Plastic fuel tank

Discussion in 'Fiberglass and Composite Boat Building' started by alyons05, Oct 12, 2020.

  1. alyons05
    Joined: Jul 2010
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    alyons05 Junior Member

    I have been in the process of removing my aluminum fuel tank from my boat after replacing stringers and various other items I am to the point of installing a new poly tank. I know I need to leave room for it to expand and have heard 3% is the magic number. My tank is a moeller tank and is sized at 49”x24.5”x7.88”. My plan is to put it into a box that is 50.5”x25.25” x 8.5”. I am going to use 3/8 neoprene on the 4 sides and on the bottom where the tank sits on the original glassed in pods in the hull. Does anyone see any issues with installing in this manor? I will have 4 aluminum hold downs one on each corner these will be free floating in a molded in pocket on the tank but attached to the stringer.
     
  2. fallguy
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    fallguy Senior Member

    You do not free float the hold downs. They should not be metallic that can cut into the tank, so make sure to champfer the ally well if that is what u have. moeller usually provides holddowns. The tank expansion is 3%. What I recommend is a 2 pound pressure test after setting the holddowns.. This test will stress all the connections before you close the sole. And you can fix anything you don't like...sand a rough edge, seal a leak, etc.

    I tested two tanks in a new build and was rather surprised at the expansion. Given a vent; it will never expand as much in real life. My test leaked one night; otherwise it went well. I tested off the supply line.

    If you mean the holddowns are not attached to the tank and only the stringer or BH; you got it right.
     
    hoytedow likes this.
  3. alyons05
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    alyons05 Junior Member

    Hold downs are attached to the stringers only and fit into a grove on the tank but not physically attached to it these are the hold downs that moeller sells for their tanks. How much fuel would I have to add to a 27 gallon tank for it to permanently expand? Is it just a gallon or two or would it need to be filled? I would like to ensure before installing the floor all is well as far as expansion goes and it’s not to loose or tight. Or would pressure testing with the 2psi be enough to ensure all this?
     
  4. fallguy
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    fallguy Senior Member

    no fuel, just pressure test with air

    cap off the fuel fill line at the exterior fitting location with a plug and hose clamp, cap off the vent line same way, install the test into your pickup supply line whip end; the test is a tee with a barb end to the pickup, a pressure gauge to the tee top, and a bike/schrader valve on the end, run the test overnite at 2 psi; the tank will balloon a bit

    I misunderstood you on the holddowns. Sounds right.
     
  5. fallguy
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    fallguy Senior Member

    All connections get double clamped, btw. The right way to do this is to reverse the clamp direction and work the screws on each clamp. Oftentimes after you tighten the second screw; you find room for the first one to tighten a bit more.
     
  6. Rumars
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    Rumars Senior Member

    Do not ever attempt what fallguy describes it's dangerous and stupid. Tanks are never tested with air, it's called a hydrostatic test for a reason. Testing for such low pressures like 2psi is dead easy and can be done safely by any amateur. 1psi = 27.7 inches of water column, to test for 2psi you need 55.4 inches of water column, so for testing you simply take that lenght of hose and attach it to one of the top fittings and hold it upright, then fill the tank with water up to the mark in the hose.
     
  7. fallguy
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    fallguy Senior Member

    putting water in a fuel tank is not what I would recommend, nor would I recommend gas

    Moeller instructs users to pressurize the tank to 3 psi and soapy water test. Noone uses water in small boat building.

    http://www.moellermarine.com/wp-con...anent_fuel_tank_installation_instructions.pdf
     
  8. fallguy
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    fallguy Senior Member

    Further, Rumars remarks are part of the reason I test to 2 psi. In order to encounter 3 psi in a real world setting; the vent would literally need to be clamped. And 3 psi is a LOT of expansion and a burst hazard is real. I have run the test up to 3 psi, but I backed it off and let it overnite at 2 pounds.

    Those Moeller tanks, if you looks close, have a number of qa sign offs. I am going on a limb here to say they probably test them all to 3.5 psi.

    Make sure to find a gauge to 10 psi. And when you see the tank balloon, watch the gauge to make sure it is registering. A bad gauge is also dangerous.
     
  9. leaky
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    leaky Senior Member

    I have installed a bunch of those - if you are really concerned, they come with hold down kits and indents don't they? Just follow their directions.

    My current skiff has one in those general dimensions. I set it down on a pad of PVC board, then on the front back & sides I bent a flap of rubber material over 180 degrees between it and the box it sits in. Basically its tight but if it must expand it can compress the rubber fold a bit.

    They will sometimes deflect on the top side a lot, basically they are so floppy they just bow up, so nothing sharp should be anywhere 360 degrees around them. Other than that they are outstanding tanks for the $$
     
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  10. Ike
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    Ike Senior Member

    I don't know where you are getting this. Tank manufacturers and boat manufacturers have been pressure testing fuel tanks and fuel systems with air for generations, if not longer. Yes, you can do a hydrostatic test, but a simple test using very low air pressure is just as accurate and far simpler to do. I have performed them myself. I been to tank factories and observed them and also observed them being done a boat factories. There is nothing dangerous about it as long as you only use 1 to 2 psi.
     
    fallguy likes this.
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