Another Leaking Diesel Tank

[Wideocean7]

Apologies in advance for yet another post about a leaking diesel tank. I’ve not experienced a holed tank before but fate has caught up with me! From the posts I have read, it seems that solutions are quite boat-specific hence creating a new post.

I have a 20-year-old 37ft aluminium yacht with 2 x 300L aluminium diesel tanks below the raised deck saloon and either side of the engine. The stbd tank has a 1cm hole just beneath a corner which I discovered last week. It had been filled with epoxy by a previous owner. The only top of tank penetrations are a 3” sender port and filling port. The sender port is on the opposite side of the baffle to the hole so I can’t see anything with a borescope. So at the moment I don’t know the extent of damage/corrosion.
My plans are for more remote and high-latitude sailing so I have to have 100% confidence in the integrity of the tanks. At the moment, I am assuming the worst case that both tanks are shot and need repair/replacing somehow. The port tank may also have similar issues but is not leaking (yet).

On the plus side, the raised deck-saloon gives me crawl-space access to the side of the tank. I could also cut-out the bottom of the locker above it (see photo) to get access to around 50% of the top of the tank. Removing the tanks as a whole would require major surgery to the fit-out and/or removal of the engine. I would really like to avoid these options. I could go through the horrible task of cutting up the tanks for removal but this still leave the question of how to get a new tank in.

The two options that come to mind are:
1. Cut-out entire side of tank and insert new plastic tanks between the baffles. The aluminium tank would then only be an enclosure and the plastic tanks will be visible and accessible from the side. I have yet to get my head around how to fill and connect the tanks and how to restrain them in place.
2. Cut-out the bottom of the locker and then the top of the tank. Hopefully this would give enough access to repair internally – say a re-weld or epoxy skin.

I am guessing that Option 1 would be much more expensive due to the cost of new custom tanks. But I also have to consider reliability and durability.

I would much appreciate views on the 2 options (or others I haven’t thought about) especially if someone has done something similar before. If the plastic tank idea works, does anyone know of companies that can make the in Australia?

Thanks Wideocean 7 (Andy)

[tkeithlu]

Thoughts, randomly after only one cup of coffee:

A third approach would be to cut out and fair all but the bottom of the tanks and sit a bladder on top of the base. Do both.

It doesn't look like a good place to be doing any welding.

I'd look very carefully at possible ways to get the existing tanks past your beautiful woodwork. Probably, they put the tanks in and then built on top without regard for ever reversing the situation, but do look.

I hate aluminum tanks, for exactly your reasons.

[gonesail]

my philosophy: if you are buying a boat .. make sure that the fuel tanks can be replaced without destroying the boat

[Manateeman]

Hi. This is a complex job but it’s been fixed before. I found 5 different models for your boat so first, which model and can you draw or post sections ( like slices of bread) so everyone can understand the layout. The more information, the more accurate the cure.
Who built it. What do they say. What kind of aluminum. Both tanks and hull.
Usually, it’s the lowest part of the tank where water accumulates. I don’t weld old aluminum diesel tanks... aluminum boatbuilders understand why. You also might get advice on magic cures from people who are not professional boatbuilders.
There are no magic cures and you are correct, it’s usually means both tanks sooner or later. I would also post on boat design.net . Lot of young smart people.
I take it the tanks are not part of the hull...yes?
If you removed the engine, is it possible to get the tanks out?
I just don’t have enough information...drawings...dimensions.
What structural elements are in the way of removal.
Lastly. Fuel in the bilge. The authorities will often ask to see the deepest part under the pretext you might be sinking, but they are looking for oil. Please retain your discharge to avoid fines. Fuel in the bilge is a another issue. Let’s fix the issue permanently. It’s a nice boat.
Mark and his crew.

[Tomodore]

I recommend exploring removing the engine, even if you end up cutting in access having the engine out will make life easier. Beware of project creep with the engine out. However if you are planning remote cruising than this could be an excellent time to have an ace mechanic look at your diesel while you install 2 new tanks. Tearing into your boat is not something you do before cruising it. Is part of cruising.

[johnn33]

A lovely boat; have you had help from the designers?
John

[johnn33]

https://www.finelineboatplans.com/br...-370-boat-plan

[Squanderbucks]

Looking at your photos above the open engine section - it appears someone my have had some of these issues before or a different issue with the engine. Because the framing appears to have been modified or replaced in sections ( especially the Port side).
Think hard about turning two tanks in to four smaller ones. Personally think that would be a constant headache managing.

I agree that removing the engine and possibly one side of the engine framing - Port side appears to already be held in place with metal brackets of some sort. That way you have a much larger hole to work through.
With the engine out a new tank of dimensions larger than the opening could be swung into place and slid over into either position. Maybe remove both tanks, have new tanks made and install the Starboard first then the Port for jockeying room.

For diesel I like fiberglass but Plastic is also good. The main reason I like FRP is that it can be made to virtually any shape and if damaged can be easily repaired or modified. Does not rot, rust, or corrode.

Building a new set of tanks allows adding custom features like clean-out hatches on each side of the baffles and a little sump to collect water in the rear of the tank.

[boatpoker]

Most aluminum corrosion on boats is poultice corrosion. Take a hard look at the hull all around the tank and ensure that whatever was causing the corrosion has not affected the hull in that area.

[canman]

I feel for you as I had the exact same issue and considered the options you are including a bladder tank.

Ultimately I realized the only proper repair was to carefully dismantle all her joiner work including cutting the floor stringers. I found a company that built new custom aluminum tanks and and then carefully replaced everything. The quotes for the work professionally done was between 15 and $30,000. Those were fair quotes considering the amount of labor hours. I decided since I recently retired I could do all the work myself and completed the entire job in about three weeks of hard work. Total cost not including my time was around $2500.

The good news is you can correct a lot of mistakes from the original installation and be reassured they will not leak for another 20 to 30 years.

[Fore and Aft]

Andy I reckon before you start getting to complicated undo those four bolts holding the tank in place, remove the batteries and see if you can wiggle that starboard tank out. The opening dimensions look similar to the tank so its worth a try.
I don't like option 1, mainly because using a grinder with a cut off wheel in a confined space can get ugly very quick.
Everyone reading this, I was on Andy's yacht last week and he might get lucky removing the tank after he has undone the bolts and hoses.
Andy it's going to be an awkward job so you might need to take a teaspoon of cement first.
Cheers

[Boatwright]

Bladder. Use your old tank as a shell. Bladders can be custom made -- look for an outfit that makes fuel cells for race cars. They will mount filler, vent, and other ports as you need.

https://fuelsafe.com/

FYI: Check and replace filler cap o-rings every year. A small amount of water will lie under the fuel and eat a hole right through an aluminum tank. There is something going on between the water and elements in the fuel (sulfur?) the doesn't like aluminum. If you have a new tank, before it's installed, consider coating it inside with Red Kote fuel tank sealer, which will also fix pinholes in existing tanks. It requires sloshing the sealer around inside the tank by rotating the tank in all directions, so probably won't work in your application. Resin patches are a problem because of the waxy residues in the fuel. The only hope for a resin patch is super clean metal, unlikely with an existing tank.

[Fore and Aft]

Boatwright the only issue I have with bladders is it looks so half done when you see it on boats. It actually looks bloody terrible when they are steel tanks, so you end up with a rusty shell with a bladder in it. To me you already have the grinder out and removed a bit of the tank so why not go the whole way and remove the tank completely. Then install two smaller tanks if access is the issue.
Cheers

[Searles]

Bite the bullet,remove the engine, both tanks check hull and associated areas tidy up wiring etc ,get new Polly tanks made to required design ,now is a good time to do alterations ,replace all the above go sailing knowing all is good .⛵️⚓️

[hpeer]

Aircraft use a 2 part fuel tank sealant. They literally make fuel tanks out of aluminum relying upon the sealant alone to seal the tank. This stuff comes in 2 viscosities; self leveling (runny) and for vertical installation (stiff). The vertical stuff will seal a 1/4” seam, 6mm.

You need to empty the tank and clean the surface on the inside. Then coat the inside with the sealant. Wait a day or so and pressure test. To seal the hole coat around it with the vertical sealant and then stick on a piece of aluminum sheet to cover the hole. If worried about the tank bottom pour in some of the self leveling sealant and mush it around to coat the bottom.

You can buy this stiff from Boat Life or from an aviation source (cheaper.).

This stuff is messy, someone else once said “You only think 5200 is messy.” That is about right.

You do need some access to work your arm around inside the tank. Maybe use a hole saw to cut a hole in the baffle? Or cut a rectangular hole in the top or side and seal it up same as I suggested for your existing hole. Or for that matter cut out the piece with the hole and replace it with a larger piece held on with the sealant.

No joke, this sealant is a real thing. Research it a bit.

http://www.ppgaerospace.com/Products...e-Options.aspx

https://www.skygeek.com/flamemaster-...k-sealant.html

https://www.rivetbangers.com/documents/tank_sealant.pdf