Stringer Bedding, cushion or gap spacing

Discussion in 'Hydrodynamics and Aerodynamics' started by member74761, Aug 16, 2021.

  1. member74761
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    member74761 Junior Member

    Hello All.

    this is long winded, my apologies.

    New to here but been a silent reader a long time.
    My question is when you set a stringer or bed it.
    I have pondered and picked this apart for a long time and I guess I need some help grasping this.
    I understand the hard spot theory but.. this kinda gets deep and twisted.
    so I keep reading the approved method is cut square spacers and place under stringers.
    Then.. We go back and fill this space with PB. (and that is a hard substance)
    Now let me take this a step further. we don't fill the gap, we make a continuous strip of cushion, say foam, cork or rubber. which would be a royal pain keeping all that in alignment.(first off)
    I'm having a hard time "REALLY" getting the value of this "attempt"
    Now if you are talking about a very minimum thickness hull, I could see this especially if no foam was used.
    But,. If we are talking about an Old boat like a boston whaler or grady white or mako and I'm talking pre-mid 70's or earlier.
    The fiberglass is pretty thick on this era of boats.
    Is this extra effort really beneficial,?
    because. we also have foam that is pretty solid when its done right, you end up making a large solid interconnected grid that has very little give anywhere.
    And then we have the tabbing that further displaces "the foot print" of the stringer further tying things down and together, and I get the leaf spring effect of tabbing as well., but you slam into water eventually,. it will dispurse around and away.
    I myself would find it a better idea to lay 2 layers of glass under the stringer 2"-6" wide and set your stringer on this type of foundation.. by stiffening the hull and foot print further.
    And I say this because, when we say things like soft spots in floor. the hull flexes, the transom has excessive movement. and so on.
    All of these things or mutter of words typically leads to a re-build or the retirement of a boat.
    but then we say we need to build flex or give in our stringer setting.
    I just don't get it.

    thanks in advance.
     
  2. ondarvr
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    ondarvr Senior Member

    As long as the plywood doesn't touch the hull you achieved your goal, the rest of it, while not irrelevant, doesn't add much.

    There are a couple of things that come into play, one that's frequently overlooked is the shrink factor. If the plywood is touching the hull, then you glass over it, it forces the the plywood tighter against it as the laminate shrinks. This creates even more of a hard spot by preloading it.

    Most small boats don't bed or space the plywood off the hull, they place the stringer grid directly on the hull and glass over it. Even though this breaks all the rules, it doesn't normally cause a problem.

    On a much larger hull sides it can be far more noticeable.

    Bedding it in putty helps by uniformly spreading the load along the full length of the stringer and gives it a slightly wider base.

    Since it's not possible to cut the plywood for an exact fit to the hull, you'll always end up with point loading in a few places along it's length.

    Again, on small boats it doesn't create much of a problem no matter how you do it.
     
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  3. member74761
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    member74761 Junior Member

    Ok, thats pretty much how I grasped things, well except the shrink factor.(I did not figure that)
    Don't you normally bed and contour radius, let that dry. and then tab and full encapsulate afterwards. (because things would be a greased monkey trying to do all at once without a jig to hold things in place)
    My boat is basically 8' x 20' , so a larger small boat, not a 15 footer and not a 30'
    I am taking great pains to match the hull just because thats my nature.
    My idea is its less PB and just better workmanship. nobody will know but me.
    But whether you have a 1" or 1/8" of PB the idea is its solid with 0 gap/full bedded contact. did I get that right?
    trying to get this right before I'm knee deep and find I have really goofed.
    But you did ease my mind, it makes little difference, and that was my feelings on this.
    After I wrote all this. the light bulb may have flickered.
    I guess you could bed with like a really good silicone, that makes the most sense to give this said cushion,. and after saying that you could radius the contour with that that now in a twisted way that adds up to give flex. and still be held in place by tabbing.
    The PB is hard and the plywood is hard. and those 2 to me, either 1 is a hard spot.
    So is the mud clearing in this way of thinking.

    Thanks for the response.
    it made me feel like I hadn't completely Lost It.
    and then I wonder
     
    Last edited: Aug 16, 2021
  4. ondarvr
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    ondarvr Senior Member

    Flexible glues, adhesives and bedding compounds can work, but resins don't typically stick very well to them. So if you get any on the hull surface you're bonding the laminate to it will most likely come loose.

    20' is in the small boat range.

    If the plywood cut 1/32" off from perfect it makes no difference than if it was 1" off.

    Since it makes little difference in how you do it, do whatever it is that makes you sleep well at night.
     
  5. fallguy
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    fallguy Senior Member

    What you are seeking to avoid is a single contact point of bh or stringer to hull.

    it is especially important in solid frp layups and on the sides of hulls of any kind

    For a moment consider building a bulkhead that has a 2mm glue gap all around. Let's say it is plywood. Now, let's change that to one side of the bh has a 5mm hump on it in one spot and when you install it; you force it. You have created a hump in the hull that is nearly impossible or darn difficult to fair out. You'd need to fill all around that spot and it may never fair to perfect..

    The best way to think about padding is will the bh/stringer push onto the hullsides; ever.

    Consider a 3/4" thick ply bulkhead on a 1/2" hull of any type of material. If the bh is big anywhere; it will push the hullsides out. That will show overtime and you will see the outline of it.

    It is all about keeping the bh/stringers off the hull to avoid that hump or many of them.

    On my build, I used padding in on place where a foam bh pressed against an frp hull with about a 0.200" thickness. The padding is 2" wide and the 1/2" bulkhead was glued to the padding which will disburse the load over 2" than 1/2".

    This picture explains better what you are seeking to avoid by gapping and gluing. A classic hardpoint where the stringer makes contact with the hull in a single location. When forces are applied above; the stringer may/will begin to show through the hull. Solid hulls are more prone to this problem than foam I have been told.

    stringer.jpg
     
  6. TANSL
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    TANSL Senior Member

    Of course, that is very good advice, but if the solution you adopt is structurally adequate, you will be able to sleep in your boat without danger of the structure collapsing. :p
    @fallguy , nice picture. What does it mean?
     
  7. fallguy
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    fallguy Senior Member

    the picture is just a cutaway of a hull and a stringer and I am explaining the concept of what to avoid laying stringers..hardpoints
     
  8. TANSL
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    TANSL Senior Member

    Sorry, I still don't understand if it is a longitudinal or cross section or what it is. I don't know how to deduce which is the stringer and what is everything else, I don't see the hard point, I don't know what a "laying stringer" is. But don't worry, it doesn't matter, forget my question.
     
  9. member74761
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    member74761 Junior Member


    Fallguy, The sketch you drew showing the point of early impact, or constant preloaded pressure is what I spent so much time sanding, grinding, shaping trying to avoid. but it takes A LOT of time..
    And I realize you can't get this perfect, there is too many changes in the hull, but you can get it fairly close.
    I had the hard spot mentally as something else. (the point along the entire stringer, being solid and the hull flexing along this rigid edge on both sides)was my thinking.
    After the feedback from here. I think I am going to change my direction. with shrinkage and temp. changes considered and everything else.
    I am leaning towards 3M 5200 for bedding. if that gets a stamp of approval.. I have never used the stuff so I have no idea how stiff it is. but reading it seems to be maybe a better solution. AND... I won't have to mix so much cabisil with epoxy.(thats always a ++++)
    I think the light bulb went from a flicker to a faint, dim glow, just barely red.
    Let me know if my new direction is a better way.
    Appreciate it.
     
  10. fallguy
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    fallguy Senior Member

    I really do not like 5200 here. The problem is the setup time for bonding something like a stringer is just too long. It will take days to set properly under a 3/4" wide stringer.

    The proper way is to make the stringer so it does not contact the hull and then use epoxy pb under it and on the edges as fillets. This is a one or two day process. You can weld the stringer with an 1/8 gap all around to the hull with hotglue. Then trowel epoxy pb into the margin, then a fillet and tapes can allbe done in one day.

    I would not trust 5200 until is cures for a week before using pb and tabbing. I am also not certain you will get good bonding between glass tapes and the 5200.

    Any pressure from the sole or hull will be more balanced across the thickened resin versus the single point I drew as an example. Probably would be about 3/4" at say 10 feet or 90 square inches versus say 3/4" in a single spot or say 1 square inch. The psi difference is 90 times.

    So a 180 pound load from above is disbursed at say 2 psi in the pb example versus 180 psi if the stringer is setup with a contact area of one inch.
     
  11. fallguy
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    fallguy Senior Member

    You are not trying to have a flexible joint. You are trying to build a joint that disburses load evenly.
     
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  12. fallguy
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    fallguy Senior Member

    Another option is a foam pad. A marine foam pad say 2" wide can be cut with 45 degree edges. Then the loading from the stringer or bh is applied at 2" wide versus 3/4". In my 10' example, that becomes 240 square inches. The 180 pound point load is then 0.75 psi.

    The argument can be made that the loading on the pad is 2 psi, but the pad loads to the hull less....
     
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  13. member74761
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    member74761 Junior Member

    Ok then,
    Well, that puts me back at my original plan, and thats fine too.
    I had that mindset for so long it won't hurt my feelings to go back to it.
    thats what I was most comfortable doing anyway.
    thanks


    this is kinda what I had considered doing, but not with foam, with 2 layers of fiberglass.
     
  14. fallguy
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    fallguy Senior Member


  15. fallguy
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    fallguy Senior Member

    Solid fiberglass would not be used in this way. The padding in the drawing is there to appreciate any loading variations.

    The padding is made from 4-5#/cuft density marine foams. It is sacrificial. You could even use xps here, but don't tell anyone I said it. Thickness of pad same as BH, width of pad is about 2-4x the bh thickness.
     
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