Microcatarmaran

Discussion in 'Fiberglass and Composite Boat Building' started by sebaseba, Apr 14, 2020.

  1. upchurchmr
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    upchurchmr Senior Member

    Forget putting joints in the middle of the crossbeams. That will be weak and very heavy.
    Just use 1 piece tubes, and connect them to the hulls with SS straps bolted down.
    Use a cordless drill to remove and attach the bolts.

    The best advice I've seen here is to make the boat longer.
     
  2. sebaseba
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    sebaseba Junior Member

    Yes I think that's the best idea. How to screw it in though?

    upload_2020-4-26_16-2-4.png

    Method 1, I would make a window through and through or a reachable box, where you could put nuts and washers.
    Method 3 I would just make the screw through whole structure.
    Method 2 I would attach the screw to the top layer, but without nuts with some sorts of technology, maybe for hollow walls.
     
  3. upchurchmr
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    upchurchmr Senior Member

    Method 2.
    Go look at every beach catamaran ever built.

    My suggestion was poorly worded.
    Put bolts in the hulls with the heads inside bearing up against larger washers.
    Then attach the crossbeam with straps and nuts.

    You will need an access port near each joint, to be able to hold the bolt head with a wrench when you tighten the nuts.

    Don't ever think about #4. Heavy and it will leak.
    #1 will also be needlessly heavy.
     
  4. sebaseba
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    sebaseba Junior Member

    OK, you actually picked method #1 by your description. Just I couldn't draw it in 3D, it's a cross-section.
    Method #2 implies you can't reach the other end for nuts.
     
  5. sebaseba
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    sebaseba Junior Member

    OK, I calculated and the price for making it out of PVC discharge pipes is greater than foam/FBR. Also foam/FBR would have a better shape and look nicer.
     
  6. upchurchmr
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    upchurchmr Senior Member

    Method 2 requires deck plate access.
    Don't even think about putting holes in the side of the boat for bolt/ nut access.
    Go look at a Hobie, Nacra, Prindle, Tornado, Sol cat, etc, etc. All 50 year old boats which work well.
     
  7. sebaseba
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    sebaseba Junior Member

    OK, I'll do it graphically. You meant this:
    upload_2020-4-27_8-53-47.png
     
  8. fallguy
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    fallguy Senior Member

    If you want to beam two canoes together; you just need to build two wood akas to the desired spec.

    lash the akas by cutting a small groove under the canoe gunwhales about 1/4" deep or less by say 1.5" wide. You run rubber lashing through, so, the top edge must not be sharp and ground to the gunnel. If your gunnel is sharp; u may need to install a plate here so the lashings can't get cut. Rubber lashing from inner tubes tied right does not move. Done wrong moves a lot. Basically come over the aka first and tighten and then tie in top again. If you start in the groove; it is wrong. To keep the aka from moving laterally; you need some type of stay. The lashing is good, but a stay is better. A crude stay is a c clamp each side of the aka. But you can make something with an angle iron aluminum bolted to the gunwhale on each side, given space.

    You may need three beams for a 16 footer.. I would guess a laminated beam made from 1/4" slabs of maple or ash about say 2" wide by 3" high would meet my gutcheck for decent conditions.

    the rubber lashing method is similar to Wharram's proven method where some give beats all break..
     
  9. upchurchmr
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    upchurchmr Senior Member

    Seebaseba,

    Yes, but you confuse me with the cross-section.
    The thickness of the deck doesn't need to be anywhere that thick.

    Good luck.

    PS, I've never see a beach cat break the deck to crossbeam bolted joint. Much as I admire Wharram, the idea that bolted joints break is just advertising. Assuming you are not incompetent in your design.
    This is a 10' boat. You could make the joint with some zip ties.
     
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  10. sebaseba
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    sebaseba Junior Member

    Yes, I've looked up the term. I think it would be more correct to call it half-section view.

    I think if I put it through sandwich FRP/foam/FRP with bolts it shouldn't break, but is interesting how this Wharram does it.

    The drawing wasn't to scale.
    From what I read on the site/books, seems that I would put something like epoxy + 200 g/m2 rowing + 2-3 cm of XPS foam + 200 g/m2 rowing. Didn't do calculations yet on that, just aluminium tubes (seems that 40 mm diameter, 2-3 mm thick for 1,8 m is enough).
     
  11. upchurchmr
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    upchurchmr Senior Member

    Don't ever put foam in a bolted joint. It will just crush and you will never know it, until the fiberglass around it cracks.
    This is a well known problem.
    If you want foam in the deck, then embed plywood of the same thickness where the bolts go thru.
     
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  12. sebaseba
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    sebaseba Junior Member

    Aha, I see. Well foams are rated for a few 100 kPa, it sounds a lot but 100 kPa is 0,1 N/mm2. I already did some calculations regarding that and it was clear that the foam wouldn't survive, except if you put quite a big washer. Using plywood there, instead of foams, would make a big washer redundant. Would make the whole thing cheaper and lighter. Thanks for the trick! :)

    Damn, there's so much to learn. Do you suggest any literature, links, yt? I'm already reading a bunch of things, but more reccomendations are always welcome.

    According to this site, EPS is better than XPS for GRP/foam construction. Interesting. (I know that PVC foams are usually used, but they are harder to get and more expensive. Just researching alternatives for now.)

    Interesting article, making Al tubes stronger by filling them with PU foam: Crushing response of square aluminium tubes filled with polyurethane foam and aluminium honeycomb - ScienceDirect https://www.sciencedirect.com/science/article/abs/pii/S026382311630372X even stronger if you add PU+Al honeycomb, but I have no idea where to get the latter.
     
    Last edited: Apr 28, 2020
  13. upchurchmr
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    upchurchmr Senior Member

    IMHO, EPS and XPS are not for structure.
    They both have little stiffness. Stiffness (shear) is required to make a good sandwich structure.
    People have used it in large thickness, but it is going to be heavy. There is just too much of it.

    A pity PVC is hard for you to get. They are certainly more expensive.

    One thing you might want to do. Make a test sample of whatever materials you want to use.
    For example, make a "plank" that represents how you would build the deck. Make it big enough to be similar to the size of the deck. Then walk on it. If it survives, get 2 or 3 people to walk or step in the same place. Then jump on it.
    Then make any alternate method and do the same.
    If your deck is only 2' wide, you might find that just a piece of 6mm ply would do just fine. It might need the occasional small stiffener bonded to the ply.

    My typical recommendation is on building epoxy/ wood boats. Gougeon Brothers on Boat Construction book - WEST SYSTEM Epoxy http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook%20061205.pdf
    This is a free download, or you can buy the hardcover book. This has been the standard for about 50 years.
     
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  14. Dejay
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    Dejay Senior Newbie

    I think it's not that people here hate XPS, it's that it's just not good enough but many are trying to "make it work" anyway. Because they see the high prices for structural foam compared to insulation foam. You can get PU or XPS foam panels that have a price of something like 0.25€ per litre of foam. The prices for PVC foam I've researched are like 2.5€ to 5€ per litre. So then the arguing begins as you go through the 5 stages of grief until you get to acceptance haha. A boat is a hole in the water you throw money in.

    Then you realize that marine plywood also costs quite a lot! The cheapest you can build with is outdoor plywood (must be WBP) and accept that it will be less nice to work with and might not last as long. Or strip planking. But everything is a compromise including time and labour.
     

  15. sebaseba
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    sebaseba Junior Member

    Thanks for the book, I'll look into it. Now I'm reading J. Teale: How to Design a Boat.

    Well I've found a few sellers now that sell all sorts of foams locally, but we're in a lockdown, due to the virus. Maybe the difference in price is not that big.
    Online the prices are just for those that sell for RC models. Those prices are crazy. :)

    The stiffness of typical EPS boards is low (50 - 70 kPa, 7 - 10 psi), but you can get stiffer ones (200 - 250 kPa, 30 - 35 psi) that are almost as stiff as the starting stiffness of PVC foams.
    XPS foams are as stiff (300 - 700 kPa, 40 - 100 psi) as PVC foams. A lot of surfboards are made out of EPS or XPS foams.

    Well those values are for compressive strength. Shear strengths are harder to find, but I found them in scientific articles.
    upload_2020-4-28_13-55-11.png
    Those codes are:
    upload_2020-4-28_13-55-44.png

    For EPS it's either this data (but is molded packaging):
    upload_2020-4-28_13-58-7.png

    So a 30 psi EPS (~200 kPa) has shear strength of 70 psi (500 kPa).

    Or maybe this is more relvant, EPS type equals to compressive strength:
    upload_2020-4-28_14-2-33.png

    PVC foams start at shear and compressive strengths of around 400 kPa (60 psi).

    So in this regard XPS/EPS is half as good, but it's offset by price.

    I think a bigger issue is that the surface energy of PS (XPS/EPS) is lower than PVC, so the adhesion will be worse, which would more likely to cause delamination.
     
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