Homebuilt: Plywood vs. sandwich

Discussion in 'Materials' started by Slowmo, Aug 30, 2004.

  1. Slowmo
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    Slowmo Junior Member

    The question here emanates from the idea of building stich & glue plywood design from foam in the same way and cover with glass+polyester (or epoxi).

    This to reduce weigt to have the hull very easy to handle on land. Extra ballast needed sailing.

    A dealer in ply-boat kits stated that:

    "Most people who tries to save weight by using sandwich end up with a boat of same weight but dubble price!"

    It's quite clear that wood has very good qualities especially to get the hull stiff and perhaps also a GRP hull should have some wood stringers.

    1. Do you think there can be a substancial save in weight by sandwich for a 16' Trailer sailer? 20%? 50%?

    2. Wich is the most cost efficient foam for smaller boats?
     
  2. Not A Guest
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    Not A Guest Junior Member

    "Most people who tries to save weight"

    But then most people are not engineers or even good builders and you must be both to save weight.

    It is possible to build as strong and lighter using a sandwich of glass and foam than using plywood.

    I suggest paying someone to do the engineering for you. That may put the price out of reach.
     
  3. mistral
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    mistral Senior Member

    probably you won't save much weight on little boats using sandwich.
    I experienced it with a trailerable 19-footer, one built strip-planked and one with sandwich (hand lay-up traditional building); both boats had approx the same weight; you can have a clear advantage if you push hard on sandwich building technology, i mean properly using unidir and multiaxial fabric, vinilester resin and vacuum bagging, to avoid unuseful amount of resin on your hull. But often this go much further the skill of a homebuilder.
    By the way, I'm not sure that good epoxy and marine plywood are so much cheaper than airex, wave roven fabric and polyester (or even vinilester) resin, at least not here in Italy :( . Not to mention that building with plywood (radius chined??) puts serious limits on hull design.

    fair wind
    Mistral
     
  4. PAR
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    PAR Yacht Designer/Builder

    Very light weight craft in smaller sizes can be had, but must rely on very expensive materials to keep the weight down and have the puncture resistance necessary in small, near shore (where things get run into mostly) boats. This is one of the great benefits of plywood and related construction techniques, good puncture resistance and strength to weight ratio in the same sheet goods type product. That's a tough combination to beat in a single material, but if you combine other materials that have some of the qualities with others that have the rest of the equation covered then a composite can be engineered to out perform typical plywood construction.

    Aside from the weight issue, the real benefit, I think, is the sheet goods angle. Right out of the box you have building material, cut, attach and slather on some goo, presto a plank, bulkhead or what ever. Composites are a different breed. Sure, design limitations in some ply construction methods aren't a problem with composites, but there are many other considerations not needed in ply that are quite necessary in composite construction.
     
  5. rjmac
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    rjmac Junior Member

    Par...

    One issue that I have been thinking about is the adhesives used to laminate the plywood together...? My concern is penitration of the epoxy is limited and so I would be dependent on the lamination adhesive.....? Could you expond on that...?

    :)
     
  6. Slowmo
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    Slowmo Junior Member

    When you got this "plywood heavy 19'er"... what did you then use?
    1. Multi-directional (strand-) matts or controlled weave?
    2. How many layers (or mm's) outside?
    3. Did you cover the hole hull inside with GRP?

    Do you think there is a problem that some foam surfaces are rough and demand (absorb) to much matrix?


    - - PAR - -
    How thick outer GRP coverage is needed for a) minimum b) fair c) safe puncture resistance?


    - - rjmac - -
    How do you mean?
    "penitration of the epoxy"

    From epoxy into /onto /compared to what?
     
  7. James Mills
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    James Mills Junior Member

    Has anyone experimented with honeycomb core sandwiched between thin plywood. Seems like it would be easy construction - built inside out. Inside layer (boat interior) of ply over temporary frames. Epoxy vacuum bag the honeycomb core onto the inside layer of plywood. Vacuum bag the outer layer (outside hull surface) to the honeycombe core. Same hull shape limitations with ordinary plywood construction, however, seems like it could be made stronger/lighter than ordinary plywood construction.

    Anybody know of any engineering references for those types of panels? i.e. x'' polypropylene honecomb epoxy vacuum bagged between x" merranti = approximately 1/2" marine douglas fir.

    In Kurt Hughes' demo construction video of a quick, inexpensive cold molding method, he mentions using a core in the hulls in place of stringers. His Cylinder Mold method is pretty neat. www.multihulldesigns.com

    James
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    I have experimented the honeycomb: it works very well on boats of a size big enough to have significant savings, paying the "complication" of the method and on small boats for the decks and floors.

    There are a lot of books about naval design and the (simple: secundary school algebra) calculations of inertia of skins. I can only counsel to read them, improve your maths and making the calculations. The book of Pierre Gutelle has in the french and I hope in the english edition examples of very useful and precious sheets of calculations.

    You can ask also technical advice to Nidacore if you show you are a serious customer with the money for your project. Sorry to say that some amateurs are worst than bugs and mosquitoes during a picnic for marine suppliers, and naval architects.

    Compounded plywood and cylinder mold are very effective methods for some hulls as multihulls. The results are be very good for a technic that a home builder can master. The variations are innumerable.

    Coming back to the thread; PAR is totally right. There is a requisite on local stresses like a jumping sailor and puncture resistance that makes foam sandwiches useless for amateur builders in the small sizes.

    A part the problem of price of the materials (foam is many times more expensive than plywood and you'll need a lot of resins, putties, and cloth), the boat wild need at least twice the work for a very disapointing result.

    The hard work hull will result ugly (it's very difficult to finish male mold sandwich boats compared to plywood which is so smooth) and probably heavier that the plywood boat.

    It's impossible to a home builder to reproduce the light high tech sandwiches, which use female molds, vacuum, oven, controlled environment and skilled workers after a expensively paid engineering study.

    Myself I use plywood and strip plank in small boats( until 30 feet), while I have the technical expertise and long experience of sandwiches. In fact, it's because I have the technical expertise about sandwiches, that I prefer plywood on simple and small projects.

    Cheap, light, strong, durable, no delamination issues, no blisters, easy to work, easy to repair and maintain. What do you want more? Use always a marine BS 1088 plywood, it makes your life nicer...It pays itself its price. Those having used Fir exterior plywood will understand me.

    An example: the team of the high tech Class C Yellow Pages used 3 mm plywood for the 27 feet hulls...cheaper, simpler and almost as effective as carbon. And they are very qualified guys.
     
  9. Slowmo
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    Slowmo Junior Member

    Inside a small boat only "exposed areas" need fibre.

    Weave can be reasonable impregnated (doing it separately) with epoxy without vacuum, but not perfectly.


    *** LOWER WEIGHT ***

    Is it even possible in practice for the homebuilder to lower the weight of a standard playwood design?


    *** Plywood + honeycomb ***

    There is one obvious problem. That is when applying a non perfect shape on the honeycomb it will collapse. Or very easily do so.

    Honeycomb is very sesitive to forces applied in other angles then the theoretical optimum.

    In the aircraft industry honeycombs allways are used in the exact shape in combination with prefabricated laminates of exact the same shape.
     
  10. mistral
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    mistral Senior Member

    1)external sheeting with 3-4mm plywood or 1500gr/m2 chopped strand mat+wave roven (CSM500 g/m2+ WR 500+ CSM500)
    2)see above
    3) yes, that would be better, otherwise you can use just two or three CSM layer where the bulkheads lean, to avoid stress concentration on planks

    anyway you con referr to Dave Gerr's book, chapter 11
    for the 19-footer take a look here if you like it
    http://www.tecno-legno-yacht.com/ide_19_in_costruzione.htm

    fair wind
    Mistral
     
  11. Slowmo
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    Slowmo Junior Member

    1500g/m2 !!! on a 19'er.... wow! I'd say that is overkill of at least 150-300%.

    No wonder it became heavy.

    Did you use any microballons in the matrix?
     
  12. amitk
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    amitk Junior Member

    Wood composit panels

    Read this document. If you want more information about wood, I advise you to download the all book, it is free and a good introdution.
     

    Attached Files:

  13. mistral
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    mistral Senior Member

    you're right, to speak clearly I'm not building that boat, I've designed it; 1500 g/m2 is due to a poor plank core thickness, don't ask me why :mad: ; that's what happen when homebuilders doesn't follow designer specifications.......
    anyway she's born intentionally as a very sturdy boat, that kind you can forget on a beach side for the whole winter without any problem

    Fair wind
    Mistral
     
  14. mistral
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    mistral Senior Member

    for slowmo
    ....oooops I'm sorry, now I recognize that i gave a totally wrong answer.
    The correct sheating for my 19-footer was 750g/m2 on the external side and 500g/m2 on the internal, otherwise she will be a good submarine....

    sorry again, it's still a hot summer here in Sardinia, maybe too hot for my brain
    mistral
     

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    Slowmo, you are asking too many questions

    Slowmo, you are asking too many questions:

    1/ If you're designing a boat, you are visibly too short in technical training, as the questions you ask are detailed in many good books. Internet is not the best source as being too fragmentary.
    Some reading is always usefuls and like piano, guitar, motorcycle driving, horse jumpimg, sailing and so on, work is needed before mastering the technical skills (both intellectual and manual)

    2/ If you're building a boat you have bought the plans, get stuck to the designer's specifications. That will save you from a lot of worries, sorrows, lost time and lost money. If it's your first boat this counsel applies at power 3!
    A lot of boats are in the garbage after "clever improvments" by their amateur builder...

    3/ Plywood is a simple and forgiving technic which gives, when used with epoxy and glass, the best ratio strength/weight/price/easy construction.

    4/ After 30 and some years of boatbuilding and engineering, from warships to high tech race trimarans passing by dinghies, pro fishing boats etc... I can say (specially in small boats) that paranoic weight savings and building complications are not always the best engineering path...
     
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