need help with a assymetrical cat- plywood and other things

Discussion in 'Multihulls' started by assycat, Apr 3, 2011.

  1. assycat

    assycat Previous Member

    HI everyone--just a new guy, wanting to build a p95 kohler designed asymetrical catamaran. stats: 30ft loa, beam 17ft single mast. etc
    in plywood/epoxy/glass. please see for the p95 stats.
    im hoping some more experienced builders could follow and guide me through the process?

    so- Going to order my wood this week so I need some advice..
    my eyes popped out of my sockets when i got the sticker shock from the price of okoume! so here is the issue.
    the boat calls for okoume so I contacted the designer and asked to use df fir instead. which he ok'ed except that he didnt say if i could replace the 8mm with 6mm which is what id asked-he just reponded with "use df fir" Mr kohler has been nothing but helpful- so i didnt want to bother him again although in a week or so if i cant find the answer myself ill call him..

    ok sorry for the long intro.

    anyway if using comparable thickness in df fir, from my understanding this adds about 5 lbs per sheet due to the fact that okoume is lighter by about 5 lbs per sheet.
    this means a weight gain of about 200 lbs.overall- to me a lot for a cat?

    my first question is- how significant is this weight gain?

    and will it affect my performance much?

    so maybe im just being too anal with regards to saving 200 lbs of wieght?

    if not however:

    in order to cut costs but still have a fast strong light boat i wondered about some possible solutions to paying 6000.00 in high grade plywood(i believe a fiberglass cored hull would be comparable in price so why build in wood then?)
    here is what i was thinking as some solutions:

    1. I could use 6mm df fir instead of 8mm(weird thickness, dont ask me why, its just whats in the BOM)-this would equal or be lighter weight to the 8mm okoume and im told that fir is actually a bit stronger than the okoume??(need verification).
    2 i thought about using 3 mm okoume and glassing both sides for strength, like a wood cored hull(?) this should balance out the weight to equal the okoume and hopefully be less cost.
    3. there is okoume b.s. 6566, about 50.00 a sheet at noahs marine, but its 9mm not 8mm what the heck is the diff other than price for the 1088? what do those designations mean in marine ply? and at 2000.00 for 40 sheets-its still expensive to me.(?)
    4. I could use red or white cedar and strip plank the hull glass both sides-this would be very light and about(?) the same cost as the lower grade 6566 okoume(?)(this idea seems good to me except that the hull is triangular in cross section, would this be a problem?)
    5. if meranti is stronger-go with a 6mm for that?

    i apologize for the long post, please bear with me...thanks

    also if there is anyone else building or has built a kohler design or built or building or built the p95-id love to hear from you

  2. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    I have,nt started my build yet but I have gone though the same process you are going though now.I have all my material for the build , I went with foam core. Get bulk prices from fiberglass supply on your foam, their site list the case prices, you will use about twice the epoxy with foam and silvertip is the one recomended by the bateau web site for foam. Compair the wood to foam price , for the little extra money you spend in my mind, you get an easyer build , lighter boat,a little more interior room, less maintance over the years ( no rot) ect. The saveings between wood and foam build is not much if any in the total cost of the boat. rick
  3. assycat

    assycat Previous Member

    Hi Rick thanks- i had to laugh as I was writing my post i had forgot to mention the core--grrr
    --i had worked with some core cell a couple years ago and let me say its wonderful stuff!! I priced out some 9mm core cell at 20 bucks per 7 square ft at Noahs. However the 6.4 mm was 14 bucks.
    The designer did say that I could go with a cored hull but the beams, i.e. the cabin and mast beams needed to be wood so they imparted strength.
    I agree with all you said about core. In fact that has always been my first choice. My main concern is--what thickness would be comparable to the wood at 6mm(1/4 inch)?? I know when i did test panel two years ago the 1/4 was VERY strong but Im not sure if 1/4 inch is too thin for the hulls and maybe 3/8th would be better but does the weight change? i.e. will the layers of glass make it more heavy-since i would assume two layers. both sides?

    what are you building??...
  4. Doug Lord
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    Doug Lord Flight Ready

  5. assycat

    assycat Previous Member

  6. rberrey
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    rberrey Senior Member

    Best case your designer should give you scantlings for the foam build, worst case you convert wood to foam scantlings yourself.I have Ed Horstmans book ( foam fiberglass sandwich construction). I will post a conversion from his book.
    cold molded = cm
    single fiberglass skin =sfgs
    sfgs, single skin refers to hulls that are built up of fiberglass laminations only, no foam.
    formulars, cold molded plywood to foam sandwich construction.
    A.ratio 1 cm to .66 sfgs
    B.sfgs X 2 = core thickness
    C. .7 of sfgs = total fg laminate

    Example using two 1/4 cm plywood skins, equaling 1/2 plus the glue and the fiberglass hull cover of say 1/16, which gives, .25 + .25 + .062 =.562 of cm.
    1. (A) ratio 1 to .66 or .562 x .66 = .371 sfgs
    2. (B) .371 x 2 = .742, or 5/8 or 3/4 core thickness.
    3. (C) .7 x .371 = .259 total laminations thickness.
    4. .259 divided into two surfaces = .129 thickness of fg laminate per side.
    He adds ,for number 4 you might add 10% to the outer hull laminate to compensate for
    the extra abuse it receives.
    I will also add there is a lot more to converting wood to foam , keel area build , stringes, ect, you can order Eds book from his web site . rick
  7. assycat

    assycat Previous Member

    I just price out 780 square ft of 9mm core is cheaper than 6mm and 9mm okoume. the plans call for 40 sheets 8mm...
    cost for 9mm core cell- 2400.00
    cost for 9mm okoume-3616.00
    assuming 1 layer of epoxy and cloth over the okoume and two layers of cloth and polyester(yes i know your all thinking stay away from poly but on core cell its great!)
    over the hull, its seems corecell is about 1/3rd the cost to do the hulls.????
    so right now it seems a core would be better-
    now assuming the core is better, i now need to find out how to do the bulkheads, stiffeners etc to make the cored hull strong- i really dont know much about a cored hull, so now it would be a learning curve...I know that you cna do hand layups of corecell..
    so this should mean not having to vacuum bag. but how do you attach the stringers bulkheads etc? to a cored hull?
  8. assycat

    assycat Previous Member

    Rick--thanks again. I just posted here that its even less expensive for core im leaning that your boat a multi-hull? thing i should mention--the hull not rounded i.e. its not a round bilge or radius chined hull...
    ill pick up the looks great...especially if in the end I use the core...
  9. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    My build is a tristar 31 , you might ask Catbuilder what he thainks about corecell v divineycell h80. You also can do a hand layup on your foam, also ask catbuilder pros and cons of male v female form or mold. I would also look alot harder at epoxy v poly. Go on fiberglass supply,s web site and price out material as well. Other sites are thayercraft, and US composites. rick
  10. rberrey
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    rberrey Senior Member

    I would think you will have to have thicker foam than wood, 9mm (3/8) would be to thin for that boat as a rule of thumb. I would bet it converts out to between 5/8" or 3/4" foam.rick
  11. assycat

    assycat Previous Member

    yes you could quite well be right about that Rick...Its hard to apply youngs modulus to composite. unless you know the given thickness of laminate and foam..
    but isnt core cell foam?? ill read again the above- i dont recall if you gave a trade name for the foam you are talking about? and at 9mm the Corecell is actually heavier than okoume!...
    im guessing the person you speak of must have had a delamination problem??
    ill look into it..but the stuff i tested was amazing--shear strength was awesome and was ductility and compressive strength, at leats on a practical level where i could hold it and hit it with a hammer etc...but I dont have the yield numbers for you.
    i just cant justify the extra cost of epoxy if using foam though...but again ill make sure -i think there are no easy answers to the boatbuilding game...
  12. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    Corecell is a good product, Catbuilder post on this site and has already raised many of the questions you have and will ask in the future. After choosing foam and build method he may have advice on his chorces. If you know the thickness of your ply and fiberglass hull cover you can use the formular I posted to convert and get the thickness of your foam core and fiberglass. Corecell is pricey, price other closed cell foam as well. rick
  13. assycat

    assycat Previous Member

    I'm on my way right now over to check out catbuilders posts...the only other closed cell foam I know is dyvinicell..ive never even been able to find it--I dont know too much about it but I havent heard the best things about it when i looked on the net. But of course I dont know too much about the product..but i do like the corecell!!..

    my boat i should iterate is not a bluewater voyager but can do crossings. its a solid boat apparently. I like the speed it offers....ill take less accomodation for speed any day but thats just me..ive never seen a woods design that i know itll be interesting to check it out..
  14. assycat

    assycat Previous Member

    I looked at some nida-core--since my cat has flat sides, it looks like a great product..and at 2100.00 for my materials its also a good deal too..
    here is what noahs says about it:

    "Light Nature provides the geometry, we provide the honeycomb composite material. Nida-Core is extruded from tough, versatile polypropylene plastic. (Other materials are available). Thermo fused to the honeycomb cells are non-woven polyester bonding scrims with a polypropylene barrier film to limit resin consumption. When encapsulated in a laminate it is recognized by the US Coast Guard as a primary flotation material in small boats. Tough The important question is: "How do these plastic honeycomb sheets hold up in the real world?" Although no lightweight sheet is the answer to every possible application, Nida-Core boasts the broadest list of positive features. Some honeycomb cores are either stiff, light, quiet or economical. Nida-Core is all of the above...and more! Quiet Polypropylene honeycomb is an effective sound dampening material that provides a very low natural harmonic due to the polypropylene's viscoelacity and cell structure, effectively dampening sound and vibrations. Boats manufactured with Nida-Core honeycomb sheets demonstrate a dramatic improvement in vibrational dampening compared to traditional lightweight sheet construction methods."

    this comes in 4x7 sheets!
    so this option seems alright as well...has anyone worked with it?

  15. rberrey
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    rberrey Senior Member

    there are other material sites besides noahs, I never could get them to deal. Go to diab ,compair their product with corecell. Surf the web for ply.
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