How strong is fiberglass/plywood sandwich plate?

Discussion in 'Materials' started by Bmsluite, Dec 26, 2023.

  1. Bmsluite
    Joined: Dec 2023
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    Bmsluite New Member

    Hey guys I'm looking to reduce the thickness of plywood for my build. For the forces I am dealing with I need 12mm Okuame plywood.

    Could I reduce this thickness if I fiberglass the material? Could I get it down to 6mm if I use three or four layers of glass? What about 3 layers of glass and one layer of carbon? My plan is to use surfboard resin.

    Would this 6m.composite sandwich material be just as stiff as the 12mm plywood? Would it be even stiffer to the point where I could drop down to 4mm?

    The only things I can find on how strong fiberglass plywood is is by watching some random YouTube videos where people test it in various way. These are usually impact resistance tests. Not stiffness tests.

    Please let me know if you have experience with fiberglassed or carboned marine plywood. Thanks.
  2. ondarvr
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    ondarvr Senior Member

    What made you choose surfboard resin?
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    +1 re Ondarvr question re 'surfboard resin' - if you are building in plywood, using polyester resin is usually a false economy in the long run - you would be much better off using epoxy resin.

    And what type / size of boat are you going to build?
    Can you post some details of it please?
  4. gonzo
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    gonzo Senior Member

    The stiffness of a panel increases with the cube of the thickness. That means that the same material would be 0.125 as stiff if the thickness is half. Another way of looking at it is that the materials would have to be 8 times stiffer to be equivalent. However, there are other considerations. For example, the toughness of the thinner panel will most likely be lower. This means that it may have less resistance to break with impact. Thin laminates are tricky. You will need to vacuum bag and use some better suited resins that what you propose. To start with, what is the goal of a thinner laminate?
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  5. Bmsluite
    Joined: Dec 2023
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    Bmsluite New Member

    Yes, I know thickness is everything. I also know that most of the stress on a part is on the exterior.

    What I am after is stiffness for the least paosible weight. And some impact resistance which is why I wouldn't even consider a foam sandwich style plate for this.

    This isn't for my boat or canoe. This is actually for my time attack car. Ground clearance and weight is everything.

    I am making a new diffused splitter

    This last one, pictured below, I made using techniques I learned from boat building. However, it weighs about 45 lbs in an area where I really do not want weight.

    I figured this time I could go thinner if I just layer it up with fiberglass or carbon. Its stiffness to weight ratio that I'm after.

    As for the epoxy, I was recommended the surfboard resin by a carbon fiber distributor. If that's not what I'm to be using please let me know what I should be and a decent quality brand to use.

    Thanks for the help

    Attached Files:

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  6. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    Take a mold off the 45 pounder
    and lay up a lightweight E-glass
    diffuser splitter from the mold.
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  7. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    If you want stiffness,go carbon.It will allow you to mould in those cute little diffuser ramps without the step that is currently not helping at all.Do those teardrop shaped blocks have to be there to ensure compliance with ground clearance rules?You could add stiffening ribs to the top surface,where there is clearance to the mechanical parts and that would give you some of the stiffness back.I'm guessing that the row of double holes is the fastenings that attach the splitter to the lower flange of the front bumper moulding and if you are generating real downforce,it might need to be restrained from flexing or your arrival of downforce will be inconsistent.Wrapping plywood may work but I would suggest you use a sheet of melamine faced chipboard as a surface to mould on as it will give a better finish.Ideally you would vacuum the ply onto the initial carbon and you could easily include the ramps by sticking down shaped blocks with double sided tape.You might even be able to add end fences for a bit more efficiency,if you have data about shape and location.
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  8. Rumars
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    Rumars Senior Member

    You are asking for an empirical solution and that's only possible by trials. Take your existing plate, put it on two sawhorses, pack weight on top and measure the deflection. Make another plate with a different thickness, measure deflection, add laminate, measure again, stop when you are satisfied.

    Your biggest problem is impact resistance and you must quantify that somehow, because otherwise it's hopeless. To determine how much impact resistance you need make a few small samples, bolt them under your car, drive a typical course, inspect for damage, decide on the minimum layup you can live with.

    My advice: make a few samples using end grain balsa 12mm sandwiched with carbon fibre. One 200-300gr/sqm plain or twill, one 400gr/sqm biax + 100gr/sqm plain or twill, and one 600 g/sqm biax + 160gr/sqm plain or twill. Bolt under the car, see what you like best from an impact perspective. Then make a full size plate and measure deflection. If it's to much, up the core thickness to the next size, repeat.

    There are more esoteric ways to get impact resistance like sheathing a nomex carbon sandwich with Al or plastic, but that's more complicated.
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  9. gonzo
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    gonzo Senior Member

    I think wood is hard to beat on stiffness to weight.
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  10. DogCavalry
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    DogCavalry Senior Member

    And really really hard to beat on performance per $.
  11. Barry
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    Barry Senior Member

    You could consider an aluminum additive manufactured piece.

    The scanning can be done with a hand scanner of the component but with the advantage of compound curves AND being able to add in internal ribbing/scantling/stiffeners to achieve the
    rigidity that you need.

    You can vary the thickness depending where you need to, ie attachment points etc
  12. wet feet
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    wet feet Senior Member

    I suspect an additive manufactured item in metal of any sort will give a bit of a boost to static downforce and polar moment of inertia.Which may well rule it out for this purpose.
  13. Barry
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    Barry Senior Member

    "I suspect an additive manufactured item in metal of any sort will give a bit of a boost to static downforce"
    I would think, but without knowing the precise details of the diffuser, ie internal stiffening and reinforced mounting points, that an aluminum diffuser would be lighter, (the OP's goal)
    and could be made stiffer (stronger if required) because all the internal structure can be incorporated as a monoque structure. ie lightweight internal reinforcemet etc
    If there is no change of the shape, there should be every opportunity to lighten the structure. With the amount of compound curves in the piece, the thickness required (to acheive the OP's requirments) in aluminum should weigh less than either 12mm plywood with additional fairing material or 6mm plywood with several layers of glass over top.

    Can you provide more information on this. Which axis does a reduction or increase in the PMI hinder race car performance? If the Polar Moment of Inertia (resistance to torsional acceleration) due to Inertia, I would like to hear more about what this entails. Certainly, if Z-Z is the axis down the length of the car, the aluminum diffuser could increase the torsonal resistance (or not) of the stiffness of the entire car, but if it is lighter then the inertial effect would be reduced.
    Last edited: Dec 29, 2023
  14. Tops
    Joined: Aug 2021
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    Tops Senior Member

    @Bmsluite Which epoxy surfboard resin was recommended by your fiber vendor?
    The 2:1 water clear and optically brightened non-blushing types may have less mechanical properties that the blushing 3:1 to 5:1 mixes.

    Also consider contacting Vectorply and getting their Vectorlam program, you could enter in various plate laminates and compare them.

    Vectorlam - Vectorply

  15. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    Lets begin with the weight reduction target;if the part weighs 45lbs at present it won't be possible to lose all of it,a reduction may be possible.Second point,an additive manufacturing machine capable of creating metal parts may exist in the OP's part of the world but It would be a surprise if it could produce a component approximately six feet wide.Should such a machine be present the part will almost certainly need post processing to make the most of it's mechanical properties.I have a feeling that by this point we may have exceeded the value of the vehicle.I would be absolutely amazed if there would be a reduction in total weight as I don't recall a single instance of a metal item replacing a carbon part on a racecar,the reverse has happened thousands of times and is quite normal.My conclusion is that even with an internal lattice structure and webs on the top surface you will still have a skin thickness comparable with carbon over (my guess) 2 square metres in a denser material.

    Gaboon/ocoume ply sheathed in either glass or carbon is light,stiff,cheap and can be stiffened by cedar or spruce bracing battens on the top surface.My earlier suggestion of a laminate on a melamine faced board could be even lighter if a foam or Nomex core is used and foam strips on the top surface were laminated in place to add stiffness.

    Polar moment of inertia makes a difference and acts mainly in two axes.In the vehicle world it is common to use X as the longitudinal,Y for lateral and Z for vertical.The rotation around the vertical is particularly significant as the effectiveness of the steering input may be effected.The input from the steering wheel will lead to the wheels pointing in a different direction and the deflection of the tyre sidewall may not lead to the vehicle immediately changing angle to match since the contact patch may change and the result is know as the slip angle.Less weight outside the wheelbase is generally seen as a good thing.For more information it may be rewarding to search for details of "contact patch" ,"slip angle" and "traction circle".

    The other harmful aspect of weight at the ends is that with suspension movement on bumps there is a tendency to pitch a bit more and this means that the aerodynamic loading varies quite a lot.The teardrop shaped blocks in an earlier post give us a clue about ride height and a change of a few millimetres would pass unnoticed in a road car but makes a colossal difference in a ground effect racecar.Its why active suspension was invented and ,because it worked ,swiftly banned.Which means that vast efforts have been put into damper tuning and progressive bump rubbers.The pitch axis will be somewhere between the wheels and will be determined by the suspension design.Weight that is added at a distance from this axis will have an increasing effect with distance and this component is about as far as you can get in terms of distance-hence another advantage to going lighter.

    I hope this post helps.
    gonzo likes this.
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