Replacing One Fuel Tank with Two

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

  1. Wideocean7
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    Wideocean7 Junior Member

    Quick question whilst I’m digesting what you’ve both said. Does it matter if it’s a poly or aluminium/steel tank. I’m thinking longevity and strength here.
     
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  2. Barry
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    Barry Senior Member

    The issue of strength is difficult to offer an opinion on as a properly built aluminum tank and a properly molded poly tank built by people who have been doing it for many years, should not cause an issue with either material.
    I am not sure if Ike is viewing this thread but you could PM him to have him offer his opinion on this. Just go the Private Conversation section, type in the contributor, and you will find him

    The issue of longevity depends upon proper installation. Perhaps someone can comment on diesel impacting the strength of poly over time.

    Polyethylene Storage Tanks: Chemical Incompatibilities https://blog.polyprocessing.com/blog/polyethylene-storage-tanks-chemical-incompatibilities

    An interesting link above. I was not aware that a poly tank cannot be "properly grounded"
    I would have assumed that if a charge could build on the plastic surface that a wire to ground would look after any PD

    Is Polyethylene Plastic Safe for Diesel Fuel Storage? https://itppackaging.com/is-polyethylene-plastic-safe-for-diesel-fuel-storage

    and another link which mentions issues with diesel
     
  3. fallguy
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    fallguy Senior Member

    My biggest concern with poly is how long will it remain odorless. I am not convinced forever. Static charges are a concern @fill or anytime anything with electric potential touches the fuel system. So, a metal deck fill gets grounded, not the hose or tank.

    The aluminum tank, itself, can have a potential and also needs a ground.

    Personally, if I were custom sizing, I'd go aluminum. It costs more, but here in the US, we have an exceptionally good ally tank builder.

    In my boat, I have gas tanks, and a Moeller off the shelf 64 gallons was like $300, free ship. No more, they got acquired iirc. I also had an aluminum tank built for the heating system (14 gallons). It is a finicky business, but I believe we did it all correctly. With a custom tank, you can squeeze it in to a harder spot better, but you also have more potential for troubles in tight spaces.

    For your situation, I'd probably go the extra mile and really check and double check the tank specs and locations for things. You don't want a straight end, when there is only room for a tank 90, for example. And the tank inspection port on the top could be located not under furniture. And if you need an access panel, for example, you want that under the middle of the cabinet, not under the side of it!

    I also understand OP appprehension with aluminum after it failed. Perhaps @Barry can comment further.
     
  4. Wideocean7
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    Wideocean7 Junior Member

    I've attached an elevation - roughly to scale -showing the tanks profile and taking into account your comments as much as I can. Because of the "overhang" against the hull, I don't know how it could be securely strapped? I could go for fore-and-aft straps which I can get access to, but I'm concerned that they will hold a full 260L tank when the boat is heeling. So maybe athwartship straps are needed - the problem here how to attach them at the side nearest the hull which is tricky to get to.

    I'm thinking more and more about replacing with aluminium not poly. Poly seems to be the best for longevity but apparently they can't have large inspection ports which I'd want after experiencing what I have.

    Whilst I'm on this learning experience, what do you mean by straps? Presumably not just the cheap ratchet jobs from a hardware store?

    Thanks again for the invaluable help guys.
     

    Attached Files:

  5. fallguy
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    fallguy Senior Member

    Go to duck duck go.

    type in 'moeller tank hold downs'

    there are several types to choose from

    if you are afraid of screwing into or toward the hull, you need to come up with some 3/4 plywood or coosa or high density marine core and epoxy bond it to the hull(not paint). Then you can screw into it with a 3/4" screw with no fear of damaging the hull. Don't use a painted surface to bond to as the strength of paint is far less. It looks to me like you could angle iron tie to the floor joists as well. Or get a bit creative making a hold down from epoxy and a core. In my boat, I installed Moeller plastic angles that were like 2" or so. One side on the tank and the other bolted through the bulkhead. But you would only be able to screw, which is fine if you have places for the screws.

    Any boat builder has scraps of the stuff. But a piece of timber has good holding power (not end grains).
     
  6. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Wideocean's sketch is very small to read, even if you click on it - here is a larger version for reference.

    Wideocean's fuel tank drawing.JPG
     
  7. comfisherman
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    comfisherman Senior Member

    Alloy hull. Use alloy tanks. Don't glue any wood to the hull anywhere and introduce a place for interior electrolytic corrosion. Don't confuse it anymore with the oddities associated with poly tanks. Moeller tanks expand up to 5% when first fueled up, and require being strapped and blocked into place. An alloy tank needs a slightly thicker set of alloy welded on to double around the tabs and that's about it. The beautiful part of alloy is that simple fact. A welded tank with a simple doubler to spread the weight and some tabs and your off to the races. Straps would introduce something over the alloy that can trap moisture and potentially prematurely erode the material. You may need to weld tabbing in the framing of the hull... once again this is the upside of alloy construction.

    As to the original question I cannot say, my industry is only vaguely regulated and as such am not familiar with the legality in your area.

    I will say this. Call whomever is welding your tanks and ask them what gap they can span between frames and what reinforcement is necessary. My last alloy tank was just picked up, the welder knew exactly the specs of what it needed for support and we planned accordingly.
     
  8. fallguy
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    fallguy Senior Member

    Electrolytic corrosion from wood?
     
  9. fallguy
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    fallguy Senior Member

    He doesn't want the aluminum because it corroded, so perhaps address that a bit if you understand it.
     
  10. comfisherman
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    comfisherman Senior Member

    I've seen more nasty white corroded aluminum from people gluing (with whatever your favorite glue is) substances to aluminum than any other single reason including bronze fasteners.

    I'll be in the yard again next week, hopefully the alloy scrap pile hasn't been hauled off. There is always a pile of corroded aluminum plate caused by various types of bad bonding solutions.

    Aluminum has its ups and its faults. The faults is sensitivity to crevice corrosion. The upside is chop the old one out, tig a few new ones up weld some pad eyes on bolt them down and you've got a solid 20 years and you'll have to do it all over again....

    Make sure your not afoul of regulations, but don't overthink the boat.
     
  11. fallguy
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    fallguy Senior Member

    Aluminum does not magically corrode under epoxy. It corrodes in crevices of more permeable coatings, like polyester.

    Furthermore, the OPs picture showed wood and what looked like a 2x2 joist, so the idea to add blocking there was not off.

    Some way or another, the 2x2s got attached.
     
  12. Wideocean7
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    Wideocean7 Junior Member

    Thanks for the feedback/discussion guys/gals (best be politically correct) I think I'm going to go with the existing design ie 90 degree brackets with one arm welded to the tank and the other thru-bolted to the frame. That way I don't have any strapping issues and can use the existing holes in the frame for bolting. The feedback to everyone's posts has been really very helpful in my getting to an answer. But I think I will go for the simplest solution - hence going for what's worked for 20 years on this boat. BUT one question. A lot's been written about alu-on-alu fittings being potentially exposed to poultice corrosion. In my case between the bracket and frame. Do you reckon I should sandwich the 2 with a non-absorbant material to reduce the chance of poultice corrosion?

    Fallguy - yep, I wanted to look at other materials because what's failed in my tank. But if I keep a clean tank, with good access ports, I think I'll go for aluminium. Thanks for staying the distance mate!
     
  13. fallguy
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    fallguy Senior Member

    You ought to consider a stripper valve. It doesn't replace hand cleaning of the tank, but it does allow for periodic, simpler maintenance.

    just saw a guy on a diesel forum I belong to ask the group what it was

    The stripper gets put where water can end up, so the tank level is designed so the valve is at the lowest and a few pumps monthly or annually does some of the work and if terrible, you have a window into the tank's health, if you will, without planning a full cleaning session.

    You can use rubber innertube to make isolation spacers, but the fastener still creates the delta for galvanic corrosion. I plan for the fastener to survive almost always, so I typically am using 316 ss in aluminum, but that's me.
     
  14. Wideocean7
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    Wideocean7 Junior Member

    Yep, if I can I'll include a stripper valve. It's a good idea.
     

  15. Wideocean7
    Joined: Feb 2017
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    Wideocean7 Junior Member

    Hi Guys. An extremely long gap between drinks! Barry suggested replacing the brass/bronze fuel line fittings with I assume stainless steel. But I can't find any supplier here in Australia. I am assuming the existing fittings are NPT but even brass/bronze ones down here are BSP. I'm not going to replace every fuel line fitting at this stage, hence searching for NPT. Any suggestions? Cheers Andy
     
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