How to calculate fiberglass thickeness for making tubes

Discussion in 'Materials' started by Wingandaprayer, Sep 13, 2014.

  1. Wingandaprayer
    Joined: Oct 2011
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    Wingandaprayer Junior Member

    I may regret starting this thread, but I hope not.

    I am in the final stages of building a glass/honeycomb 19m sailing catamaran of my own design. So far, I am very pleased with how my design ideas are working out in practice, but we're not in the water yet, so who knows.

    It has always been planned that it would be driven by either rigid wing sails or (fallback position) junk rigs in a biplane configuration. The masts, or stubmasts, have about 7 1/2 foot of bury below the roof and are located immediately next to massive bulkheads to which they will be bonded.

    Without going into all the details just now, I need 40' ~ 45' of mast above the roof. I may have to add 7 1/2 feet to that number, depending on whether the mast is slipped over a stub mast or goes straight through the roof to be bonded to the bulkhead. The bottom of the junk rig/wingsail will start approximately 5' above the roof.

    I am exploring the idea of making the masts from fiberglass tubes. I figure that I can use paper concrete forms for the mandrel, to keep things true and round. The masts will be made of fiberglass and epoxy laid up on the paper tubes. Then the tubes will be extracted (or not?).

    I want to make the masts as large in diameter as possible, while at the same time not incurring more windage than necessary. This is because the larger the diameter I use, the thinner the wall sections can be. Each mast will carry the equivalent of 600 sq. feet of sail. While this may seem small, the wingsails make up for it in efficiency gained.

    So, what I need is somebody who is good with working out engineering specs for this type of situation. The mast can be anywhere from 10" to 18" in diameter. I have the space for that range. Although I am sure I have left out some important detail or other, I am open to serious suggestions from experienced builders/engineers.

    I know there are experienced and wise designers, builders and engineers who read these threads. Those are the guys I want to hear from. Discussions of pros and cons about how to implement my idea are welcome, but there are also some flamers on who always have an opinion about everything, regardless of their knowledge, experience or common sense. Please find another thread to set on fire.

    Help please.
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Besides getting the calculations for the mast, you should explore construction techniques and see if it possible to achieve the proper quality. Most masts are spun, that is they have a mold that turn around and the fibers get wrapped around it. Then the resin is infused. You could hand lay it, but the weight will be much higher. Do you specifically want a fiberglass mast? Aluminum is reasonably cheap. I think that with hand laid fiberglass a steel mast would probably be of equal weight.
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    WingandaPrayer--Welcome to the forum.
    First off, I'll direct you to my website and the article on the state of the art of free-standing masts and wingmasts, here:

    What you have proposed--large diameter fiberglass masts--cannot be done well, for a number of reasons:

    First, fiberglass is way too flexible and stretchy. Your masts will bend way too much and the sails will invert (camber in the wrong direction--they'll bend to leeward instead of to windward.) A high-quality fiberglass laminate will have a modulus of elasticity of 2.0 x 10^6 psi if you're lucky. Carbon fiber has a modulus of elasticity of about 8.0 or 10.0 x 10^6 if you're doing well as a do-it-yourselfer, higher if you have your masts built professionally.

    Second, the idea of going for as large a diameter as possible is bad, it creates too much form drag. You want the smallest diameter consistent with strength and stiffness so that you get the least amount of form drag for the best performance. Actually, for your size boat, 19M (62.3'), 10" to 18" seems a bit small. For a stubmast-wingmast configuration on this size boat, you might even be a bit bigger. On my Globetrotter 66 (20.1 M) design, the stubmast at deck level was 17.25" OD and the wingmast was 27.2" OD. The big difference between these is because there has to be enough room for the bearings, which are mounted on the stubmast above deck. Each mast on that cat schooner design carries 978 ft^2 of fabric sail area, which including the wingmast itself, comes to 1,100 ft^2 total sail area per mast.

    Third, going for large diameter and thin wall will make your mast prone to buckling. That is, the mast wall will collapse before the stresses get up to full load. This happens with any material. For composite masts with mostly unidirectional fiber but some off-axis fiber (±45° and 90°), I have always used a wall thickness-to-diameter ratio of 0.03 (3%). This is based on years of full-scale destructive testing.

    So, what you are going to need to do is build the masts with carbon fiber--it's really the only good material for doing this on the size masts you want to build, and you want to engineer the diameter, wall thickness, and taper for the best strength and overall mast stiffness. This will give the boat the best performance.

    Have a look through my website at some of my mast designs and boat designs with wingmasts. I have adopted the wingmast-over-stubmast technique as the best way to make good performing free-standing masts for sailboats.

    One more thing. For monohulls, I (and practically all other naval architects) always design my masts to the full righting moment of the boat (plus a safety factor of 3). The maximum righting moment of the boat determines the maximum bending moment that the masts can ever see. For a multihull, however, the maximum righting moment is huge, by comparison, and this occurs when the windward hull starts flying clear of the water. To design and build the masts to the maximum righting moment on a multihull necessarily makes the masts heavy and expensive--two things that are anathema to multihull sailors. Therefore, most designers will design mast strength (whether stayed or unstayed) to about 60% of the maximum righting moment. Just be aware of that in your calculations.

    This is from the horse's mouth--you won't get better advice than this.

    I hope that helps.

  4. Wingandaprayer
    Joined: Oct 2011
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    Wingandaprayer Junior Member

    Thanks Eric,

    I kinda hoped you might chime in.
    Now (re)thinking.
  5. rob denney
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    rob denney Senior Member

    I agree with Eric about the sizing and the material.

    We have probably built more unstayed multihull masts >15m/50' than most and I have helped a lot of amateur builders build their own.

    Building the masts yourself is not very difficult if you understand vacuum bagging, pressure moulding or infusion, but expensive if you stuff it up.
    An alternative is to have them filament wound. Because the tow used in filament winding is about half the cost of the uni sheet material, it may not work out more expensive, particularly if the winder already has a mandrel the right size. Another advantage of filament winding is that it is easy to incorporate joins so the mast can be shipped in pieces which fit into 40' containers. Not just cheaper shipping, but less likely to be damaged.

    Let me know the righting moment of the boat (more relevant than the sail area, a sail plan and any other requirements and I will quote to engineer and build it. If you would rather build it yourself, I can supply build instructions, email help and low cost carbon.


    rob denney
    1 person likes this.

  6. Wingandaprayer
    Joined: Oct 2011
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    Wingandaprayer Junior Member

    Rob, thanks for your kind offer. We are still in the planning stages and nothing is set in concrete. I will PM you with some details.
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