Sandwich wood core strenght

Discussion in 'Materials' started by TomE, Sep 21, 2005.

  1. TomE
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    TomE Junior Member

    Hello all! New to boat construction, and trying to make my learing curve as steep as possible... Have all these questions that need answers... :rolleyes:

    My day-dreaming tends to point strongly in the direction of a wood/epoxy high performance cruiser, that I can build my self with professional drawings and guiding.

    The first thing I want to know a little more about is how much performance I can expect from such things one can asseble as a DIYer. If my expectations are TOTALLY off, I will have to reconsider things.

    How does stripplanking and cold moulding compare to the GRP composites commonly used in high performance production boats, all of similar weight pr. suqare inch/whatever?


    I found this equation for calculating flexural rigidity of foam core:
    http://boatdesign.net/articles/foam-core/ - under sandwich priciple

    Can this equation be used with wood cores as well (stripplanking/epoxy/glass)? If so, can anyone give me a calculation on, say, 1/4" cedar with 2 layes of 6-ounce fabric each side?

    How would this compare to, say, two layers, 90 deg intercecting 1/4" cedar coldmoulded with epoxy?

    Post is getting too long, gotta stop - more questions coming if someone is willing discuss.... :p
     
  2. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Vi kan ta dette pÄ norsk, men for de andre:

    Strip planking with glass and epoxy on both sides produces very light and strong boats. The wood wil contribute to global longitudinal stiffness of the hull, this is not the case with lighter cores.
     
  3. Karsten
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    Karsten Senior Member

    I think you have three "challenges". One is to find a lines plan that is suitable. Most naval architects can probably help with the lines if you are prepared to pay. Then you have to engineer the boat to ensure it is light, stiff and therefore fast. I would insist that this is done to ABS. There are other codes around that are easier to use but because they are less accurate they have greater safety factors and therefore your boat ends up to be heavier. This should narrow the list of naval architects down quite a bit if you want to go down that route.
    The last challenge is to find somebody who is able to help when you got stuck with the building process. Maybe there is a local builder who is willing to give a few tips?

    To the materials: As you can see from the equations the core mechanical properties don't contribute much to the bensing stiffness. The thickness of the core is most important. Because the core should be thick you want a core that has a low density. Cedar (strip planking) is pretty good but foam is better. Please note that you can also plank the hull with foam strips instead of wooden strips.
    For the skins you want a material that has a high strength to weight and stiffness to weight ratio. Again wood is pretty good, glass fibre is slightly better and carbon or similar is best. In your case I would also consider the skill level required to come up with a good hull. It's usually easier to slap on a few layers of fibreglass instead of cutting veneer, fitting it and glueing it on. but again if the local yard makes cold moulded boats and you are a carpenter this might be different.
     
  4. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Plywood skin over stringers is a very quick and light building method. Dudley Dix' Didi 26 for example has a (light/empty) displacement of 1000kg with 400kg lead bulb. That is lighter than most other boats of simmilar length and inside volume (accomodation). The performance is simmilar to Melges 24 and J80 according to the LYS number.

    (LYS = Lidingö Yardstick System, a swedish empirical handicap system)
     
  5. TomE
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    TomE Junior Member

    Maby I should give a defined outline of what I'm after - a cruiser of 23-25' to accomodate en engine of 500-550 fwhp. Total weight should preferably kept under 1200 kgs including engine (all alu V8).
    Money-no-object, this would be perfect: http://www.vandamwoodcraft.com/frames/alphaz.html

    A scaled down, and simplified "replica" would be nice. :)

    I have been wondering about doing a double skin glassed strip plank hull, with a core of pour-in PU foam, with common (but downsized) frames between the skins.
    Is this a construction method tried out before? I have not come over any as per yet...

    Your thoughts?
     
  6. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I still think the quickest and cheapest way to make a hull like that is plywood skin over pine stringers and plywood web frames. I think you should put the frames inside the skin to get them stiff enough. I don't know the weight and cost of teh engine, but materials for hull, deck and internal structure could be 500kg of okume plywood, pine stringers and glass from Devold AMT at an average cost of 60NOK or 10USD per kg.
     
  7. TomE
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    TomE Junior Member

    Karsten: I will probably use a architect to design me a one-off, or buy drawings that can be easily adapted to my needs. I can see myself drawing/making a hull that will probably float, and do a decent top speed, but there is no way that the boat will be performing the way I want it to do under all conditions. My main considerations are performance in adverse conditions and stability/manouverability at high speed. Anyone can cut a 24 degree v-shape, and copy somone elses keel-line, but when the shape of the bottom are changing from concave at the bow to straight, or even convex at the stern, and incorporating steps/vents I'm in over my head.

    Thor: I agree with plywood probably beeing the quickest and cheapest. But the boat will also look cheap and simpe when finished. This is mostly my feelings more than actual hands-on experience, but I see problems forming plywood in three dimensions (like with sheet metal). Strips can be twisted and bent at the same time and make the beautiful lines I'm after - the ones that makes your eyes wet by looking at them. ;) I think the perfect wooden boat will give assosiations to looking into the eyes of the woman one loves. But now I'm my daydreaming has been carried way off...

    Back to issue - I have some considerations with regards to putting foam in an marine environment. It does not really appeal to my "common sense", or "farmers wits" as we say around here... Especially when used under the waterline.
    Anyone have experiences that would make me sleep well at night after putting it into my hull?

    Also; can you guys give me more information on ABS and LYS systems? Pros and cons, where to find the details etc...

    Thanks for all inputs!
     
  8. Karsten
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    Karsten Senior Member

    Very nice boats on the webpage! Some info on cores so you can sleep well again.

    I'm not a motoryacht expert but I recon a 500HP engine in a 25' hull should make the boat go quite fast. This will lead to quite considerable slamming loads in the hull bottom depending on the height of the waves.

    Usually cores are not used in this area because of bonding, toughness and core shear issues. If the skins are not properly bonded to the core they can disbond and distroy the sandwich effect. This is especially a problem if you glue the core to the female molded outer hull skin because you can't see if there is a proper bond between the core and the skin. If you use male frames, strip plank the hull and cover it with fibreglass you should be o.k. Using epoxy also helps.
    Because the slamming loads are high the shear loads in the core are also high. Therefore you need quite a strong or very thick core. There are foam cores with different strength ratings so this is not a big problem if you use the right foam.
    The toughness in the core is important because slamming is not a constant load. Under changing loads a brittle core can crack and again distroy the sandwich effect. Therefore I wouldn't recommend cross linked PVC cores.
    You shouldn't worry too much about water in foam cores. The cells in the foam are closed. So if one tiny cell is "flooded" the water can't go anywhere. Wood is worse in this respect.
    I first thought you were talking about a sailing boat. For a motorboat like this I would use the DnV rules for high speed and light craft (www.dnv.com). They are not really made for small boats like this but should give you a good result anyway.

    If it was my boat and money wasn't an object I would use:
    Hull: Strip planked with CoreCell A foam and glass - epoxy skins
    Deck: Foam core with carbon - epoxy skins. The carbon will look very nice in combination with the wood. I woldn't use carbon in the hull bottom because it makes the ride harder because it's stiffer.
     
  9. Raggi_Thor
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  10. FAST FRED
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    FAST FRED Senior Member

    I have built deckhouses ,cabin tops , some decks ect from foam core.

    For any shape that can be created in ply , a usefull system is to build a simple set of jigs to hold the outside in shape.

    Lay the foam out on any flat surface and glass ONE side, then trim and lay in the mold with the glass surface to the outside.

    When all the pieces are done , carefully tape and then fully glass the inside of the boat.

    Install some bulkheads and pull the shell from the jig.

    Finally glass the external joints . This gives a tough hard chine boat that is really quick to construct .

    FAST FRED
     
  11. D'ARTOIS
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    D'ARTOIS Senior Member

    What comes from Michael Peters is only the best - I met him once in Holland and his designs are close to sensational.
     
  12. TomE
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    TomE Junior Member

    D'ARTOIS: It's very reassuring to know that the design that struck me as close to perfect actually is. Even though his designs are way beyond the scope of this project, it may be used for guidance and something to stribe for - get as close to as possible within the limitations I have - cost/skills.

    Karsten/Raggi_Thor/Fast_Fred: It is very interesting to compare the advice you two give; I consider both good - plywood is most certaily the most cost/time efficient, and strip planked foam superior in strenght but more costly. Strip planked wood may be somwhere in between I guess?

    Is it possible to give some (guiding) hard numbers to compare the three methods - mechanical properties/cost/weight/manhours?

    I do like the approch to start thinking in lines of a money-no-object, and see what I can get. Then start compromising - I consider it often to be the case that one can reduce the strenght-to-wight by, say 10-15% and save 50% on materials cost. If I then can add 20% in weight and have close to full strenght and still have the weight under the set 1200 kg I'm getting closer to what I want.
    Then again; what is possible to attain with with hig-tech materials is probably WAY more than what I really need for this kind of boat...
    I kind of want things to be "bullet-proof" - the boat should be able to take more beating and slamming than the pilot driving it. Like the guys at Bowler Offroad; they where building a car to compete in the Paris-Dakar rally, and one of the few problems encountered during testing was one of the pilots shaking a lung loose... I like that kind if engineering... Can't help it... ;)
     
  13. D'ARTOIS
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    D'ARTOIS Senior Member

    Looking a bit deeper into your project, you better sta away from sandwich-constructions. Rather use aramide skins between layers of plywood; already in the late '80 s we have tested this material at the Bruynzeel factory, where we did a variety of bonding tests together with the engineering of new and very strong laminates.

    A solid laminate is in my opinion not only stronger than a sandwich laminate but also less vulnerable to shifting loads, the more if you are using such terms as "bullet-proof".
    DSM has some newly developed fibers that are ultr strong and can absorb high impact-loads and that is exactly you do require.
    A sandwhich construction might have some all-over strength properties, high impact loads are not their forte.
    Secondly, interlaminar delamination is hard to detect and often occurs in places where inspection is dificult.

    We have used aramide between veneers ase so[called "gluecarriers" some epoxies have difficulties to adhere evenly - a gluecarrier insures the best possible bond.
     
  14. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Strip planking with norwegian pine (stronger) or some kined of cedar (ligter) may cost as much as 50USD per square meter if you buy ready made strips. The cost may be 1/10 of that if you can make the strips yourself.
     

  15. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I feel safer in a solid skinned boat (if wood + glass = solid) than in a sandwich hull. Even with a lot of frames and stringers you can allways see what's going on.
     
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