Proper way of bedding stringers to fiberglass hulls.

Discussion in 'Fiberglass and Composite Boat Building' started by holorinhal, Feb 15, 2018.

  1. holorinhal
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    holorinhal Junior Member

    I have an 18 ft Advantage Jet Boat and am in the process of replacing the stringers, bulkhead and transom.
    Question is on bedding the stringers to the hull.
    On previous projects, I have bedded the stringers with a few layers of CSM and polyester resin, weighted down, to cure.
    On researching process, for bedding, I have been reading of other methods, and ensuring the wood stringer is not bonded hard to the hull. By shimming the stringer from the hull with foam or rubber pads the filtering and tabbing, to bonding with materials such as construction adhesive, such as liquid nails or Pl adhesive, to epoxy and Cab.
    The stringers, when removed, wet bonded with a hard translucent resin type material, but was not solid, almost as if it was parted in from the side.
    So my question is what is the “PROPER” way to bed or bond the stringers to the hull?

    B80458F7-79A4-44C9-8A7C-A7932BB86DE9.jpeg C39FD043-C008-4E39-95F6-2FA792070536.jpeg 2D678582-5768-4A36-9AE3-2E3252BEAF2E.jpeg
     
    Last edited: Feb 15, 2018
  2. Yellowjacket
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    Yellowjacket Senior Member

    The idea is to prevent stress concentration where the stringer or bulkhead meets the skin.

    Realize that the stringer or bulkhead is stiff in the direction perpendicular to the skin, which is (relatively) flexible. If you don't tab the stringer the skin will flex and the stress of that flexing will be high right at the junction with the stringer. The tabbing should taper in thickness as you go away from the stringer. The idea is to increase the skin stiffness the closer you get to the stringer and this added thickness reduces the stress at the edge of the stringer so now you don't a stress concentration and resultant cracking at the edge of the stringer or bulkhead.

    You don't necessarily need the strength of cloth here, so using CSM is ok, but you want it to be layered so the thickness increases as you go toward the stringer. Depending on how much pounding you're going to see and how stiff the stringers are relative to the skin determines the thickness and how wide the tabbing needs to be.

    In your case I'd look at similar boats and figure out what works in terms of total thickness at the stringer and taper it out over an inch or two and you should be in good shape.
     
  3. holorinhal
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    holorinhal Junior Member

    Yellow jacket, thank you for that explanation.
    I do understand the tabbing process.
    I have always tapperd/ staggerd my tabbings, but never real have been explained the reason in relation to stresses.
    I was explained and taught that the stagger was for each successive layer to get a new bite on the hull, so it is good to know it’s relation to flexing stresses.

    However , my question is aimed at the initial setting or bedding or bonding of the wood part/ stringer, to the fiberglass hull, before the tabbing process is taken place.
    So I am referring to the joint between the bottom of the wood stringer and the fiberglass hull.

    What is acceptable in an 18’ v hull
    To bond the stringer to the hull? Pl adhesive, liquid nails, thickned resin?

    Should there be a gap at the joint, between the bottom of the stringer and fiberglass hull?

    I past projects the stringer was bedded in strips of resigned CSM and the stringer set down on top and weighted, until cure.

    In recent research I have been seeing that this may not be the correct way.
    I have seen that the method should be to bedd the stringer with a foam or soft pad, creating a non hard contact between the bottom of stringer and fiberglass hull.
    In essence leaving a gap, filled with Pl adhesive or epoxy or poly, resin thickened with cabosil or talc.
    Is the same principles applied to main bulkhead members, too, having a gap, all the way around the bulkhead, even at the bottom?
     
  4. holorinhal
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    holorinhal Junior Member

    In boats like mine, of my size and erra, it was and is common for the stringers to be covered or over wrapped with 1 layer of 7oz CSM and two layers of 6oz cloth.
     
  5. fallguy
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    fallguy Senior Member

    Another thing you can consider is using a pad under.. The stringer tabbing is typically a few layers and I would probably do something like 8 - 6 - 4", or 8 - 6 -6 or 6-6-4 with overlaps of 2" min. Some of the other posters here would advise you better, however.

    Here is a link that shows a pad. The pad is a way to spread the load to avoid hardspot.

    Foam Phantom Building Method | Bateau2 https://bateau2.com/howto/foam_phantom.php

    EDIT: overlaps of 2" min just means put 2" min on the stringers and 2" min on the hull, sorry if this was unclear
     
    Last edited: Feb 16, 2018
  6. fallguy
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    fallguy Senior Member

    The padding is to avoid the bulkhead or stringer showing through overtime, but probably also has structural benefits I haven't been taught like a NA might have been. Type of hull is a factor. I was told my foam hull will probably not show through at the bulkheads.
     
  7. ondarvr
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    ondarvr Senior Member

    Most small boat builders don't do anything to bed the stringers, they just drop the wood right onto the bare hull and glass over it.

    So starting from that point, anything you would be doing is a potential improvement, but not all that important or critical.

    The only potentially important aspect is to not create a hard spot, so that means 100% contact, or zero contact with the hull.

    Glueing the stringer to the hull is unimportant, and using a flexible adhesive of some type does nothing.

    So you can improve upon one of the very few things that high volume builders do to cut man hours and speed up production that doesn't appear to cause the buyers of these boats problems.
     
    Last edited: Feb 16, 2018
  8. fallguy
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    fallguy Senior Member

    why are your stringers shiny and black? hope you plan to sand them
     
  9. PAR
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    PAR Yacht Designer/Builder

    Place your stringers where you want them, then draw a line around their perimeter or use some tape, to define the "landing" area of each stringer. Apply a semi-flexible goo, like 3M-5200 or 101 or Sikaflex, but not construction adhesive (too hard) in the taped off area. Place the stringer on top and weigh it down into the "bedding", leave to cure. When it's cured, you're ready for tabbing.
     
  10. ondarvr
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    ondarvr Senior Member

    In a glassed over stringer the bonding properties of a soft product used to bed the stringer is almost irrelevant. Due to the high elongation of these soft bedding adhesives the entire load is place on the laminate, so the laminate needs to fail before the adhesive comes into play and offer any potential benefit, but being the weaker of the two, if the laminate failed, the adhesive will fail right away too. If the contact patch was larger under the stringer it may help, but on a typical plywood stringer there isn't enough area to handle the load.

    Not that they're bad to use, it just doesn't add much strength, so the actual bedding product, if one is used, isn't that important.
     
  11. PAR
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    PAR Yacht Designer/Builder

    I agree, though gluing the stringer initially with a semi-flexible goo has it's advantages, if only a convenience, not related to the structural requirements. Additionally, the "bedding" can be used to make a fillet, again to make the installation go easier.
     
    rxcomposite likes this.
  12. holorinhal
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    holorinhal Junior Member

    Lol, that is the old stringers. I had previously Gelcoated them black, for Astetics. Those stringers are now cut out of the boat.

    The new stringers, will be either wood to show through the glass or a carbon fiber overlay, for astetics only.
     
  13. ondarvr
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    ondarvr Senior Member

    My comments were more towards DIY guys that think glueing stringers to the hull with pricy adhesives is helping to make it stronger.

    It's good to have something there to bed the stringer, but since nothing is used from the factory, anything is an upgrade.
     
  14. fallguy
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    fallguy Senior Member

    Bed the stringers in cabosil and epoxy, the same as the filleting material like ondarvr suggests.. I like to spot hotglue bulkheads and force the glue into the joint. Or like PAR suggests, you can spot the stringers with something more flexible. Stringers you need some other method of holding in place of course, hotglue would be a pain. So make a line where they go for ref and put the glue down. Think I'd make a batch of about ... 9-12 ounces of epoxy and I use fast for bulkheads, but you can go more like a slow for getting the stringers set. Once they are set, run the fillet maker over them and trowel them on off on each side. I trowel about 4" each side. I epoxy a washer onto a stick the size I like to make fillets. I think mine is a one inch washer, or it might be 1 1/4", can't remember...might have two. While it is setting up, you can wetout the tapes with slow epoxy on a table and bring them into the boat on a board or cardboard. If too much time goes by, prewet the fillet areas with a 3" roller and plain slow epoxy; otherwise the tapes tend to stick so well you can't move them. I cut my 3" rollers from 9 1/4" rollers on a bandsaw. Use some thickster gloves and put out the tapes as quick as you can using your hands to lay them. Bring them all into the boat on a board. Have some paper towels in the boat as well and a bucket for trash. After you finish one stringer; do the other one.. A helper is nice as you will be pushing the time envelope a bit.

    I was pretty tired when I posted, so I made some edits this morning.
     
    Last edited: Feb 17, 2018

  15. ondarvr
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    ondarvr Senior Member

    He's using polyester resin, so epoxy is not what he would want to use, he can do the same thing with polyester though.
     
    fallguy likes this.
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