Skin thickness

Discussion in 'Fiberglass and Composite Boat Building' started by Tungsten, Dec 25, 2013.

  1. PAR
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    PAR Yacht Designer/Builder

  2. rxcomposite
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    rxcomposite Senior Member

    Tungsten

    Maybe this will help in analyzing where to tweak, single skin or foam sandwich.

    Basically, for single skin laminate, the low modulus of the laminate provides the impact resistance by flexing or by adjusting the panel aspect ratio. For sandwich laminate, you can either go for high modulus skin to spread the load upon impact or you can opt for a resilient core that "gives" like a spring so the load is not transferred to the other side of the skin.

    You can download free the latest edition of Marine Composites by Eric Greene and Assosiate if you want to analyze further.
     

    Attached Files:

  3. SamSam
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    SamSam Senior Member

    What are you making?
     
  4. Tungsten
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    Tungsten Senior Member

    This would be ideal,what sort of core would I use?My thoughts on thinner cores were they would flex too much causing stress on the skin to core bond... eventually delaminating.
     
  5. Tungsten
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    Tungsten Senior Member



    [​IMG]

    16-17' river drift boat.
     
  6. SamSam
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    SamSam Senior Member

    Oil canning is not a bulge outward as when standing on a thin skin, it's the water pressure bulging the bottom up/inward. You can put in foam ribs and cover them in glass to make unsupported panels smaller, but when sliding over rocks and things you have fairly abrupt flexible and hard areas.

    You can also just put in extra layers of glass on the bottom. Something for your boat might possibly have a 1 x 5' strip on the centerline, followed by a 2 x 6' strip followed by a 3 x 7' strip. Something that would feather out the reinforcements. That method of reinforcement is more even and doesn't have the hard spots of ribs and stringers.

    You can make a female mold very easily with this stuff, white tileboard.

    http://www.lowes.com/pd_16605-46498...L=?Ns=p_product_qty_sales_dollar|1&facetInfo=

    You make some frames, line it with some 1 x 1 or 1 x 2 stringers every 8" or so and then line it with the tile board. It's already gloss painted, so a few coats of wax and some PVA and you're ready to laminate. I only used it with polyester, if you were to use with epoxy you would want to make a test.
     
  7. Tungsten
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    Tungsten Senior Member

    So moving onto some tests of solid panels,i have some 1.5oz chopped matt and some DB cloth.I will try infusing these with Ve.

    Should I be putting a layer of cloth between each layer of matt?Or can I use the matt as a core so to speak?Meaning say a few layers of clothe on each side of 2 layers of matt.

    Another question I have is I have some woven roving that's sitting could this be used in the middle of the layup?Somthing like a few alternating layers of matt and roving then the stiched glass on the outsides.
     
  8. Herman
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    Herman Senior Member

    Strengthwise the CSM does not bring much. Thickness, yes. From a structural point of view I would take a lower weight CSM, or no CSM at all.

    The CSM is there historically to prevent resin rich areas which due to shrinkage can delaminate. The CSM weight should be balanced with the coarseness of the reinforcement used. Traditionally a woven roving 800 grams (do not care to calculate to imperial now) is alternated with 1.5oz CSM. This seemed to work well.

    Now think about double bias multiaxial. Much smoother than woven roving. So CSM weight can come down. (a 600 grams multiaxial can be backed by 0,75oz CSM for polyester hand laminate)

    Now think about vinylester. Lower shrinkage. CSM weight can come down.

    Now think about infusion. Much more compaction of fibers, reducing resin rich interply areas dramatically.

    One of the boats I made was for a government agency (as in: ***** boat driver does not own the boat, and does not care either, only knows full throttle or idle, poor engines).

    Original boats were hand laminated with multiaxial and CSM. Lots of problems ripping the laminate. They brought the problem to an engineering firm, which recalculated amounts of fiber needed. Much higher of course. However, there was a fixed weight limit, so either infusion or prepreg needed to be used. At that moment I was called in. Boat was alternated between biax +/-45 and woven roving. No CSM. Infused with VE. Boats were 10 kgs underweight. More importantly, the small series of boats I made, are the only survivors, and have no hull troubles.
     
  9. rxcomposite
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    rxcomposite Senior Member

    This is in consonance with what Herman is saying. Forget about CSM if you are infusing. CSM in between layers are used to reinforce the adhesion or strengthen the matrix bond in between layers and works best when hand laminating and when the in between layers are almost dry or tacky.

    When infusing, the fibers are compacted and the "glueline" is reduced so there is no need for reinforcing the glue line.

    The key words are HIGH STRENGTH and LOW MODULUS. That means also high strength in all directions so make it isothropic by combining +45/-45, and 0/90 fabric. Low modulus means relatively high stretch fabric. WR comes to mind as the undulations due to weaving gives it the little "spring" in each element. Being connected together, it also has a strong mutual influence on each others during tugging and pushing. Although it does not have the best packing efficiency because of the bulk created by the undulations, it is best to choose many thin layers to build up bulk rather than a heavy cloth.

    Watch out though, thickness is the enemy of flex so some prefer the use of Uni's or stitched biax. Because of its packing efficiencies (less resin content), it always come thinner and more flexible. Although it is "stronger" when loaded in the directional stress, uni's are very sensitive to off axial load. Whack it in an off axis load and it delaminates. Again, it should be isothropic and many thin layers for best results.

    For example, a hull laminated with a +45/-45 direction will be very strong in the twisting department but hit it with a projectile at 0 degree, (a debris floating by in a 0 degree direction) and it delaminates. Uni's (or stiched biax) lose about 73% of its strength/modulus in the longitudinal direction.

    Suggest you make some panel test. What is the beam of you boat and what is your estimated displacement or weight you plan to carry?
     
    Last edited: Jan 6, 2014
  10. Tungsten
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    Tungsten Senior Member

    Excellent,thanks for the help.

    Hull width in middle about 54"(1350mm) by about 14'(4270mm)Beam at gunnel 70"(1778mm)LOA about 17'(5180mm)

    My plan is to make a male mold including a rounded chine of approx. 6" in length.Extra glass will be added in the chine area to get too about 1/4"(6mm) in thickness,there I will attach sides by stich and glue.Sides will be either plywood or foam core.

    I plan to copy some of the production boats available,they have a double hull in the center portion say 6" in from the chine a second raised floor is added.This does 3 things,allowes for a flexable hull to move or flex over shallow areas and the camber of the top floor to shed water to the chines to keep the floor dry.Lastly this makes a flat spot to stand on as the hull is rockered on each end.I haven't yet planned the raised floor ,In a typical day people are in and out of the boat this brings in a lot of water from there wading gear.
    Approx passenger weight,is about 350Kg.

    https://www.google.ca/search?q=driftboat+hulls&rlz=1T4ACAW_enCA358CA360&tbm=isch&tbo=u&source=univ&sa=X&ei=P47JUqfLFo3boASQhIDQCg&ved=0CGQQsAQ&biw=911&bih=368

    Thanks,
     
  11. rxcomposite
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    rxcomposite Senior Member

    Tungsten,

    From the information you gave, I tried to do some preliminaries. Attached my notes.

    It appears that with a double bottom, you are going to have deep transverses and combined with the center girder, the framings will be stiff. This is because the EI was raised due to deep transverses and the combination of floor and bottom panel effectively forming a box. I was thinking more of a limber structure or a "flexible" boat. By flexible, I mean without deviating from the rules of limiting span to deflection ratio of 150 for the plates and 200 for the primary frames/webs.

    Maybe you could come up with a sketch of midship plan and let's see how the numbers turn up. So far I am getting 1.75:1 ratio for the panel. Good if it is 2:1 but its workable in the meantime. The side panels needs dimension so we can define the panel dimensions. Looks like we need a side longitudinal. There are forum members who have spreadsheets or software that can participate in calculating panel thickness, strength and deflections.

    With a 0.65 glass content (infusion) I am getting 8.6 mm for the required keel plate thickness, 5.2 mm for the bottom plate, and 4.2 mm for the sides, single skin laminate built up with WR and 300 gr CSM on the outside for the tie coat to gel coat. This is without correction for ply angle layup and use of VE for the resin. None yet for impact strength adjustment.

    The 2.6 lb/sq inch bottom pressure was calculated with 615 lb total displacement and 4-5 knots speed. needs some iteration.
     

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  12. Tungsten
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    Tungsten Senior Member

    Rx,thank you for your time.

    Typically the raised floor has no supports under it,from what I can tell looking at some of these type of boats when you remove the drain plug the hull in the center is about 1/8" thick.As you move towards the edge say 4" away the glass there is much thicker as these kinds of boats take a beating on the chines.

    Here is a sketch,

    [​IMG]

    This is how the center part floor typically is.A raised floor is also in the bow area that normally goes right to the sides,same for the rear passenger.These end up higher so the water flows off down into the center area.This also makes a nice flat spot to stand on.The center raised portion of the floor will be tabbed approx. 5" in from the chine.This will probably be made from 1/2" Corecell to keep the weight down.

    Average flow of the rivers I float are about 7-10 kmh,

    The key to all this is getting the floor to flex when going over shallow gravel areas and hitting things but not flex so much that when pulling on the oars the floor also flexes.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Ok. Looks like a simple wide beam running the length of the craft. Seems good enough for 2" thickness. With a cored bottom craft, might be a good idea to core all the way as the sides are only 20" deep.

    Corecell is good choice as it is a high strain core and does well in impact resistance test.

    Will try to do another calculation as my initial calculations (single skin) showed that resin infusion (0.65 Gc) increased the modulus of the skin requiring 3 more layers to be added whereas with hand lamination and Glass content of 0.50, the stress criteria was quickly met.
     
  14. Tungsten
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    Tungsten Senior Member

    My last boat was cored hull,was just too stiff.When hitting small rocks the boat high centered and dented the foam.My guess is my outer skin needs to be so thick that I might as well just do a solid floor and get some flex back.Probably cheaper also.
     

  15. rxcomposite
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    rxcomposite Senior Member

    So true. As Ad Hoc pointed out in post 20, sandwich construction is not the way to go when designing "flexible" boats. To a certain degree you can make it flex but always, the core suffers by absorbing the impact. Adding more layers on the outer skin would only make it stiffer and heavy.
     
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