alldodge]

I would say it depends on if the return line empties close to the top of the tank or if it goes closer to the bottom. If on top, good, on bottom not so good

 

[Maclin]

On the 5/8", is there any way an elbow fitting can replace that or be adapted.

 

[model14]

I have to figure out a way to tell where the return line terminates without putting fluid in the tank. It is a 90 deg non-removable elbow at the tank top. I will call a tank manufacturer and find out where they normally terminate a diesel fuel return line.
​The only elbow which might work would be a 90 deg hose piece. There is no room for brass fittings. I will research that.

 

[model14]

Just got off the phone with a Moeller technician. Diesel return lines terminate at the top of the tank, so it should work as a vent.. Now is 3/8" a big enough ID for venting the tank? Logic would say yes, especially with only 90 HP versus a 300 HP diesel. Any comments on this aspect would be appreciated.

 

[alldodge]

My opinion is that is plenty.

 

[ajgraz]

Page 34 of this document suggests that 3/8" vent may be too small to prevent pressure build-up inside the tank while filling:
http://www.uscgboating.org/assets/1/AssetManager/ABYC.1002.01.pdf

 

[Bondo]

3/8" a big enough ID for venting the tank? Logic would say yes, especially with only 90 HP versus a 300 HP diesel.

Ayuh,.... The Problem ain't with the motor suckin' fuel outa the tank,....

It'll be when yer tryin' to fill the tank at the gas pump,.....

 

[model14]

Duh, didn't think of that. Back to the drawing board.