Replacing One Fuel Tank with Two

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

  1. Barry
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    Barry Senior Member

    I would NOT use this type of "something more substantial than hose clamps-------------similar to the port-starboard equalization line" as there is an inherent design flaw in what you have there. I expect a reply along the lines of "it has been like this for x amount of years" and I would come back with, "you have been lucky"

    Over time aluminum can crack at weld joints with flexing at stresses below its yield strength. Ie bend it back and forth many times within its elastic limit and it can crack. The lower equalization line is a candidate for this because your boat
    is constantly flexing due to loads on the hull. There will be some movement between tanks and you should not rely on the fuel line to supply rigidity to the twin tank arrangement. I believe that ABYC even allows an upper limit of movement of fuel tanks to be under 1/4 inch. So a flexible hose between the two tanks, in my opinion, is a pre-requisite to a sound installation, not a design short coming.

    If you are not comfortable with hose clamps, similar to the million of hose clamps that hold water out of hulls at through hull connections you can upgrade this joint between tanks. A couple of options would be to
    use a swaged fuel hose with a coupling, extra long hose barbed fittings to allow for additional friction ridge at the hose to spud interface which would also allow for say a third redundant clamp, or upgrade the type of hose clamp from the standard
    worm drive and perforated strap type normally used. There are higher standard clamps available.

    Regarding your through hull inspection ports.

    Keep in mind that I am paranoid about fuel leaks. A company that I owned in the late 70's built auxiliary fuel tanks (gas and diesel) for pickups as well as equipment transfer tanks that lived their life being bounced around harsh roads. They were built with steel with more fatigue resistance than aluminum. At our peak year, we built over 3,000 tanks. Of these we might see 5 or 6 come back with weld failures.

    While you could build the tanks out of 1/8 material, this is quite flexible and will require more internal support and baffling to minimize movement. If you can handle the extra weight, you could consider a thicker material for increased rigidity and stronger welds.

    If the thinner sheet thickness is used, there will be distortion from the welding and you need to ensure that the inspection hole surface is flat.
    Prior to welding up the tank, cut the hole, weld a ring if you are going for a round hole, on the inside of the tank surface, drilled and tapped and at close intervals as backing as a sheet "straightener". The outside cover will have the
    same bolt layout of course. The gasket will have to be diesel inert, not cork and rubber, again check the ABYC manual as they more than likely have a specification. Use stainless fittings through out

    As far as the tank construction, some suggestions.

    The main profile of the tank will be an inverted U to limit weld beads at the bottom of the tank. At the top ends of the U, have the fabricator put in a 1 inch return lip. ie the top is open at this time.

    The ends would be cut to fit inside inside the end profile. Ie you can then inset the end piece say 1/4 inch to allow for a fillet weld as compared to a corner to corner bead. As the top is still open, you can catch quite bit of these beads from the inside.

    Install internal stringers and perhaps a baffle, I am not sure of the length of your finished tanks. Internal stringers would be 1 inch by 3/16 flat bar welded longitudinally along sides, ends and bottom to reduce movement.

    The top is then placed on the 1 inch return to allow for another fillet weld and a doubling of sheet thickness for this weld which you can only catch from one side. If you have installed a baffle, you can cut a slot in the top precisely over the top of the baffle and weld the top to the baffle

    To reduce corrosion, the tank, needs to be raised or sat on plastic spacers to inhibit the tank from sitting on anything wet. ABYC covers this in some detail, certainly under the gas tank section.
     
    Last edited: Jun 7, 2021
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  2. Barry
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    Barry Senior Member

    Is that a brass valve on your equalization cross over? Change this over to SS or aluminum
    I am not a fan of welded tabs to a tank for fatigue and weld crack issues. Straps would be my go to choice.
    I notice that you are taking a large reduction in fuel reduction due to the splitting of the tanks. The side profile of the tanks may be modified from rectangular to maximize volume.
     
  3. fallguy
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    fallguy Senior Member

    Barry is the man on this stuff. Pay attention to his posts, I can't recall if I have disagreed with him, perhaps one time.
     
  4. fallguy
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    fallguy Senior Member

    How can he strap if he is unwilling to crack the floor open Barry?
     
  5. Barry
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    Barry Senior Member

    I opened his revamped drawing from Post #30 and noticed the tab mounting. Depending on tank skin thickness, bolting the tank through tabs rigidly to the floor will more than likely cause issues. All the load due to acceleration etc from the hull will be held pretty much entirely by the short welds on the tabs. If the skin thickness is maybe 1/4 inch, which is excessive, perhaps it would not be a problem. Any time you weld into the center of a sheet which is flexing with and
    create a "hard spot" you run the risk of weld cracking. A couple of straps will permit limited movement and remove this potential for failure. It appears from his Post 30 picts that if he has room to screw in a bolt through a tab, then he would have room to install a strap.
    Regarding straps, They can be a preformed aluminum strap around the tank or what we have done on jet boats which we anticipate may be grounded abruptly, is to use threaded rod (ready rod) attached to a hull stringer up the side of a tank, then use
    say a 1 1/2 x 1 1/2 x 1/4 inch angle across the top. And then just tighten everything up. We had several customers come to zero speed abruptly, occasionally causing permanent deformation, without this ready rod type of "strap" causing fuel tank integrity failure.
     
  6. fallguy
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    fallguy Senior Member

    Not sure I follow the instruction Barry, but no need for me.

    I still think cracking the floor is best.
     
  7. Barry
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    Barry Senior Member

    I would tend to agree as the reconstitution of the deck is probably feasible. He appears not want to go that way. His primary goal seemed to be to was not to touch the deck structure and will accept a reduced fuel capacity.

    If his primary goal was to have the same fuel capacity, then cutting the deck is the way to go. Perhaps there is an economic factor involved, ie he has to buy at least a single tank per side and perhaps splitting the tanks
    might come at a cost of say another $800 per side plus a bunch of his own time which may be within his parameters.

    We had our 490 moored over a winter storm, two of the bumpers actually broke, allowing the swim grid to have vigorous interactions with the dock. We hired out the repair and when completed you could not see a difference between the
    original 14 year old grid and the repaired grid. The original gel coat had lost its sheen and the glass guy matched even that. We were not there that day, so I do not know if he scuffed with 1000 grit or just used a low gloss gelcoat.
    There is no doubt in my mind that a qualified glasser could do the re and re and an observer would not know the difference. I would be curious of the cost of this method.
     
  8. Wideocean7
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    Wideocean7 Junior Member

    Thanks very much Barry. That's really great information. I need to digest it then I'll post again if that's Ok?

    Cheers

    Andy
     
  9. Wideocean7
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    Wideocean7 Junior Member

    Guys. I'd prefer not to start cutting up the cabin sole if there are other options, but if that's needed I'll do it. I've stayed away from this option because the size of a new single tank is limited not so much by whatever hole in the floor I make but by the companionway. I'm driving up to the boat this week and will take more measurements and post again. Thanks again.
     
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  10. Wideocean7
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    Wideocean7 Junior Member

    Hi Barry and Fallguy. It's taken me a while to re-measure and sketch out some options. I hope you're still around! To cut to the chase, I am now leaning towards what you recommended and cutting out the floor. I thought I'd be able to keep decent capacity and avoid cutting floors with the 2 tanks, but new-found limitations means that I'd be lucky getting 200L total, compared with 300L now. That's far too much of a loss for me. Barry - thanks for all the detail you shared on this option which was extremely helpful to my being able to figure this out. Fallguy - yep, you were right! But I think I had to come to this decision myself.

    So when I lift the boat in a few months, I'll remove some furniture, cut out about half of the floor above the tank and then cut-out the leaking tank. If my measurements are right I'd be able to install a new 260L tank that will also be able to fit through the companionway.

    So some more questions about tank material and how to secure it...

    I've read that poly is the most durable/suitable for a diesel tank. Would you agree or not?

    Then, how to secure it. I wasn't correct in previous posts that the existing tank was tabbed and bolted to the frame. What's actually the case is that the existing tank has a 90 degree angle bracket welded along both sides. The bracket is then bolted to the frame - see photo/sketch. The distance between the two frames that support the tank is 880mm.

    So if a poly tank is the way to go, how would I secure it, especially when it's spanning an 880mm gap between frames? I know that Barry prefers strapping but how would that work so that the tank doesn't move?

    Really appreciate your help on this. It's been a nightmare trying to figure out!
     

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  11. Barry
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    Barry Senior Member

    I am assuming from your sketch that the bottom of the new tank will be at or above the height of the existing boat frame to which the current tank angle iron is bolted to.

    A possible option would be then is to build a flat surface that attaches to the same elevation as the boat frame is that spans the 880mm. You will have space underneath this to run some ribs to inhibit/restrict/ minimize this new plates
    deflection and then strap the new tank to this flat plate. While there is a lot of info on how to isolate an aluminum tank at the bottom to reduce corrosion, I suspect, but you need to check that the poly tank can sit on say a treated plywood,
    metal or fiberglass surface. I am not a wood boat builder so would be hesitant only because it seems plywood and a marine environment often causes the plywood to rot etc with moisture sitting on it.
    If it were me, I would probably take a 1/4 inch piece of aluminum as a base, run some 2 inch stiffeners on the bottom of the plate, say 2 or 3 --2 x 2 by 1/8 inch angle , with one leg of the angle at 90 degrees to the plate and the other
    forming a channel, ie parallel to the plate but 2 inches away from the plate.
    Bolt the aluminum into the boat, similar bolting location as the exiting tank, and then attach the straps to the plate.

    Of course, the wood contributors may come up with an easier wood solution.
     
    Last edited: Jun 11, 2021
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  12. Wideocean7
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    Wideocean7 Junior Member

    Brian. You're a legend and if I were over there, I'd buy you a beer (or two, or wine, or a cuppa' tea). I will digest and repost. Cheers Andy
     
  13. Wideocean7
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    Wideocean7 Junior Member

    And I meant Barry!! Too may scotches.
     
  14. fallguy
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    fallguy Senior Member

    Barry has a lot of great content on the forum.

    Boats are always a lot of work to build well and sometimes shortcuts ain't [sic] shortcuts.

    Most of the poly tanks you purchase will have a means to hold the tank down. In my poly tanks install, 2x64 gallon same both hulls, Moeller included plastic holddowns. The holddowns still allow for some expansion and they were basically like angle irons. I simply bolted them, but I did have access to both sides of the holdown bulkheads. If you don't have access; perhaps lag screws will work, but I'd avoid punching through the bh/stringer. If you have some size discrepancies, you can epoxy a piece of wood to the bulkheads and temp screw it while the epoxy cures. Typically thickening the epoxy a bit is done. (If your bulkheads are thin; you can add some 3/4 timber)

    The floor can be done really well. If you have a problem like a chip out, there is always epoxy. To minimize fuel odors, if you are going to make an access panel of the entire tank area, I recommend you allow for a neoprene seal at the cover. This is more work, I know, but adding cleats for the lid is required to stiffen the sole and you can screw and finish washer into the neoprene and if it is neoprene foam, it will compress a little so you can line the floor up. Get the seal before you cleat anything so you get heights right. Something like 3/16-1/4" foam would be nice and allow for some compression and seal away from any odor concerns.

    Barry can address the odor issue poly vs aluminum perhaps.
     

  15. fallguy
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    fallguy Senior Member

    typical on left, alternate for space issues right, the timber can double as the floor cleat, btw

    but usually fills are on tank top so should be room for typ

    also, double clamp all fittings with clamps facing opposite, test the install to 2-3 psi before closing-it is a little scary when the tank balloons in poly


    92C6E428-DB86-4356-98F8-7A7C827FC5AE.jpeg
     
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