Best length for 3 layer veneer fiberglass covered monohull

Discussion in 'Boat Design' started by pironiero, Aug 6, 2020.

  1. pironiero
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    pironiero Junior Member

    So im currently working on plans for building a boat and i heard that after approximately 50ft aluminium hulls are getting much more stronger and lighter than wooden, covered with fiberglass hulls, can anyone give me some info on that because aluminium is not an option for me and i want to get somewhat best weight to toughness ratio.

    Thanks and sorry for stupid questions.
     
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  2. bajansailor
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    bajansailor Marine Surveyor

    No worries Pironiero, there is never a stupid question on here.

    If aluminium is not an option for you, are there any other materials available that would be viable options?
    (apart from fibreglass covered wood)

    What type of boat are you currently designing / planning to build?
     
  3. pironiero
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    pironiero Junior Member

    im sorry to say, but veneer is the only option for me, part from planks or plywood, my father has already built couple of boats, 11m sloop and 15m ketch, i asked him about hull material and his opinion is its better to use 3 layers of 4mm veneer for building a hull instead of plywood strips or wooden planks.

    i want to build a somewhat fast racer-cruiser to sail around the world for 1 time at least and an option to live aboard would be much appreciated

    also im considering using lifting dagger keel with bulb on the end of it.

    i heard that there is an option of building a hull using foam planks but i literally have no information bout it.
     
  4. Rumars
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    Rumars Senior Member

    1sqm of 12mm thick wood epoxy x 750kg/cum = 9kg/sqm + 600gr fiberglass + 600gr epoxy = 10.2kg/sqm + fairing + paint = ~11kg/sqm
    5mm Al plate = 13.5kg/sqm
    Both need stringers and frames so in the end I would say it's about the same total weight per square metre of skin.
     
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  5. pironiero
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    pironiero Junior Member

    but on AL boat bottom part of the hull is much thicker then top which is usually somewhere around 4-5mm im talking about 50ft boats
    what about rigidity?
     
  6. Rumars
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    Rumars Senior Member

    There are two strategies, increase plate thickness or increase interior supporting structure (stringers and frames). Metal boats often use different plate thickness and wooden boats more stringers, it's a question of what is easier to build, but you can employ either strategy, it's the designers choice.
    When comparing materials we will asume stiffness to remain the same, otherwise we compare apples to oranges. The materials will behave differently on impact and abrasion, but that is outside normal sailing loads and to be considered according to SOR.
    Have you priced out singke skin fiberglass/polyester with top hat stringers and frames?
     
  7. pironiero
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    pironiero Junior Member

    im currently working on it but lack of experience takes its toll
     
  8. Rumars
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    Rumars Senior Member

    Have you calculated the loads you need to design to? Comparing materials weight can only be done to a common baseline, otherwise you don't know how thick the aluminium plate needs to be (plus stringers and frames) to achieve the same stiffness (or strenght, or whatever property you decide is important) as XYmm wood epoxy (plus glass, epoxy, stringers, frames/bulkheads). Same for fiberglass, cored or not.
     
  9. jehardiman
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    jehardiman Senior Member

    First of all, there is no magic size for the selection of a hull material. And generally, the selection of hull material is driven by other reasons than weight because it usually not a significant driver of overall vessel weight.
    The hull material must resist the primary (bending) and secondary (hydrodynamic and hydrostatic) loads the hull is subjected to while, for working vessel, being able to absorb nominal damage and be easy to repair. While there is an approximate maximum size for a wood/wood+glass hull, it is significantly greater than 50ft. The original, and still ongoing reason, to choose metal over wood was the significant increase in internal volume.
    Without a detailed hull shape and SOR, it would be almost impossible to say which material would make the " best weight to toughness ratio". All easily worked materials; cold moulded wood, various composite construction, aluminium, and steel would make viable hull materials for a world spanning cruising vessel of the 15-20m size. Other materials, such as titanium, carbon fiber, and ferrocement, are also viable as hull materials in this size, but each has unique advantages and dis-advantages. However, in my own personal opinion based on a study I did for an "all latitude" world cruising yacht of ~45 feet, steel won out due to material toughness, ubiquity of material, and ease of repair.
     
    Last edited: Aug 7, 2020
  10. gonzo
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    gonzo Senior Member

    For cruising around the world, ease of repair wins a lot of points. Steel can be welded underwater.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously the actual design shape of the hull, will largely dictate which materials are used, compound curves, largely excludes metal for most amateur builds.
     
  12. pironiero
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    pironiero Junior Member

    Guys, thanks for suggestions but metal is not an option for me...
     
  13. fallguy
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    fallguy Senior Member

    Honestly, a hull of 3x 4mm veneer for a 15 meter hull offshore racing boat sounds a bit light, but the glass sheathing will add to it. Any defect in the hull laminates means you are potentially thinner. I didn't want to post; devil made me. I suggest you read Elements of Boat Strength by Gerr and follow along by learning the requirement in solid glass and then translating that to ply. Or purchase a plan from a designer or even a study plan which may at least give you a bom or hull material.
     
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  14. pironiero
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    pironiero Junior Member

    okay, thank you very, much

    also have anyone here read principles of boat design by Lars Larsson, Rolf E. Eliasson and Michal Orych 4th edition? someone recommended it to me and i wanna buy it, but in my country the lowest price on this book is about twice as much as the price in US and we do not have amazon here so...wanna hear opinions on it too, or maybe suggestions on books about boatbuilding/designing using modern technologies and materials.
     

  15. Rumars
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    Rumars Senior Member

    It all depends what is under the skin. For a pure monocoque where the plating takes all the loads 12mm is not enough for a boat that size. But if you add stringers and frames (bulkheads) the equation changes, and 12mm can be plenty, structurally speaking. Cold molded boats of this size are usually not pure monocoques since it makes more sense to build them over bulkheads and stringers (longitudinally framed), so it all comes down to how many and what size stringers.
     
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