Under floor tank breathers

[Big Munga]

Hi guys, I'm going to fit an under floor tank in my Top Ender, I've seen some boats with the breather hoses venting under the floor, I wasn't sure if this was a good idea to have fumes building up in a confined space? Would I be better to have them venting up under the gunwales?

[noelm]

Yes, vent to the outside, make sure the vent line has a "U" bend at the top to prevent water getting into the line, don't use a complete loop.

[Big Munga]

3 hours ago, noelm said:

Yes, vent to the outside, make sure the vent line has a "U" bend at the top to prevent water getting into the line, don't use a complete loop.

Thanks mate, that's what I thought. I was however thinking of having the vent on the inside of the hull but tucked up under the side deck?

[noelm]

You could do that, but it's only partially vented "outside" you will get fuel smell on warm days, most wont care, but now and then, a deckie who is feeling right on the edge of sick, it might just tip him over!

Edited March 22, 2022 by noelm

[zmk1962]

In addition to the considerations Noel has highlighted, also remember that fuel vapour is much heavier than air and will settle and collect in lower sections of the hull. I’d prefer to vent outside the hull especially if there is any enclosure. Open runabouts on the other hand by nature of design are well ventilated and can get away with breathers in the deck area…. But again I’m not a fan. 
cheers Zoran
 

[Smobaby]

Best option would be to run the breathers out in the motor well where all the other fumes are.

[pegz]

I have mine venting in the motor well

[campr]

You may have this already, but I believe that a good water separating fuel filter (with bowl and tap) is mandatory with underfloor tanks.  I have never had any issues myself but I like to keep my tank full to minimise condensation.   Ron 

[slowjigger]

On 3/23/2022 at 7:50 PM, campr said:

You may have this already, but I believe that a good water separating fuel filter (with bowl and tap) is mandatory with underfloor tanks.  I have never had any issues myself but I like to keep my tank full to minimise condensation.   Ron 

Condensation is a myth according to some (and I tend to believe it). You can get water through the breathers if you don't have a trap in the line. It's possible to pick it up from the service station too. A poorly fitting filler cap is another potential source. 

Edited March 25, 2022 by slowjigger

[slowjigger]

Another point is to have the vent line going at least slightly up all the way (apart from the water trap). If you have any low points they can fill with fuel and the tank won't breath properly.

I would vent it outside of the boat, no point risking any vapour build up and explosion. Also the vents should be angled so the open part is pointed towards the stern to minimise water pickup.

Edited March 23, 2022 by slowjigger

[Big Munga]

Thanks for all the responses guys, much appreciated. 

I will have a water separation filter on the line from the under floor tank to the outboard, what trap would you suggest to put on the breather line? 

I have two 16mm breather outlets on the tank so two lines to deal with, maybe I should have only put one in......I can always cap one I guess.

[wrxhoon1]

Vent it outside, on a small boat at the rear on the outboard well is best . As Noelm suggested bend the vent hose in upside down U so water can't go in the tank.