Cold moulded Brazilian cedar V's E glass and Vinylester

Discussion in 'Sailboats' started by willfox, Sep 13, 2009.

  1. willfox
    Joined: Oct 2008
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    willfox Junior Member

    Just wondered if anyone had an idea which boat would have the higher displacement if both were identical with same factors of safety. one built with say eglass and one Brazilian ceder. Does not have to be these particular materials but just trying to guage loosely how the weight would vary using composites and cold moulding techniques. :confused:
     
  2. apex1

    apex1 Guest

    The cold moulded wooden hul ist stronger and lighter than a glass boat.

    The wood you refer to (Cedrela fissilis) is not very common in boatbuilding but not a bad choice. But make sure, you get that species! Names vary and there are lots of weaker species on the market for most of the known names.

    Regards
    Richard
     
  3. Steve W
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    Steve W Senior Member

    I agree with Apex,the cold molded boat will be lighter.It takes a lot of effort in engineering and resin to glass ratio control as well as cored construction to build a composite boat as light as a wood boat (assuming modern wood techniques of course) I read an article decades ago in an Australian magazine where the late,great multihull designer Lock Crowther observed that of all the cats built to his ,i think they were called Spindrift 37,40 and 45ft designs to date,all of the cold molded versions had come in under weight while none of the composite versions had.A very good book that compares the weights of various methods is Dave Gerrs "The Nature Of Boats" Chapter 47 is titled "believe it or not,wood is best" where he devotes the entire chapter into proving with the relevant mathmatics involved that pound for pound wood,(he uses douglas fir in his example) is lighter.After going through all the calculations he states,and i quote. "amazingly,and contrary to everything you have probably read and heard,wood is structurally far more efficient than ANY of the building materials known anywhere.Its even more efficient than titanium and advanced graphite fiber and spectra composites.In other words,all things being equal,you can build a lighter structure out of wood than anything else,for the same stiffness" sounds good to me. That is a great book btw,i give it two thumbs up.
    Steve.
     
  4. Paul Kotzebue

    Paul Kotzebue Previous Member

    Two of the issues to consider when engineering boat structures are strength and stiffness. Wood is a relatively stiff material, and E glass is a relatively strong material. E glass needs a core or lots of internal framing to meet scantling rule (such as ABS Guide for Building and Classing Offshore Racing Yachts) requirements. Wood needs to be heavy enough to meet the strength requirements. In my experience the weight of a cored E glass boat is about the same as a cold molded wood boat, if both boats meet the ABS guide. A single skin E glass boat without internal framing will be very heavy.

    This is from the USCG Guidelines for Review of Structural Plans for Wooden Vessels:

    "(W)ith cold molded shell and deck plating, the cold molded wood plies in which the grain runs parallel to the ply, are generally laid at plus and minus 45 degrees to the longitudinal axis of the boat. Tests conducted by ABS Americas have determined that the ultimate strength in the principal axis of a panel laid up in such a manner is about 22% of the ultimate strength in the principal axis of a panel in which the plies are laid parallel to the principal axis. Thus, without the builder conducting material tests on the cold molded wood laminate, the modulus of rupture value to be used in the design calculations must be 22% of the particular wood species’ established modulus of rupture."

    What that means is diagonal planking has to be thick enough to account for the reduction is strength of wood laid diagonally vs perpendicular to the stiffening members.
     
  5. willfox
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    willfox Junior Member

    ok...brilliant, thanks guys....so am i right in thinking that the real reason for using composites is for a cheaper production process and lower man hours for creating the hull?
     
  6. Paul Kotzebue

    Paul Kotzebue Previous Member

    You're right if you're comparing a simple E glass structure to wood. However, advanced composite structures using pre-impregnated carbon fiber materials and vacuum bagging, etc are very expensive and very strong.

    There is no way a wood structure is as strong a one built with carbon fiber.
     
  7. willfox
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    willfox Junior Member

    At the mo im doing parametric research into classic yachts...pre 1960 which are all of wooden construction and comparing these to the modern classics of today. The 3 companies im looking into are Hinkley, Morris and Spirit. Spirit use Brazilian cedar where as the other use scrimp with with e glass carbon and kevlar. Spirit Yachts are definate performance yachts with high ballast ratio's and Slenderness ratio's where as Morris are much more in the Rassy, Najad, Oyster region. Hinkley being more classic oriantated with lowest slenderness, and highest SA displacement etc...
    I am currently putting together my dissertation for my degree at university. I am intending to design a 'sell up and sail' classic mid 50 foot, pure cruiser intended for round the world (the right way use). I'm really keen on designing a cold moulded vessel because I really want to learn more about the construction technique. I dont know if this is appropriate way to build this sort of yacht considering I have the option to use whatever I want! Maybe I should really be designing in composites as these have much better impact resistance?
    What would your opinion be?
     
  8. foxy
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    foxy Junior Member

    Paul is correct when he says that for equal scantling requirements the weight of an e-glass foam cored composite and a wooden boat will be comparable. I've engineered many boats in wood and in GRP and wrote an article on this subject for Professional Boatbuilder in 2001.

    For that article, Rodger Martin allowed me to include test data that he had run, when designing Gray Wolf, at Gougeon Brothers on the hydromat they developed. It showed that contrary to popular belief, cold molded construction is neither strong, nor stiff compared to cedar/strip composites or E-glass/foam panels. In fact, the E-glass/foam panels were just slightly better than cedar/strip and the cold molded panels failed at about 45% of that.

    The article included laminate schedules for two similar boats in the 36 foot range. The hulls were within 10kg of each other as built.

    There are some good reasons to build in wood and good reasons not to. It depends on your preference and availibility of materials in your area. And it depends on what type of boat you are building.
     
  9. gggGuest
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    gggGuest ...

    Certainly SteveW's conclusions are at odds with all empirical experience with smaller race boats...

    Few people would disagree with the statement that "traditional" uncored glass fibre was probably technologically the worst boat building material ever given widespread use. Its only real advantage was cheapness for low cost/low skill batch production.

    As foxy says basic cored construction (foam or cedar) with a good quality glass skin is roughly on a par with wood. Engineering wise they bounce around the same sort of area with advantages and disadvantages all the way round.

    I don't know of Mr Gerrs, but much as I like working with wood, his sums must be way off the track. In the development dinghies "achieveable by the amateur" minimum weights have plummeted since carbon fibre/epoxy became readily available to the amateur.

    However for your "sell up and sail" project strength and stiffness may not be the key factors in the same way they are for a racing craft. Things like maintenance overhead, ability for self repair or repair in areas without first world engineering facilities, things like that are likely to be at least as significant factors. These less tangible factors are likely to be key...
     
  10. apex1

    apex1 Guest

    At the time Dave Gerr wrote his statement he was dead right! Today with some further developed techniques he would make the exemption of carbon composites.
    But wooden boatbuilding is not the same as it was too! The old, simple method of cold moulding is not state of the art! With a clever designed composition of materials, fibre orientation and building methods we are pretty close on the heels of carbon and far far ahead of any glass composite! But still call it cold moulded or strip planked (which way so ever the choosen method derived from).

    Regards
    Richard
     
  11. Steve W
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    Steve W Senior Member

    gggGuest, Dave Gerr is a naval architect and author of numerous books and i believe he is currently the head of the Westlawn school of yacht design,maybe youve heard of it. If you want to build a ultra light weight PRODUCTION raceboat then the only practical way to go is a cored composite structure in female molds but when building a custom boat you have options.Given an equal budget im not sure which method would come out the lightest but it would be close. Dollar for Dollar wood will be way ahead.
    Steve.
     
  12. foxy
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    foxy Junior Member

    At the time when GRP construction was new, people did not understand how to use the material so well and it was pretty hard to produce a small GRP boat that equaled wood in weight and stiffness. Forty years later, we know a lot more about engineering in various materials and computers allow us to crunch numbers quickly and explore more possibilities.

    Cost of materials varies widely and what is cost effective in NZ or Brazil may not be what is cost effective in England or the US. You have to include labor in the mix as well and both hourly rate and productivity enter into that.

    Wood will certainly be cheaper in an area where wood is easily obtainable AND you have a work force that does a lot of wood construction in the type of boat you are building. GRP will be cheaper in other areas. If good quality timber is not grown locally, you are going to pay a lot to get it and the labor force may not be so efficient at building with it.

    People building race boats usually build them to win and will spend accordingly. We have several builds with strip planked/composite hulls, but opted in all cases for foam cored composite decks because we could build them lighter that way. Hulls usually come out about the same, and internal componants almost always come out lighter in foam composite than plywood. This is in building to ABS or ISO 12215 scantlings, not some arbitrary assumptions.
     
  13. willfox
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    willfox Junior Member

    Thanks so much for everyones help on the topic. This has given me a huge amount to think about.
     
  14. BeauVrolyk
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    BeauVrolyk Sailor

    Willfox,

    I have sailed all the boats you're mentioning and of those the Spirit (especially the Spirit-46, which is really their performance model) is by far the lightest and fastest. I don't think S&S ever really intended the Morris boats to be "fast" in any ultimate sense. But, this summer we had a Spirit-46 planing along at 18k down wind without a lot of fuss and bother.

    That said, and as much as I like both cold molded and light glass boats, for a classic mid-50s cruiser intended for round-the-world I would strongly suggest you go in a completely different direction. This is because round-the-world cruising is a long long long way from bombing down the Solent with the chute up. You'll want reserve strength and resistance to damage that neither of the construction technologies you're talking about will have. I would strongly recommend you look into a double bottom steel hull.

    I sailed the 65' Tom Wyle design "Saga" from San Francisco to New Zealand and back over the course of four years. She had previously been around the world twice. Since I sold her, she's gone to Europe and back from San Francisco through the Panama Canal, and is currently doing her third trip around the world. In the course of all those miles she has run aground hard a few times, tapped some rocks, and lay on her side a half dozen times. In most of these events, which thankfully didn't happen whilst I was her owner, she got away with only a few dents and some scratched paint. If any of these events, except the laying on the side bit, had happened to the Spirit there would have been extensive damage, as there would have been to the glass boats you're considering.

    Modern construction techniques rightfully stress light weight to gain performance. But, there is very little benefit to finishing an 18 day passage from San Francisco to the Marquesas a day or even two days earlier. There is a tremendous advantage to knowing that the keel won't come in through the bottom of the boat when a big wave drops you on a sinker off of the Bay of Island! All boat designs are trade-offs, and I would be completely with you if you were talking about a weekend boat, short distance cruiser, or race boat. But, you mentioned going around the world, and that changes the parameters entirely.

    Whilst one could certainly sail a Spirit-46 around the world, as one could with the Morris, there would have to be a level of caution on the part of the skipper that just isn't exhibited by the cruising skippers I've observed in over 40 years at sea. These boats aren't built for what you're considering, and their construction technology (while quite correct for what they are intended to do) isn't adequate. Have a look into steel construction. For a one-off it's by far the least expensive, it's astoundingly strong, and if done right isn't awfully heavy.

    Best,

    BV
     
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  15. apex1

    apex1 Guest

    That is just to back, by any means!



    said the wood epoxy builder........................


    Regards
    Richard
     
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