Replacing One Fuel Tank with Two

Discussion in 'Boat Design' started by Wideocean7, Jun 2, 2021.

  1. Wideocean7
    Joined: Feb 2017
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    Wideocean7 Junior Member

    I've been a regular visitor to this forum but this is my first post. It's been recommended to me as a good place to raise this topic.

    I have a badly corroded diesel tank on my 37ft aluminium monohull. I don't want to line it with one type of goo or the other, nor do I want a bladder. And I can't get it out to repair it. So I'm looking at cutting it up and replacing it with 2 tanks (not yet decided if poly, aluminium or stainless). It has to be 2 tanks so they will fit into the gaps available.

    My thinking is to connect the 2 tanks so they function as one. One tank would be the "Master" with fill, fuel draw and fuel return ports. It would be connected to the "Slave" with a pipe at the bottom - possibly incorporating a sludge box if room permits. Both tanks would be vented.

    Can anyone see any problems with this set-up? Comments greatly appreciated!

    Cheers
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Welcome to the Forum Wideocean.
    Is your badly corroded tank made from aluminium?

    Re how it's replacement will be two tanks, I presume from your description that they will be side by side, rather than having one sitting on top of the other?
    I think that I would be looking to have them built out of welded poly, and then no worries re future corrosion.
    Re commercial builders of poly tanks in Australia, Google found these folk - they note that the price of a custom size would be about 65% more than one of their standard size tanks of the same capacity.
    Rigid Water & Fuel Tanks for Boats RVs & Caravans https://www.marinedirect.com.au/water-fuel-tanks-rigid-marine-quality/~-150

    Can you post a photo of the location where the tanks will be installed?
     
  3. fallguy
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    fallguy Senior Member

    Afaik, not allowed or not advisable.

    why would you want a potential leak at the bottom of two tanks?
     
  4. comfisherman
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    comfisherman Senior Member

    As someone who made a lot of money cutting out rotten aluminum and repairing old alloy boats, I find it hard to believe that the tank rotted without damage to stringers or plate. Assuming the the tank is a tight access area? Is there any chance of some pics to see what were up against when giving advice?
     
  5. gonzo
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    gonzo Senior Member

    The tanks should not be cross-connected at the bottom. Diesel engines have a fuel return, which makes multiple tanks more complex. The easiest way is to have a switch valve to use one tank or the other. The valve needs to be double, to switch the feed and return simultaneously. Also, each tank needs a fuel level gage and fill inlet.
     
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  6. fallguy
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    fallguy Senior Member

    Also, fuel fills at tank are supposed to be accessible by all rulemaking bodies, so technically, you'd not be able to meet that requirement because you could deck plate the joining pipe, but not be able to remove it which is the point of the rule.

    Just use two standard fills and two vents and use valves from the supplies. You can probably use a remote valve even.

    Or, hire a tig guy to come help you make a single tank replacement possible.
     
  7. Wideocean7
    Joined: Feb 2017
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    Wideocean7 Junior Member

    Thanks all for your replies.

    I hope I've answered your questions and thought it would help if I described the current set-up:

    There is a 300L aluminium diesel tank against each hull and either side of the engine. Both have their own fill, vent, fuel draw and return. The 2 fuel draws are connected before they go through the FilterBoss. The returns are also connected. Then there’s a pipe connecting the bottom of both tanks presumably to equalise the fuel level between tanks. There are valves on all connections so one or the other tank can be isolated. The boat is 20 years old so I am assuming that this system has worked. But there aren’t inspection ports, which is probably why its’ so dirty.

    I found a ½” hole in the bottom weld of the stbd tank and after much investigation, cut out part of its’ side. After removing up to 1” of sludge, the bottom is very heavily pitted. A marine welder has had a look and told me that the tank has to be removed (which it can’t intact) for him to properly clean, tig weld and pressure test.

    An upside is that the tank is located beneath the raised deck saloon so I can get to the side relatively easily. The photo shows the tank hiding behind the wires below the cabin sole. Getting into the top will mean stripping out a lot of fit-out and cutting floors which I want to avoid.

    So the idea is to cut-out the tank and install 2 new ones which will be of such dimensions to fit through the cabin floor. See attached sketch. I’ve not shown attachments or support yet.

    Further help would be greatly appreciated!
     

    Attached Files:

  8. fallguy
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    fallguy Senior Member

    You just don't want to connect the tanks on the bottom. This is unwise and potentially illegal because one is a fill to the other which requires an inspection port which is impossible per your drawing. Also, if the fitting leaks, where does the diesel go?

    What I recommend is you call and hire a local marine surveyor to help you navigate a proper rebuild that will also allow you the means to polish the fuel, etc.

    Another option you may consider is a fill tank and a polish tank. One tank is raw fuel and the other tank is filled with purified fuel. This requires the place to do the filtration, but is another idea. The dirty tank would be the one you may need to access for future replacement.
     
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  9. Wideocean7
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    Wideocean7 Junior Member

    Thanks Fallguy. Why do you say that inspection ports are impossible per my drawing. I've shown one in both tanks. Cheers
     
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  10. fallguy
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    fallguy Senior Member

    your slave tank is filled on the bottom and that fill is not 'accessible' per most rulemaking bodies

    of course, I could be wrong if you can put inspection ports under the tanks
     
  11. Wideocean7
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    Wideocean7 Junior Member

    Not sure if your second sentence is a joke?!:) I must be very dim because I still don't understand why my proposed side inspection ports are an issue. But this seems to be a side issue to the one about linking tanks.

    I do take on board the comments about the risks that ports at the bottom of a tank pose if they rupture/break. But I can only work with what I've got and aside from the possible leak problem, I can't see why my sketch wouldn't work. I understand that there are very strict codes that all ports on petrol tanks must be at the top. But from the little I've read, that doesn't apply to diesel.

    I'm happy to be put straight on any of this. I'm just hoping someone can help me get to a solution. It might not be the "perfect" answer, but I don't have the budget for perfection. It's doing my head in trying to figure out a practical solution!!

    I've had a look on the web for someone to help me design things. But so far no luck.

    Cheers - and thanks for spending time on this. It's much appreciated.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    There are products that can line the tank, provided you can get access to apply it, which appears to be an issue here. Google "tankguard"
     
  13. Wideocean7
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    Wideocean7 Junior Member

    Thanks Mr Efficiency. I know I said originally that I'd prefer not to use an internal lining but I have to keep an open mind. Do you know if internal linings have been used as a long-term solution? Thinking about it, I'd be able to apply an internal lining by cutting away the side of the tank, but have no idea how to seal up the opening I've cut away.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    That tankguard stuff is a type of epoxy that withstands many chemicals, and has reinforcing material in it to make a tough membrane, I know people who have used it, I think what he did was cut an access hole maybe two, then when finished he pop-riveted a plate over the holes and proceeded to seal around that with more of the tankguard goo. But that was from the top of the tank
     

  15. Wideocean7
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    Wideocean7 Junior Member

    Thanks for the lead. I'm trying hard to keep my mind open to all options!! I just Googled "Tankguard" which seems to be a Jotun product but it doesn't say for aluminium (more for steel, concrete). But I take on board your comments about accessing the tank to apply it.
     
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