Carbon vs fiberglass for autonomous transatlantic boat

Discussion in 'Fiberglass and Composite Boat Building' started by andy47, Oct 10, 2016.

  1. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    I am designing a 2-metre autonomous sailboat that will attempt to cross the Atlantic ocean. I am trying to find the best material for a foam-filled hull, a rigid foam-filled sailwing, a keel with lead weights and a rudder. I am considering between fiberglass, carbon fiber and Kevlar. The cost of material is not important in this project. Based on several articles and forum threads I have read, I decided to use these materials:

    1) Hull: fiberglass

    The hull has to survive extreme conditions like storms and hurricanes, but also impacts (e.g. rocks, icebergs, a floating piece of wood, transportation, etc). As I understood, carbon is more brittle and it would not tolerate impacts as well as fiberglass. Fiberglass is also more flexible than carbon fiber. However, I have seen similar projects that use carbon fiber instead of fiberglass.

    2) Sailwing: carbon fiber, then one layer of Kevlar and then another layer of something (carbon fiber?) for UV protection

    The sailwing has to be lightweight (!) and rigid (perfectly symmetrical), therefore I go for carbon fiber. The outer layer of Kevlar would improve abrasion resistance and prevent breaking the sail in a strong wind or high ocean waves.

    3) Keel: carbon fiber and a layer of Kevlar

    The keel must be rigid and it's absolutely critical that it won't break apart.

    4) Rudder: carbon fiber

    It must be rigid and lightweight.


    I will appreciate any feedback.
     
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    fiberglass is tougher, will withstand impact damage better and is more resiliant that carbon fiber. if regular groundings or impacts are a concern they often put a layer of fiberglass over several layers of carbon. If impact resistance is not a concern, and light weight is, than there is no reason not to use all carbon fiber, particularly if cost is not a considration.

    The only reason to use fiberglass is cost considerations, it can be made as strong as carbon, but it will weight much more. so carbon will always be the best choice to save weight.

    Consider that you are designing to a desired hull strength, so the amount of fabric, resin and hull configuration will define how much thickness you will be using. There would be little weight savings if you just put the same amount of carbon fabric as you would fiberglass. Most of the weight is the resin, which also means a build method that minimizes the amount of retained resin would be lightest. infusion or vacuum bagging it will minimize retained resin.

    Good luck with your project.
     
  3. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    Thank you very much!

    I have to experiment with a different number of layers to find out when fiberglass is comparable to carbon fiber and what are the weight savings. I didn't find any estimates.

    I am still worried that carbon fiber may be too stiff for the hull. The weakest point is part of the hull where the mast is attached (the mast will go through the hull from top to the bottom). The mast is obviously made from a much stronger material than the hull. As you can imagine, strong forces that act on the sailwing try to break the part of the hull that is surrounding the mast. If the hull is made from carbon fiber that is not thick enough or the acting force is strong enough (imagine a 7-metre wave attacking the wing), it can result in a catastrophic failure. However, if the hull is made from fiberglass, it can just temporarily bend without permanent damage. I am not sure if my assumption is correct.
     
  4. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Both glass and carbon laminates will bend.
    What matters is how thick they are.
    You really need to try and get some engineering done on this, not guess or ask others who have never even seen a drawing of your boat/ sailrig.

    Even the best engineer will have made assumptions as to what you are actually building.
    Without a drawing even perfectly true information may not work for you.

    Do you know how to sail?

    Its not obvious to me that the mast is stronger than the hull. Especially if you reinforce the hull properly. On real sailboats the mast generally fails before the hull, but that depends upon the details of the design, and the idiot who is sailing.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You might get pinched for being responsible for a hazard to navigation !
     
  6. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    The autonomous boat is too small and lightweight to be a hazard for a big boat. There are plenty of objects of similar size and weight floating around. It may be a hazard for kayakers that would try to cross the ocean, but they accept that risk.
     
  7. andy47
    Joined: Oct 2016
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    andy47 Junior Member

    Sorry I don't have a drawing (the design is still not final), I just need some rough overview of the materials - what's best for the hull and what's best for the sailwing. I just ask whether my assumptions are correct. The material strength differs by the manufacturer, so either way, I have to test different layer combinations before making the final product.

    The mast is stronger than the hull because it can be made stronger. On my experimental boat, it is made from a thick carbon fiber tube and the lower part of the mast is reinforced by a stainless steel rod. It is just easier to make a strong mast (in my case, unbreakable) than a strong hull. If it has to break, the hull will break first.
     

  8. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    I think I would rather have the mast break than the hull, it can still float with a broken mast, maybe not with a broken hull. But I don't sail, too lazy.
     
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