Help with Replacing the Belly Tank in a 25' 72 Sea Ray

Discussion in 'Boat Design' started by Bugmanx, Jul 14, 2004.

  1. Bugmanx
    Joined: Jul 2004
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    Location: California

    Bugmanx New Member

    I need to replace the belly tank in a 1972 25' Sea Ray. An old college buddy and I used to take this boat to Catalina all the time to dive for lobster. We started smelling gas and cut out the original tank, about 90 gal. We ran a temporary on deck tank for a while. I want to put another tank back in the belly. Years later I can't remember what the original setup was like.
    There is a long cutout about 12" by 72" long and the depth varies as the hull tapers. The entire bottom surface is the inside surface of the hull, so I don't want to mount anything directly on it. I guess I should have the tank made out of stainless for best results. I can't find a plastic tank that gives me the maximun capacity while fitting inside the hull.
    I can't design the tank until I know how I need to mount it.
    Any help will be greatly appreciated.
    Jeff
     
  2. dereksireci
    Joined: Jun 2004
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    Location: South Carolina

    dereksireci Senior Member

    Aluminum Tank

    Jeff,

    The easiest thing may be to install a custom aluminum tank. There are many tank builders who can build a one-off. Stainless is not a suitable material for fuel tanks in most cases due to the fact the alloy undergoes changes during welding. Make a template of thin plywood of the forward and aft sections of the compartment. I usually have about a 6" flat on the bottom so it doesn't come to a point where contaniments can collect. It's also good for your pickup(s) to be over the flat. Let the tank guy suggest where to put baffles.

    Now lay out the top of the tank: fill, vent, bonding wire tab, sender and pickups. There are many acceptable ways to mount the tank, the best depends on the boat and the preference of the installer. One way is to size the tank so that you can lay it over some a non-metallic, non-moisture-absorbent, non-abrasive material suitable for the purpose, e.g., neoprene, Teflon, and high density plastics. I've seen a piece of scrap rub rail used. Make a drawing and send it to tank builders for a quote. If you're scared, make the thing out of 1/4" plywood to make sure it will work out the way you have planned. It should not support anything. Some people like to primer the tank, others think it's a waste of money.

    One way to attach it to the boat is to have the tank builder weld L-shaped material to the top. The other leg of the L is vertical and has holes drilled to accept screws that go into the stringers. There are other good ways too. In any case: Fuel tanks shall be installed and restrained so that the fuel tank does not move at the mounting surface more than 1/4 inch. Some people pour in foam, some gasp at the idea. Foam makes it a challenge to remove the tank but the tank will not move if it is foamed in. I'm sure others will add some good ideas to help you.
    djs
     
  3. Bugmanx
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    Bugmanx New Member

    Sounds good, what about corrosion?

    Thanks Derek,

    Your suggestions gave me some good ideas. The original tank was steel.
    I've heard the stories about foaming in aluminum. Holding moisture against the tank causing the aluminum to corrode through in a short period of time. I guess if I could seal the tank air tight then no moisture woud condense against the surface. I was orginally thinking that I would make a removable cover for inspection and service rather than sealing in the tank, but I'm not completely against sealing the baby in. I like the idea of having the entire bottom surface of the tank resting on the bottom of the hull so the weight of the fuel is being evenly distributed. 720 lbs of fuel bouncing up and down might cause a problem if the tank is suspended with an air gap around it.
    What about the sides? Should I put neoprene or something in between to prevent chafing there as well?
    Thanks,
    Jeff
     
  4. dereksireci
    Joined: Jun 2004
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    dereksireci Senior Member

    Tank Supports

    I don't suggest mounting an aluminum fuel tank hard against the hull because in rough conditions the hull may flex in this unsupported area in between bulkheads. It will be nearly impossible to match the shape of the hull exactly with the tank. Therefore it will only make contact at certain points and that's where the problems will occur. Better for you to decide the contact points. If you place the supporting material under the baffles and at the ends of the tank it will work best. You can usually tell where the baffles are by looking closely at the tank, and verify it with the builder.

    As for the sides, neoprene won't hurt and hopefully your mounting technique will prevent the tank from moving and contacting the stringers.

    djs
     
  5. Jones

    Jones Guest

    Do not use neoprene on an aluminum tank. The contact areas will collect water and corode. Aluminium needs either air or the absolute absense of moisture. If You intend to foam in the tank, you should first prep the aluminum and coat it with an epoxy primer and urathane top coat. You are much better off cutting 2" wide strips of 1/4" plastic and aranging them in a herringbone pattern down each sheer pointing aft tward the keel every 10-12". Leave the keel open. Bed the strips in 5200 on both sides and set the tank. The tank now is in contact with air except where it is sealed to the hull. The weight of the fuel will hold the tank in place. Allow 7 days for the 5200 to cure. If you are a sceptic fab up some blocks for the sides of the tank and attach them in the same fashion. Pardon the spelling and don't rely on any of my advise. I am drunk or at least wish I was
    High and dry
    Stuart
     
  6. lprimina
    Joined: Jan 2004
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    lprimina Senior Member

    Alright stupid question, What about a fiberglass tank? would that work out for him?
    I have been thinking about using fiberglass tanks. I got the specs from the coast guard and asked some people at Hatteras (supposedly you use special cloth and resin).
     
  7. dereksireci
    Joined: Jun 2004
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    Location: South Carolina

    dereksireci Senior Member

    FRP Fuel Tank

    A fiberglass tank would work fine. They are built with special fire retardant resins. Since in this case only one tank is required, the extra step of building a mold for an frp tank is not justified in my opinion. I'd go with aluminum.
     
  8. Terry
    Joined: Feb 2004
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    Terry Junior Member

    Aluminum is your best bet. 3/16 5086 and you can weld it up in just a couple of days. Just make sure you pressure test before you put it in the boat AND DON'T USE FOAM!!! Well made FRP tanks are good, but they take a long time and the cost of the resin will make you choke. I've got 30 yr old aluminum tanks on my WellCraft and there is absolutely no signs of corrosion. They have good air flow all around. Good luck
     
  9. Bugmanx
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    Bugmanx New Member

    Air Circulation

    Hi Terry,
    How do you achieve the air cicuclation around the tank? What do you think about Jones's comments about suppoerting the tank with 2"X1/4" strips of plastic.
    How did you achieve air circulation around the tank while providing support to the hull?
    Jeff :cool:
     

  10. Terry
    Joined: Feb 2004
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    Terry Junior Member

    My fuel tanks are wing tanks mounted up high, so the installation is different than yours. I would go with 1/2" thick strips of hard rubber to support the tank or you could make a cradle out of wood. The main thing is to not let water accumulate anywhere around the tank and to keep the tank from shifting. 33 CFR Chapter 1 Part 183 has all of the Coast Guard Regs regarding boats and tanks.

    Another option is a plastic tank. It will need more support than an aluminum one, but will not corrode. Ronco Plastics is the best supplier that I have found with the most shapes and sizes and reasonable prices. They will even install the fittings where ever you need them. I haven't used them for fuel, but I have used their tanks for holding tanks.
     
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