Fitting inspection port to alumni. fuel tank

[skyflyer]

Following a nasty blockage last summer I have removed my fuel tank (2.3mm aluminium) and a look inside with a probe camera shows it thoroughly gunged up with black sludge.

Simply sloshing various solvents around the tank will not dislodge it. It needs to be scrubbed!

However the only access is the hole for the fuel sender gauge (~50mm) which is inadequate for cleaning.
I therefore want to fit an access port in the top face of the tank - in fact I want to fit two, as there is a vertical baffle nearly all the way across the tank - so I can get my arm in and clean out everything properly.

To do this I reckon the diameter of the hole needs to be at least 5 inches so i can get my arm through to the elbow and still have room to move it around inside the tank.

The question is, will 2mm aluminium plate be adequate strength (using a suitable gasket of course - I plan on using 2mm Viton) or is it likely to distort under the liquid pressure as the fuel in the tank slops around in heavy seas etc.?

My gut feeling is that as it is sited on top of the tank it will be OK. had it been in the side that would have been a different matter.

There is also the issue of whether 2.3mm aluminium gives adequate thickness for the securing bolts. The old fuel sender unit uses self tapping screws with a hex head (looks like a bolt when in place and somewhat different from a normal 'self tapping screw'); the new one I am installing is supplied with machine screws, which implies tapping (not a problem).

Do I buy machine screws or self tappers for my inspection port cover plates?

Ideally I want to set this up so in future I can clean the tank in situ, within the boat as removing it was a bit of a herculean task requiring much dismantling of surrounding boat!

 

[lw395]

Makes little difference whether it's on the top or the side. It must withstand pressure from sloshing about. Probably far more than the hydrostatic pressure involved.
The top is still the better place in case you need to open it with fuel in the tank.
I would probably stiffen the edge of the hole and use a lot of small fasteners.

 

[PaulRainbow]

The tank is liable to distort and be difficult to seal. Make the cover out of thicker plate and have some strips of 5mm x 20mm drilled and tapped for the plate to be fastened with machine screws. Hold the strips in place with a couple of small, countersunk, screws.

This obviously means that round hatches will be more difficult, could you fit a single hatch, say 10-12" x 5" so that it's centrally over the baffle ?

 

[PetiteFleur]

Definitely in the top face and 6" diameter or larger would be better. Better still if you can get the tank out and do it in the workshop - much easier if out of the boat. Have just done a friends tank, took it out and flushed with water but it was not badly fouled. It might be better if you tap holes in the tank and fitting a bolt from inside, tightened and then use washers and nylock nuts to hold the plate down.

 

[Pete7]

You could use a Henderson Hatch.

http://www.hendersons.co.uk/boattank/Tank-access.html

 

[Seajet]

I don't see tapping such a thin tank top working or am I not understanding ?

Whatever you do, dropping nuts or washers into the tank would be on the serious side of undesirable.

How about dinghy inspection hatches, a la RWO ? While plastic, the inner ring bolted on with nyloc nuts would have quite a stiffening effect, the outer rings are already countersunk but one could drill out a touch for larger less fiddly bolts & nuts, the bolts semi-bonded with Sikaflex or Araldite as long as the tank is dry and no bits allowed to drop into the tank - NB check these or some other adhesive sealant are compatible with your fuel.

If diesel I once had to clean the large stainless tanks under the side decks of a 1960's motorsailer, the bottom of the tanks had a layer of thick clay-like residue which had to be scooped out by hand - might be worth planning with that sort of thing in mind as a worst case scenario, also one should hopefully have the inlet and outlet within arm's reach to clear debris, possibly in a hurry, so two hatches if necessary.

 

[benchmark]

Sounds like a job for Seabuilt

http://www.seabuilt.com

 

[Yellow Ballad]

I would consider getting an aluminium 5mm (or so) ring welded on before cutting out the hatch in the tank (in the center of the ring). Then you have a thicker plate to tap threads in to and it gives a bit of strength to the hatch.

 

[PabloPicasso]

You could use a Henderson Hatch.

http://www.hendersons.co.uk/boattank/Tank-access.html

That looks like a neat simple solution. I wonder how much they cost?

 

[Tranona]

I would consider getting an aluminium 5mm (or so) ring welded on before cutting out the hatch in the tank (in the center of the ring). Then you have a thicker plate to tap threads in to and it gives a bit of strength to the hatch.

That is what I had done to my stainless water tank. Worked well.

Alternatively there is a good range of screw type inspection hatches available from www.tek-tanks.com

 

[skyflyer]

Interestingly the tank label says it was pressure tested to 3psi. Assuming a 1.5 safety factor that suggests an anticipated makimum working pressure of 2psi. Over a 5 inch aperture that's a pressure of about 40lbs

The seaview product is very expensive compared to a sheet of aluminium a gasket and a dozen screws.

I'm not sure I'd trust plastic inspection hatches any better and you still have the same problem of attaching it to the relatively thin tank body

Reinforcing ring welded in place seems the best way to go

Thanks all

 

[vyv_cox]

Another alternative to welding a strengthening ring on is to make (or buy?) a C-ring. The ID is the same diameter as the hatch opening, the OD doesn't matter within reasonable limits. A gap of an inch or so allows the ring to be inserted into the tank. It could be made from mild steel, say 5 mm thick, drilled and tapped to avoid the dropped nut problem. The fuel gauge on my tank is fitted like this.

Here's an example https://wema.co.uk/collections/flanges-fittings/products/fls-u-flange

 

[skyflyer]

I had thought of something similar Vyv, two semicircular rings, but I can see this would work better. But what stops the ring dropping into the tank when you remove the inspection port? Any glue or sealant is liable to be degraded by the diesel?
Also I had assumed a steel/aluminium combination would produce galvanic corrosion although that problem will exist anyway due to s/s screws

 

[lw395]

I had thought of something similar Vyv, two semicircular rings, but I can see this would work better. But what stops the ring dropping into the tank when you remove the inspection port? Any glue or sealant is liable to be degraded by the diesel?
Also I had assumed a steel/aluminium combination would produce galvanic corrosion although that problem will exist anyway due to s/s screws

As Paul said in post 3, the ring can be screwed to the tank with some small countersunk machine screws.
SS screws smothered in sealant on a diesel tank are not prime candidates for galvanic issues IMHO.

 

[PaulRainbow]

Why on Earth would you have a ring made and pay someone to come and weld it in place ? Welding ally this thin will 100% cause distortion. Welding an ally tank used for fuel may very well cause other problems, due to contamination of the metal, not to mention it has flammable gunk in the bottom.

If you have made a ring all you need to do is fix it in place from inside the tank, like i said in post #3. If you make the hole square or oblong the job is easier and apart from sourcing the plate to make it all it's a simple DIY job for anyone with a drill, jigsaw and some taps. Cheap, simple and it will work. Use 5mm ally plate and 5mm x 20mm ally strips inside the tank and you won't suffer distortion and sealing problems.

 

[vyv_cox]

I had thought of something similar Vyv, two semicircular rings, but I can see this would work better. But what stops the ring dropping into the tank when you remove the inspection port? Any glue or sealant is liable to be degraded by the diesel?
Also I had assumed a steel/aluminium combination would produce galvanic corrosion although that problem will exist anyway due to s/s screws

When I remove mine I substitute one or two longer screws to hold the C-ring in. It's a bit of a fiddle but not done very often.

 

[PaulRainbow]

But what stops the ring dropping into the tank when you remove the inspection port?

Countersunk screws, as per post #3 or don't remove the last screw (next to the gap in the ring), then you swing the plate around so you can get your fingers under the ring, or just leave it all swung around.

 

[skyflyer]

Ok Rainbow, youve convinced me that a rectangular hatch with backing strips is the way to go.

Next question though is whether to have one hatch either side of the baffle or a larger one that spans it. the baffle is welded to top andd bottom of the tank and has gaps at each side to allow fuel to pass. So by making an opening that spans it i am weakening the top edge attachment.
I could fit some channel on the inside of the hatch to support it but thats getting over complicated again i think

 

[PaulRainbow]

Ok Rainbow, youve convinced me that a rectangular hatch with backing strips is the way to go.

Next question though is whether to have one hatch either side of the baffle or a larger one that spans it. the baffle is welded to top andd bottom of the tank and has gaps at each side to allow fuel to pass. So by making an opening that spans it i am weakening the top edge attachment.
I could fit some channel on the inside of the hatch to support it but thats getting over complicated again i think

Given that, i'd fit two hatches.

 

[PCUK]

Ok Rainbow, youve convinced me that a rectangular hatch with backing strips is the way to go.

Next question though is whether to have one hatch either side of the baffle or a larger one that spans it. the baffle is welded to top andd bottom of the tank and has gaps at each side to allow fuel to pass. So by making an opening that spans it i am weakening the top edge attachment.
I could fit some channel on the inside of the hatch to support it but thats getting over complicated again i think

There won't be any top edge attachment. They had to fit the top last and therefore had no access to the top edge of the baffle. You're overthinking this. It's a simple job, don't complicate it for no reason.