Impossible Mission: 200 Gallons Diesel Tank Sealing Project

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Boater On Thames

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I finally got the chance to start my boat repair DIY projects because of the lockdown.

The first thing is to seal one of the 200 gallons aluminum diesel tank. It has a little leak which I think it's the bottom. But after I got all the tools in place, then I realised this is impossible to seal it because the tank can't remove, and it has multiple rooms inside! I can't even clean it properly! And I don't know if is it okay just fill the sealer in it without cleaning it. Even that work, but I will still can't make the sealer cover the whole bottom because I can't even shake or make any movement on this big tank.

This seems like an impossible job. Any input is welcome. Many Thanks!
 
Can you see the specific position where the leak is ?

There are some impressive epoxy sealants that might work, but I expect that first you would have to get the tank completely drained and completely de-greased around the leak area.
 
This is the problem. I can't spot the exact leaking position. I have checked two open sides and see all dry. Another two side impossible to see because they have no gap at all. So, I can only assume the leaking is from the bottom. But then I found can not see the whole bottom of the tank even drain all the fuel out. Because the tank is not a one room structure. It is separate to several rooms inside.
 
If the tank is empty you can use the fuel outlet to drain it.

Get a tub of drain stain in a visible colour, mix a little in a bucket of water and tip into your tank and fill slightly each day until you see a leak, this will leave a coloured stain at the leak and down your tank where it runs.
Drain your tank through the fuel outlet and drop the stain into the water (non toxic) and if you cannot see the leak you follow it back to where it starts, if it is on the tight side of the tank then you need a borescope to find it.
 
The tank has no drain outlet. It similar to the pictures I post below (winscreen shot from a youtube video). But mine is three times bigger than the one show in the pictures. It only has a few holes on top which for fill pipe, pump out pipe, and gauge sensor. The whole tank hidden under the after cabinet bathroom floor. Very limited access.

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I just went to the boat and took 3 pictures to show how limited the access is. The tank is right under the closet, toilet and bathroom. There is only one square hatch can see a tank's corner only. And another round hatch on the bottom of the closet to access to the gauge sensor. So the only way I can pump the diesel out and put anything in the tank is through the gauge sensor hole.

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Well the good news is the deck/floor is wood. The only way to be sure the tank is properly fixed is to take it out and either fix it really well and test it. Or get a new one made. If you do anything else you will always be worried at the slightest sniff of diesel. Also when it comes to sell new tanks are a plus point.

For me it's jigsaw time. Take the bed apart and out, then make a really big square-ish hole you can work in and get the tank out. It's often easier to repair a really big hole then mess about with a small one. You will probably have to disconnect a lot of pipes and wires - just label them up, take pictures and take your time. It's possible the footprint of the bed or slightly less may well be big enough

Repairing the deck- reinforce the edges and add some extra supports/cross beams. Cut a new piece of marine ply to fit jobs a good un!
 
I have sat here and thought through your problem for some time. There is no easy answer I can offer, sorry.

There are two ideas I have had and neither is very palatable ...

1 Extensively dismantle the interior and remove / cut away the structures above say 1/2 the top allowing you to cut a hole sufficiently large to allow you to open and then remove all the internals baffles. I would then approach Tek Tanks to make say 3 smaller sub-tank sections of plastic that can be cross connected, or if the interior can be fully cleaned of all sharp edges then a tank bladder.

2 This sounds worse, but without knowing how the boat is built it is not possible to gauge how dire this would be - cut a hole in the side and extract the tank through it. Again have Tek Tanks make a new one to fit the shape left behind, then make good the access hole.

What is amazing is that this boat's maker never even considered what an owner would do in a fuel contamination situation, let alone your problem of a leak.
 
Thanks, mates.
Cutting interior is difficult. Because the tank is in the starboard side which wholly under the bathroom rather than the bed. The port side tank under the bed is fine luckily. And the bathroom is a one big piece plastic structure which impossible to remove without damage it. And the tank is too big to move out through the door.

I was thinking to clean the tank by using a pump, then just pour in the sealer to the bottom hopefully the leak is on the bottom. Now consider the baffles inside, and I can't shake the tank anyway, so this idea that sealing without remove the tank become impossible to me.

So, now I think my only option is to cut it from a side way to remove the tank if I want to fix it. Or, maybe just leave it empty, and just using one tank only.

My boat layout below. The No. 20 in starboard side of the stern is the tank I am working on.

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Just thinking outside the box (because the only way to repair this is to dig it out of its hole) - do you have anywhere in the boat you could install a new tank and leave this useless item where it is?
 
There is no room to install another one. But the existing port side tank also has 200 gallons capacity which I think it's big enough if not doing a long trip.
 
I suspected that may be the case but clearly worth a review. Well maybe just soldier on without it and plan to rip out and replace (probably cheaper than repair) at a time convenient to you in the future.
 
The only easy cutting area is the closet which next to the toilet where housing the round hatch for the fuel gauge sensor. The closest looks like cover about 25%of the tank only.
 
Would suspect that trying to clean the tank in situ is going to fail and be a waste of time and money.
Either the original tank must be removed and plated by welding or at the very least access is needed internally to abrade inner tank surface to enable any sealer a chance to adhere to surface.
Knowing where you are boating ,would be very inclined to simply put union into good tank fuel line and run both engines from it. At 8 KPH. a single tank will probably last aseason at least.
Using a cheap diesel pump (£15.00) transfer the fuel from leaky tank to good tank, fill the leaky tank with water to balance boat or simply move stuff to lighter side.
Over time try to repair/extract tank at your leisure . ?
 
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