Plywood vs foamcore-how do they compare?

Discussion in 'Boat Design' started by Eric770, Feb 11, 2011.

  1. Eric770
    Joined: Feb 2011
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    Eric770 Junior Member

    I'm considering building a 20' power boat, something along the lines of an Atkins Rescue Minor or a Tolman skiff. Fuel economy is important to me so I was considering building with foam-core rather than plywood to keep it as light as possible. I've been unsuccessfully looking for some kind of comparison between the two materials. Can anyone provide some general information such as weight, cost, building time. These boats are generally made from 3/8" ply. Any books or documentation that might help me in designing a conversion to foam-core would also be appreciated.
    Thanks
     
  2. CaptBill
    Joined: Jan 2010
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    CaptBill CaptBill

    With foam its simple to thicken the wall thickness, glass it all in and you then have a double wall composite. Inter connected inner and outer as one piece. The thicker the wall the stronger. You would be surprised how rigid the foam alone is, with the right thickness. Look at the Boston Whalers concept.
     
  3. tunnels

    tunnels Previous Member

    Just be careful with this statement (The thicker the wall the stronger.)There are a few differant ways to interprate Stong! . The thicker the core the more ridged yes, but stronger ???If you build a ridged boat in the rough it could simply break up But if you build a less ridged boat with flex it will ride the rough better and stand up to more punishment !
    A panel that bends and gives a little will absorb shock loadings yet it will still be strong !!:?:
    The same panel built with the same 2 layers of glass but a thicher core could break !
    Lots of thought has to go into building a boat and choosing the right materials for it !!
    Same if using 3/8 ply ! which would you choose 3 ply or 5 ply ?? which is more ridged and which is stronger ?? But they are both wood and both the same thickness and could be the same wood !! GET MY DRIFT ??:confused:
     
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  4. PAR
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    PAR Yacht Designer/Builder

    Foam core in the size you are talking about will not be lighter then sheathed plywood, assuming the sandwich composite is engineered to a similar level of stiffness and strength.

    The Rescue Minor get's it's fuel efficiency from a unique hull shape and limited top speed. It's not a light weight boat.

    The Tolman get's it's efficiency by shape and construction, though not a light weight, comparatively lighter then a Rescue Minor.

    The regular Tolman and wide body have quite moderate deadrise, so are not off shore vessels. To some degree (18 in fact) the jumbo model can handle rougher and deeper water, but requires more power for the same level of preformance of the lower deadrise models.

    Changing build methods, particularly a cored composite structure, requires considerable understanding of the strains, loads and issues involved. Sandwich structures in particular, are fairly well engineered if you want a light, stiff and strong structure.
     
  5. DCockey
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    DCockey Senior Member

    Shear strenght of the foam core needs to be considered.
     
  6. CaptBill
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    CaptBill CaptBill

    All your points have validity and are definitely to be considered. Although the 'parametric wall' concept allows you construct a 'tuned' wall to address those very concerns. That's the appeal. I only spoke generically for clarity of the concept.

    Also when I said light I spoke of the potential. You can go many routes with this process. Boston Whalers are not light. They show the basic idea though, just not that side of it. And were they to 'break' in half they don't sink. Having 'unsinkable' as a marketing term seems to sell them some boats btw.

    The key thing to mention is EASY to build. Plus it is piece by piece build. One man can do the whole some foam, epoxy, glass tape, and a pair of scissors. Oh yea, you have to step up to the plat and invest in a good quality bread knife.

    When you get finished and remove the foam and clean it up, you will be considering going with a clear coat finish just to show off.
     
  7. CaptBill
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    CaptBill CaptBill

    PAR you pointed it out perfectly. It puts the ball in the engineers corner. The build itself will be simple. Make small composite parts that snap together like LEGOS. Everything gets 'printed' directly from a 3-d file, piece by piece.

    "Just put it together"

    HAHAHAHA
     
  8. CaptBill
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    CaptBill CaptBill

    Here's the idea in a nutshell

    [​IMG]
     
  9. DCockey
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    DCockey Senior Member

    Can you provide a description of the "parametric wall" concept and how one uses it to construct a "tuned" wall? Also a reference would be helpful. It sounds to me like a design method which uses sophisticated analysis tools.
     
  10. tunnels

    tunnels Previous Member

    Shear strength when using foam is vitally important!!! Always over build and use a better core specially for the bottom . Do your home work and look at all thats availible and read everything you can find !!!
    Look carefully at the girder and stringer system for the inside and the size of the panels you will end up with when everything is in place !!.
    The glassing in of all these things is also vitally important as it also adds strenght to the finished panel !!.Along with what its made from and how it laid out ! if a core shears it is heart breaking and a major to repair . Stringers and girders made from wood need to be sealled before fitted to guard against getting wet and becomig heavy and going rotten . Same applies to the transom , need to be made from good quality materials and 100% sealed , Bolt holes are where the water gets in mainly as most are below water level when the boat is sitting in the water . Lots to consider for the project as a whole . The hull is the easy bit ,once built its everything that goes onto and inside of that takes lots a time and money and need to be well thought out a long way in advance !!. :D:p:p


    :eek:A boat is a hole in the ocean which you keep pouring money into forever !!:confused:
     
  11. gonzo
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    gonzo Senior Member

    Never heard of a "tuned wall". For that matter, there should be no walls on a boat.
     
  12. CaptBill
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    CaptBill CaptBill

    I am attempting a demo now using Rhino and Grasshopper. Basicly we loft an inner and outer shell both with slightly varying curves ( no specific offset).

    I am definitely not as adept with Rhino as you are DCockey and have a simple problem you can probable answer for me. Why when I loft three curves (2 side gunwale lines and the 'belly' line (?)' For some reason the open side is one of the sides and not the topsides. What am I missing here?
     
  13. CaptBill
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    CaptBill CaptBill

    DCockey

    Here is what it's doing. I have tried changing the selection order of the lines. Still comes out like this. Something simple right?

    [​IMG]
     
  14. FAST FRED
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    FAST FRED Senior Member

    You will have good resale value with a foam core.

    A plywood covered boat is just a plywood covered boat , hardly most folks dream boat.

    FF
     

  15. PAR
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    PAR Yacht Designer/Builder

    You will have good resale value on a foam core if there's no delamination or other issues.

    Resale value has much more to do with boat care then build method, particularly if it's titled composite, rather then wooden.

    Most "inspections" now include multiple "build types" check boxes on the title application form. If you check the composite box, rather then the wooden box, your insurance and resale value will jump substantially. The last one I did included Florida's new form (for this sort of thing) and the options were: wooden, sheathed wooden, composite, GRP, metal (please state), other (please state). Most accept the weight of the materials as sufficient enough to determine build type. In other words a 300 pound plywood raw hull, that has 100 pounds of epoxy and 'glass on it, can be called a composite build.

    The inspection I refer to is the initial, Fish and Game or Natural Resources or DMV officer, that comes buy to look over your project, before the paper work is sent off to the the state for a HIN assignment. All states require this inspection, though the department that over sees the process, changes with each. Only a few states require you to bring the boat to them. Most will come to you and give the boat a "once over" look before asking you to sign off on the paper work.
     
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