underfloor fuel tank question?

[Nathanrb4]

one of my mates took me out on his boat last weekend and it had a lift up panel in the floor that he kept his fuel tanks (25L plastic type) under and out of the way. it was a good idea for keeping them out of the way and freeing up floor space but i`ve always thought this wasn`t a good idea because it could let fumes build up under the floor and then become a hazard. he`s had it like that since he brought it 5 years ago and never had any trouble with it. just wondering if i`m right in thinking this? or is this something i could safely use on my boat?

[NQCairns]

The proper way is to build a sealed box that the tank sits into, that way fumes can only escape above deck and no leak will find a way into the bilge or other portions of the hull before it is noticed (hopefully).
I have a 25 litre tote tank as my second tank under the front casting deck in the tinney, it sits on the V keel with nothing from there to the bilge -not a good idea but it happens all the time.nq

[deb]

NQ absolutely spot on! Look back you will find the problems I just went through with a poly tank, leaking on a fibreglass! There are photos in the post.
The long and the short of that episode. The manufacturer replaced and installed the poly tank free. BUT! When it was installed, the dealer pointed out, that the tank needs expansion room, we are in Qld, the tank swells a long way in the heat, not to leave it with more than a quarter in it....

I'm saving up now, for a complete inbuilt ssteel or aluminium replacement.

When they are sealed under the floor, unless the unit they are sitting in is fully sealed (this one wasn't!) they do leak through the bilge, eat away any foam filling, ultimately leak eat out through the bung. (Our boat is under cover all the time, rarely undo the bung!)

Lesson in new boats well learned for us ;)

[dnej]

I have mine under floor, or rather part under part out, with a carpeted covered box over the top. Goes part under the transom, and part into the floor area.

I had an aluminium drip tray made up with 4 mm plate. It is 50 mm high, suspended from the floor with 50x 50 aluminium angle.
The plastic tank sits on the tray. All is removable, for cleaning purposes
Regards david