Pre-Preg and Balsa...

Discussion in 'Fiberglass and Composite Boat Building' started by AppleNation, Jan 6, 2009.

  1. AppleNation
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    AppleNation Junior Member

  2. marshmat
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    marshmat Senior Member

    In most respects, good end-grain balsa is superior to any of the foam cores.

    I wouldn't even consider foam core for use with prepreg. If you're going aerospace-style with the structural skins, you generally go aerospace-style for the core as well; that means honeycombs or balsa. It also means elaborate, time-consuming engineering analysis.

    Many of the foam cores on the market are only suitable for light-duty non-structural parts. Only a handful are actually stable and durable enough to use in a boat hull. They're basically just a filler material to thicken (thus stiffen) a fibreglass panel. If you're putting a lot of effort into engineering the skins- as the use of prepreg implies- you may as well go with a better quality core, lest you leave a major weak link in the system.
     
  3. Herman
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    Herman Senior Member

    Stiffness-wise, a balsa core and carbon skins (wet laminated or prepreg) are unequalled. Balsa has a nice shear modulus, and good compressive strength, which helps the skin to stay in column, when in compression.

    Balsa is not used in aerospace (satellites), because of the continuous freeze-thaw cycles, but for high-tech boats, why not? Thin skins can be a factor, but for racing boats not too big an issue. And besides that, ABN 1 (Now Delta Lloyd) is racing the Volvo for the second time, with 3mm skin thickness (and honeycomb core...)

    Balsa has a bad name due to rot-issues, but a well built boat with balsa core is something to be treasured. It is just that installing hardware requires more care than usual.
     
  4. masalai
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    masalai masalai

    works for me if you follow the instructions of the manufacturer - then, no rot and no leaks of "venting" - mainly from the mouths of builders who cannot read the flippin instructions

    Result is light robust and durable - see my link and ask Bob Oram, He will put you right...
     
  5. tinhorn
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    tinhorn Senior Member

    When I used 3/4" balsa as a core, I'd create "hard spots" from oak where hardware would be attached.
     
  6. masalai
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    masalai masalai

    Everything is glued - no screws or bolts - EVER - - - - for things that penetrate the hull cut a hole with a hole saw then put a hex-key in your electric-drill and remove a further half inch of balsa but leave the pre-glassed bit then epoxy up a bit of woven tape making a bit of a sausage shape and carefully and diligently insert that in the area you removed the balsa (and leave NO AIR BUBBLES) and then insert your through hull fitting (plastic/glass that will adhere to the epoxy) and let that set before adding some internal epoxy-wet-tape to further fix the item on the inside.... I haven't done that yet so I hope an expert comes along to clarify this

    Chain-plates are made of glass/epoxy and each of the 6 will more than carry the weight of the entire boat (NO BOLTS, NO SCREWS - JUST GLUE "epoxy" and various compatible tapes with no air bubbles or holes that are not filled with tapes and stuff - this is all defined in my build instructions from Bob Oram - which I haven't got yet... Do it right and you have a light, strong and waterproof boat.... - If you must bolt something on - - bolt it to a piece of pre-glassed and sealed ply that will spread the load and any through holes are re-sealed so NO water can ever get in to the ply EVER, then glue that ply in place - easy to do when done right the first time, if not your boat will rot within 2 years, cause YOU did not do it right when you built it....
     
  7. masalai
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    masalai masalai

    Balsa cored panels work very well if your designer knows how to do it properly, - - **** it up and you will kill someone sooner or later, - - because of your careless approach... Get advice from someone who builds and knows how to do it right first time, - - for there is no repair facility when you are mid ocean in a rotten boat that your negligence allowed to happen.....

    I spent time looking carefully at several different builders work using Bob Oram designs before I committed, and I hope his son will be on the build team.... Learn, observe, understand why and PAY for competent expertise - not a ****** who says he knows but lacks multiple build experience....
     
  8. Herman
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    Herman Senior Member

    Oh, one thing I forgot: To get optimum results, use weight selected balsa, in the range of 100 kg/m3 (sorry guys, imperial drives me crazy, but useless to discuss here). Unfortunately lead times on 100 kg/m3 balsa are about 12 weeks here, so planning ahead does not hurt...
     
  9. AppleNation
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    AppleNation Junior Member

    Thanks guys.. very informative.... and confusing...

    What if I adapted tinhorns approach and had honey-comb or core-cell in the places where I am placing winches and inhull fittings etc.....

    How would that impact overall stiffness and performance?
     
  10. masalai
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    masalai masalai

    Go and get a small sample of each from your supplier (4" square - just to see it), then you will understand honey-comb etc., - never on external surfaces.... - - ATL (Australian supplier), explained the reason to me, and your supplier should be able to advise you - otherwise they are too incompetent and know "sweet-f-all", to be considered a supplier....
     

  11. Herman
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    Herman Senior Member

    No honeycomb under fittings or winches. Compression strength is way too low.

    You could use Core-Cell in 200 kg/m3 or heavier under fittings, or alternatively use Airex T90 (in 240 or 320 kg/m3, and at half the price)

    If the fitting is really heavy loaded, I would opt for a single skin laminate, and a generous backing plate, bedded in epoxy paste (so no point loading from uneven laminate thickness)
     
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