Soft fuel tank..?

The hatches in my cat are so small I can't get a fuel (unleaded gasoline with outboard oil) tank inside.
I found a 20l black "polyvinyl" bag cheap. It is used for sun-heating water for shower.
My question is whether there are any issues with using the material for fuel? Will it dissolve or anything? What is the worst that can happen? I will use a "pre-filter".

 

The worst that can happen is that the bag bleeds fumes causing the death of you and everyone aboard in the resulting explosion and fire ......

There are approved bladders for fuel. Get an approved one like these rather than risk a fire or explosion.:

http://www.cyber-bridge-marine.com/marine-bladder-fuel-tanks-auxiliary-tank.html

http://www.atlinc.com/US/marine.html

This is not a place to go "cheap".

 

The shower-water bag will most likely not hold up to fuel and oil. It would be almost certain to leak, deteriorate, and eventually cause a massive fuel spill and probably a fire.
If you need a bladder-type fuel cell, get one that's designed for gasoline, designed for use in an enclosed space on board a boat, and certified by the appropriate authorities. Go cheap here, and you could blow up the marina.

 

I echo what both Jehardiman and marshmat said. A fire on your boat can spoil your whole day. It's not worth doing this on the cheap. I have investigated far too many fires on boats that were simply a matter of people not doing it right.

 

I think that is going to spoil more than just his day if he is anywhere near it at the time.....

 

Thanks a lot. I will use the hard tank on deck until I can afford a proper soft tank.