Help Needed with stringer mods to fit Underfloor fuel tank

Discussion in 'Boat Design' started by stevenway, Feb 23, 2023.

  1. stevenway
    Joined: Feb 2023
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    Location: australia

    stevenway Junior Member

    Hey guys

    im after some advice and help i want to fit an under floor fuel tank to my 5.5m leeder , I have cut the floor open where tank needs to go but I will need to remove a bulk head for it to fit , I have noticed my stringers and bulk heads are entirely hollow fibreglass and i am m not sure how i would go about strengthening these to allow the removal of bulkhead any assistance or advice would be great photos attached
     

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  2. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    Its there for a reason, its a main structure part, even when hollow. Better off with 2 smaller tanks that fit either side. Personally, I would not be cutting it out, and i would be repairing the hole you put in it.
     
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Well, as long as the transverse BH is just between continuous longitudinals and the hull skin is not discontinuous as well; you can stiffen the entire section with more laminate. You will also need clearance for tank fittings and can laminate a spanner across the top to support the sole in that margin. Make it hardwood and it can offer some work as well.

    I'm not going to spec the layup for you, but I'd probably go 1/4" thick minimum which is a lot and I'd wrap up the sides of the remaining transverse beams and stringers on one inch staggers, so for 6 layers of glass; something like 2,3,4,5,6,7" overlaps or at least 2,2.5,3,3.5,4,4.5. I expect you'll do less.

    That transverse BH looks pretty beefy. It is hard to consider removal for me, but since you seem intent on doing it; that would be what I'd do. If it is not all that thick; you could probably do a bit thinner on the fix.

    Some of this also depends on hull thickness, boat speed, typical operating conditions... For offshore, hard use beef it up to good and thick..
     
  4. mudsailor
    Joined: Mar 2013
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    mudsailor Junior Member

    This is on a little 5.5m boat…….that transverse is doing basically nothing unless you go wave jumping…..and even then if it broke you probably wouldn’t even notice………cut it out and glass the stringer where it’s cut out and it will be fine
     
  5. stevenway
    Joined: Feb 2023
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    Location: australia

    stevenway Junior Member

    Hey guys, I know you guys said it was bad idea but I have cut out the transverse beam and part of another one to be able to get my tank to fit as attached now for the repair I am unsure if I should be using chop or mat and how many grams , polyester or epoxy if you guys could give me some advice would be great. The videos I have watched talks about using 1708 but in australia that doesnt help me much thanks 20230304_151208.jpg 20230304_151143.jpg
     
  6. fallguy
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    fallguy Senior Member

    600/225 is the same stuff

    how much vertical clearance do you have after hooking up hoses and where is the fill and vent going? You don't want the fill to be lower than the vent and you cannot vent inboard....

    but if it were me, I'd buy some 6" wide 600/225 tapes and lay them at the area removed and front and aft where the tank sits

    you only have 100 hp, so that boat is not screaming and pounding that much

    if you have some vertical clearance, then 4-6 layers of tape to replace what you removed, or simply patch the holes and add some tabbing at the bottom of the stringer

    but first talk about the fuel system and tell us if you have space; otherwise it is sort of pointless
     
  7. fallguy
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    fallguy Senior Member

    Don't cut a hole in the top of the stringer for the fuel lines!
     
  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    As per Fallguy
    Where is the fill hose access?
    I would keep the vent to the front of the tank unless the vent hose is attached at some point to the filler hose close to the top of the fill. With this comment I am assuming that the boat will sit slightly bow up at rest and when planing.
    You want to have the air space in the tank open to atmosphere through the vent line. Well. to the cannister with the new rules
     
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  9. fallguy
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    fallguy Senior Member

    Don't you have the tank orientation backwards there friend?

    You don't want to be running on empty with the fuel pickup in front..
     
  10. stevenway
    Joined: Feb 2023
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    Location: australia

    stevenway Junior Member

    Hey guys , I would have about 40mm above hoses to the floor. Thanks for the reply. So my idea was to some how have the hoses running to back of boat and have breather higher then filler neck. I have drawn a rough photo of what I was thinking might work but I would need to have a hole in that rear transverse beam

    cheers boat fuel tank fuel lines.png
     
  11. stevenway
    Joined: Feb 2023
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    Location: australia

    stevenway Junior Member

    I also didn't realise the filler would need to be higher then the breather in my head I thought it would need to be the other way round otherwise fuel would leak from breather when filling
     
  12. stevenway
    Joined: Feb 2023
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    Location: australia

    stevenway Junior Member

    The fuel pick up has a long hose inside that sits about half way in tank. I could possible make this pick up hose longer to accommodate? I kind of need the tank this way as there is more clearance to have a remote fill; otherwise I would have to fill from a hatch inside boat which would mean spilled fuel would go into deck. Def open to suggestions
    cheers
     
  13. fallguy
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    fallguy Senior Member

    Only remote fill, a spill into the bilge is bad.

    Fill is higher, swan neck the vent... if you overfill, the excess fuel runs out the vent and you know to stop.

    Access panel is required above the access points in the tank.

    Leave the tank as planned. The pickup should be at the aft end; generally.

    For the repairs, get some 600/225; the concern now is that you return integrity to the stringers.

    Pull the tank and post a picture, please.

    Is that a cored hull? I see some margin between the outer hull skin and the inner hull skin, unless it is just where the BH was...you don't want to leave that open.. Or maybe that was a stringer web they placed... at any rate, get us a picture..
     
  14. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    In general terms, assuming a stringer web and a gap, all the gap gets filled with epoxy putty made with fumed silica. Then the hole you made removing the transverse BH gets filled with epoxy putty at about 1/4" each hour, then you can clean it all up and glass up the sides of the stringer across the entire area. So, give us the dimensions of the bottom and distance up the sides and a picture.

    Basically, you need to rebuild the stringers because you have greatly weakened them.

    I would use epoxy for the repair. If you know any glass guys; then you could use an ester, but I'd avoid that for amateur hour and spend money on 60 minute epoxy.

    I can do a pretty good estimate of the resin and glass needs for this small job. You'll need to make the cutaway on the stringer super flat, so grind away anything that won't allow a 2' level to lay flat across the hole in the stringer.
     

  15. Barry
    Joined: Mar 2002
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    Barry Senior Member

    The problem with the vent is THAT IT HAS TO HAVE NO-NONE-NADA-ZIP LOW SPOTS BETWEEN THE TANK AND THE VENT or fuel will find its way in ( a little rough water) and inhibit venting. From the factory our last boat had a bit of a dip in it and the final 1/4 of the tank would take 15 or more minutes to fill, the fuel would back up out of the filler and generally create a mess. The dealer finally after two attempts did a reroute and they say that now it will fill. That being said, proof will come this summer when the boat gets in the water. This is extremely important.
    I am concerned that you will have the same issue with the fill going all the way to the back as well. It will be difficult to keep a smooth slope throughout that distance.

    You would be better off to put the fill on either gunwale. The 1 1/2 inch id hose should be able to be ran up to this point along side the vent. It APPEARS that there is enough width to make up the joint. Take a wire and run a ground from the stainless filler to the top of the tank to a common ground. Even if you have to create a wide spot at the gunwale to make this happen. You could build a bit of a dam to ensure that any overflow goes over the side as compared to into the boat


    On any of the longer tanks that we built, we put in two vent fittings on top of the tank. One at the front and one at the back. The front to back was connected to each other and the highest vent tank fitting was then T'd into the line to the vent
    fixture on the hull. The vent fixture on the hull can be put on the side of the boat. The screen turned rearward and down. Mind you, I am not sure what new rule vent fittings for the side of the boat look like. Do not forget Fallguys comment, put a loop in the vent line to keep any water out that may splash up to the vent

    The vent Id is normally 9/16 or 5/8 inch, the vent spud will determine this.

    Also, I did not see a fuel filter on your draw line. I would recommend a cannister, screw on type as compared to the relatively ineffective inline plastic units.
     
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