Cutting out underfloor fuel tank

[pundy14]

Hi all, 90ltr aluminium underfloor tank in a fiberglass boat. Unfortunately it's fully glassed over (no removable floor to access tank fully). There is an inspection port (where filler/breathers are attached) and a small port for the sender.

There is a strong smell of fuel fumes in the inspection port, which has taken me on quite the journey over the past 12 months, replacing/resealing sender, replacement of hoses, using camera to try and fine fuel leak, tipping boat up for 24 hours to try and see if there is liquid fuel...tried just about everything with no sign of a leak. To put it bluntly - I just don't trust the fact I smell fumes when I don't think I should do...The fumes should come from the breathers (at the back of the boat).

So I want to cut it out. Obviously I'll drain the tank, disconnect and cover the hose connections - but I am nervous about the actual cutting process.

My head tells me I use the inspection port to try and line up where I'd be cutting. Then slowly slowly does it and cut out a hole big enough to get the tank out. Once out, I'm getting a boat builder to repair any stringer damage I might make, and replace section of floor with removable section (which I should have done in the first place...).

My head also tells me I don't really know what I'm doing ...

Anyone done this before? If so, any advice? Things to do? Things not to do?

Cheers

[Noelm]

Cut a few tanks out over the years. There’s quite a bit at play here, first off, is there a decent gap between the top of the tank and the floor? If so, adjusting a saw to just cut the floor will be the way to go. Draining the tank is necessary, but……seeing as you have fumes under the floor, hitting a screw of similar might create sparks which could prove “exciting” chances are it will be OK, but……. You “might” be able to remove any inspection ports and run a fan or something through the holes? Failing that, you can just cut a decent hole, then air it out and continue cutting another day when/if all the fumes are gone.

[pundy14]

Thanks Noelm - re the fan idea. I know fumes are bad... but I could certainly look to using a fan in the floor as I cut. Do you think this would make it significantly safer?

[disorderly]

I wonder if, because you only have an inspection hatch sized opening if you were to stick in your air compressor nozzle and blast all the fumes out..

Surely its just like gas where there has to be a certain percentage of gas/air mixture before it becomes flammable so in theory if you dilute that mixture right down then the likliehood of explosion is almost zero..

So let us know how that goes for you mate...but if you lose your eyebrows dont come sooking because you took advice from a random stranger on the internet...

[pundy14]

Thanks! No sooking here - just a fella trying to build a boat!

[disorderly]

Good luck with it mate...

Just be sure to sort it out so that you re-install a Hatch cover that can be more safely and easily opened for inspection and maintenance...

Even my chinese tinny has easy access..i had a 19 year old German girl on a gap year pull the hatch cover off and clean up the sika around the edges for the same reason you are doing it...strong smell of fuel so I wanted to see what was happening in there and change out the hoses and clamps just to be sure...

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[Dignity]

If you can get hold of a nitrogen or carbon dioxide cylinder and pump that in it would be safer. Here are a couple of figures to consider.
1. The upper and lower explosion limits of gasoline-air mixture are about 0.86% and 4.3% HC respectively according to the experimental results, and the critical explosion concentration is about 2.5% HC.

2. the ignition of gasoline-air mixture under various concentrations of premixed N2 and O2 in the storage tank was carried out, and the results show that the ignition of gasoline-air mixture with the concentration of 2.0% HC is impossible when the concentration of O2 is below 18.80%.

This is from this site:
https://benthamopenarchives.com/abst...de=TOPEJ-9-150

I had a tank that had the tiniest pin hole in it and couldn't locate it. We'd pressurise it and soaped it up but nada. Replaced it and the old tank sat in a yard for a while till that pinhole actually decided to shoe itself through a touch of corrosion.

Whichever way you go take care.

[pundy14]

Thanks for the further advice. I'm convinced this aluminium tank must have a pin hole. I made the mistake of letting the boat builder glass the tank completely in without easy access - just dumb on my behalf. Just ignorant at the time

They tell me they pressure tested it and it came from a reputable manufacturer, but the fact I can so strongly smell fumes tells me there has to be a hole somewhere. Having replaced the hoses etc more then once in the hope it would fix it, and overfilling the tank to try and see any liquid fuel, which only resulted in the fuel expanding and ruining the gel coat as it all came out of the breathers ... it's a mess.

Going to drain the tank. Use fans. Slowly cut around it, then get boat builder to fix the floor with a removable piece.

Trap for young players I guess

[billfisher]

Thanks for the further advice. I'm convinced this aluminium tank must have a pin hole. I made the mistake of letting the boat builder glass the tank completely in without easy access - just dumb on my behalf. Just ignorant at the time

They tell me they pressure tested it and it came from a reputable manufacturer, but the fact I can so strongly smell fumes tells me there has to be a hole somewhere. Having replaced the hoses etc more then once in the hope it would fix it, and overfilling the tank to try and see any liquid fuel, which only resulted in the fuel expanding and ruining the gel coat as it all came out of the breathers ... it's a mess.

Going to drain the tank. Use fans. Slowly cut around it, then get boat builder to fix the floor with a removable piece.

Trap for young players I guess

Filling the tank with water is a common strategy for dealing with the explosion risk.

[chris69]

Thanks for the further advice. I'm convinced this aluminium tank must have a pin hole. I made the mistake of letting the boat builder glass the tank completely in without easy access - just dumb on my behalf. Just ignorant at the time

They tell me they pressure tested it and it came from a reputable manufacturer, but the fact I can so strongly smell fumes tells me there has to be a hole somewhere. Having replaced the hoses etc more then once in the hope it would fix it, and overfilling the tank to try and see any liquid fuel, which only resulted in the fuel expanding and ruining the gel coat as it all came out of the breathers ... it's a mess.

Going to drain the tank. Use fans. Slowly cut around it, then get boat builder to fix the floor with a removable piece.

Trap for young players I guess

Try to not cut to close to the stringers so your builder can put a lip in to sit the cover on .

[Noelm]

Filling the tank with water is a common strategy for dealing with the explosion risk.

It’s always better to get blown up by a blast of water……I can assure you, it will still explode if things go bad.

[billfisher]

Thanks for the further advice. I'm convinced this aluminium tank must have a pin hole. I made the mistake of letting the boat builder glass the tank completely in without easy access - just dumb on my behalf. Just ignorant at the time

They tell me they pressure tested it and it came from a reputable manufacturer, but the fact I can so strongly smell fumes tells me there has to be a hole somewhere. Having replaced the hoses etc more then once in the hope it would fix it, and overfilling the tank to try and see any liquid fuel, which only resulted in the fuel expanding and ruining the gel coat as it all came out of the breathers ... it's a mess.

Going to drain the tank. Use fans. Slowly cut around it, then get boat builder to fix the floor with a removable piece.

Trap for young players I guess

If it was me I'd just pay a bit more and let the boat builder cut it out.

[billfisher]

It’s always better to get blown up by a blast of water……I can assure you, it will still explode if things go bad.

It's a common practice when welding tanks that have had fuel in them as it's hard to get rid of the fuel (fumes) completely. No air (oxygen) means no explosion.

[DATCOL]

Fill with water with a good amount of Detergent or degreaser leve a few days then drane then when cutting out put the air hose in the tank so small amount of air is pumped thru the tank while you cut

COL

[Dignity]

If it was me I'd just pay a bit more and let the boat builder cut it out.

Yep, that's the way I'd do it also. The bloke who did mine
emptied the tank, blew lots of air through it, filled it with water and then pumped Argon gas into the tank and cavity it sat in. Also had a big fan on the deck making sure fuel fumes didn't pool anywhere. He'd seen guys hurt through doing less than that, remember your tank might be empty of fuel but the cavity it's sitting in may not be. A lot of boats had holes drilled in the cavity floor to allow any water that got in to drain out. Maybe yours doesn't.