Belly Gas tank and stringer design

Discussion in 'Boat Design' started by aaronhl, Aug 3, 2015.

  1. aaronhl
    Joined: Aug 2012
    Posts: 204
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Central Connecticut

    aaronhl Senior Member

    I am restoring this boat and before I put the floor on I want to mount a belly (underfloor) gas tank in the "ski locker" area of the hull. I will be removing the fake stringers in the middle and fabricating stringers that will connect to the transom with knee supports. The boat has a core down the center of the hull, am I able to rest the gas tank directly on the core using some ventilation strips?

    The custom tank will be approx 7 feet long x 10" wide on the bottom and 16" wide on the top. Also about 7" high.

    Can you experts give me some ideas on how to mount the tank into the boat so it can breath and move appropriately?

    I am also thinking of adding all the fittings in the rear by the bilge pump for easy access and the vent line to go towards the front of the boat out of the side. I would really like to be able to cut the tank out of the boat if I need to in the future too.

    [​IMG]

    [​IMG]
     
  2. gonzo
    Joined: Aug 2002
    Posts: 14,509
    Likes: 660, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Those are not fake stringers, but structural members. You can mount a plastic tank directly on the bottom, with some rubber padding. The gas gauge and fittings need to have access. That can be done with a small plate on the deck.
     
  3. aaronhl
    Joined: Aug 2012
    Posts: 204
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Central Connecticut

    aaronhl Senior Member

    Those middle stringers are just 1/8" thick fiberglass. Seems like all they do it hold the floor up.

    So you are saying it's okay to sit the tank directly on the core with rubber strips in-between? Should I use a tab on the gas tank to mount to a stringer?

    I just want to do this right so I don't have to cut the floor out again, but in an emergency I want to be able to cut it out, without taking the top of the boat off.
     
  4. Barry
    Joined: Mar 2002
    Posts: 1,319
    Likes: 153, Points: 63, Legacy Rep: 158

    Barry Senior Member

    It sounds like you have an option to put a tab on the gas tank to mount the tank. Which means that you are perhaps going to get one built??
     
  5. Barry
    Joined: Mar 2002
    Posts: 1,319
    Likes: 153, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Out of what material???
     
  6. gonzo
    Joined: Aug 2002
    Posts: 14,509
    Likes: 660, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The 1/8" stringers create an I-beam with the bottom and the floor.
     
  7. aaronhl
    Joined: Aug 2012
    Posts: 204
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Central Connecticut

    aaronhl Senior Member

    I see what you are saying about the stringer, If I was to replace them, I would build similar stringers except about 1" thick and mounted directly to knees connected to the transom.

    Yes, I am will be having an aluminum gas tank built. I don't know if I will need mounting tabs because I plan to sandwich the tank (with strips on each side) between the stingers, core, and floor. I will also make bulkheads at the front and back of the tank to connect each stringer so the tank doesn't move forward or back.

    I will also have a vent at the front of the tank and run the hose to the front of the boat.
     
  8. Barry
    Joined: Mar 2002
    Posts: 1,319
    Likes: 153, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Due to the length of the tank and the fact that it is aluminum, I would not use tabs welded to the tank to bolt or screw through the stringers. The boat will flex during operation and it is possible that the tabs will be subject to work hardening then cracking, ie leaks

    It would be better to install straps over the tank and bolt these to the stringers to negate these tab stress concentrations

    You require " a non metallic, non-moisture absorbent and non-abrasive material, neoprene, Teflon or high density plastic permanently bonded to the bottom of the tank where it contacts the bottom of the tank supports . (ABYC) This goes for the straps that you have to hold it in place. Permanently bonded by an impermeable, non-hydroscopic adhesive
    A polyurethane adhesive, preferably marine grade.

    You require 1/4 inch of space between the bottom of the tank and the bottom of the surface which is this case is the hull floor

    24.10.9 ABYC: Metal tanks shall be installed where they cannot be reached by normal accumulation of bilge water in the static floating position


    Filler hose must be attached by two clamps, minimum width 1/2 inch. Ensure that the clamps are stainless which is common but often the worm screw is not. The screw can corrode over time so buy the ALL stainless marine type

    Draw and vent lines, Single hose clamp are allowed but for hoses less than 7/16 the minimum band width is 1/4 inch and 7/16 through 13/16, minimum band width is 5/16

    It is a very good idea to buy the exact sized clamp, ie a 1/2 inch band width will not really work on a very small hose as the screw assemblies natural bend is too large

    Minimum vent diameter is 7/16 and THE SAME ON THE TANK


    Use only stainless fittings OR aluminum fittings, and for screws to hold the sender in place and the hose fittings. NEVER COPPER OR BRASS

    You will need baffles in the tank, perhaps 3 of them full height with openings at the top and the bottom. The open area of the baffles should not exceed 30 percent.

    You require a draw tube, ie you cannot hook the fuel line into a bottom fitting of a tank. If there was a fire, and the hose burned, the fuel would gravity feed into the fire. Also, ABYC requires an antisiphon fitting to reduce this risk. Aluminum fitted draw tubes are available at many marine supply stores

    All fittings have to be at the top of the tank including the fill. You might have some difficulty with your layout.

    There are new filler vent requirements that do not permit open venting of the tank to atmosphere. I believe that the Perko website has information on this

    Due to the length of the tank, I would recommend two vents, one at the front and the back. They can be common. This is due to the fact that say the vent is at the front but due to boat loading the bow is down, vapor trapped in the aft section of the tank can push fuel up the vent into atmosphere. You can put two top fittings in, tie them together then take on line up to the vent to reduce this risk

    Rubber hose, filler or fuel lines, can allow vapor to permeate through it which in a vented automotive application, is not a problem. For marine applications, purchase only
    marine rated hose.

    The filler neck, and tank require grounding "so that its resistance to the boats ground is less than one ohm"
    Not sure about this to a fibreglass boat but perhaps it has to do with static electricity.

    Several years ago, a fellow was filling a plastic 5 gallon gas tank in the back of his pickup and the static electricity ignited the gasoline vapor. Now the rules require the plastic tanks be placed on the ground before filling.
     
  9. aaronhl
    Joined: Aug 2012
    Posts: 204
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Central Connecticut

    aaronhl Senior Member

    Awesome information, that was extremely helpful

    I am afraid if I have a rear vent and need to fill the boat in the water, since the engine will be heavy the gas will be floating towards the back and leaking out the vent. The boat should always be level on the trailer so I don't see a front vent being an issue. The Front vent line will be about 8 ft feet long to go up the center vee and go out the side to a 90 degree vent fitting
     
  10. Barry
    Joined: Mar 2002
    Posts: 1,319
    Likes: 153, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Perhaps I was not clear. Certainly there are rules regarding where a rear vent deck/hull fitting that is mounted on the side of the hull with respect to the intake of the engine can be placed so vent gases cannot get into the engine compartment but this is not what I was trying to explain

    On the front top of the tank and the rear top of the tank, install the minimum 7/16 inch vent fitting, hook the vent line from the rear fitting and take it to a T beside the front vent fitting, and hook it to the front vent fitting. From the T, run it up to the hull/deck vent fitting

    I would purchase the deck/hull vent fitting before having the tank manufactured as many of the vent fittings are 5/8inch and you want your custom tank manufacturer to put in the correct fittings to match to your deck/hull vent fitting

    The vent also serves as one of the means to get rid of air in the tank when filling the tank. The vent line from the front tank fitting HAS to be ran so there are no low spots
    ( which can fill with fuel) which will then inhibit the air from getting out of the tank when filling. Otherwise the fuel can splash back at the nozzle or result in an extremely slow fill.

    While a common fill diameter is 1 1/2 inch, I would recommend if you have the room to use a 2 inch as this makes the tank much easier to fill

    The draw tube should be at the back of the tank
     
  11. aaronhl
    Joined: Aug 2012
    Posts: 204
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Central Connecticut

    aaronhl Senior Member

    Gotcha so you are saying to still run the rear vent up to the front of the boat. No I have to figure a way to run it where it doesn't drop down

    I think the mounting straps would be too complicated for the shallow area I am working with. Do you think I'd be fine if I supported the tank on 3/8" strips up from the core and had a front and back bulkhead to stop the tank from moving forward/back. But leaving maybe 1/2" of free air on the top of the tank to the bottom of the standing floor? Or do you think the tank will jump up and down?
     
  12. Barry
    Joined: Mar 2002
    Posts: 1,319
    Likes: 153, Points: 63, Legacy Rep: 158

    Barry Senior Member

    The tank could still move around.
    And yes, am still saying to run the rear vent, tied into the front vent so you still have one line going up to the deck/hull vent.

    Not sure how much room you will have beside the tank so perhaps a strap that wraps 2 sides and the top is not feasible, but perhaps a bracket across the top bolted to the stringer would work.
     
  13. Woobs
    Joined: Jul 2015
    Posts: 44
    Likes: 5, Points: 0, Legacy Rep: 49
    Location: Newmarket, Ont

    Woobs Junior Member

    I'm no expert but, the aluminum fuel tank on my Donzi is foamed in place. It absolutely does not move (also, very difficult to remove) and the foam supports the bottom of the tank.

    There is a specific type of foam that is used for this application.
     
  14. Barry
    Joined: Mar 2002
    Posts: 1,319
    Likes: 153, Points: 63, Legacy Rep: 158

    Barry Senior Member

    An interesting contradiction in the wording in the ABYC standards

    ABYC 24.10.11.3 "the fuel tank must be supported in the boat by means other than plastic foam"

    Then 24.10.11 "if a metallic fuel tank is encased in plastic, the tank material may not be a ferrous alloy"

    then 24.10.6 All non-integral tank supports , chocks, or hangers shall be separated from the metallic tanks surfaces by a non-metallic, non-moisture absorbent and non-abrasive
    material suitable for the purpose, (eg. neoprene, Teflon and high density plastics) permanently bonded to the tank surface with an impermeable, non-hydroscopic adhesive

    Woobs, Are you sure the bottom of your tank sitting on the foam?

    To me encased in plastic, might mean say a tank dipped in plastic, ie a thin coating to prevent corrosion. re 24,10.11 otherwise if they mean a tank totally surrounded by plastic foam, ie foamed in place, then they would violate 24.10.11.3

    If you were to foam in an aluminum tank and the foam bond was not 100%, you could get moisture at the interface resulting in accelerated corrosion.
     

  15. aaronhl
    Joined: Aug 2012
    Posts: 204
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: Central Connecticut

    aaronhl Senior Member

    Through my research I found it is bad to foam it in because of trapping moisture. AND the whole point of me ripping the floor out of this boat was so replace a couple hundred pounds of wet foam and floor. So no more foam is going in this boat

    Barry- do you have a link or picture of some mounting straps?
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.