Can I build 50' free-standing masts out of composite?

Discussion in 'Fiberglass and Composite Boat Building' started by Seafarer24, Aug 15, 2011.

  1. rxcomposite
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    rxcomposite Senior Member

    The formula looks very simple. The EI defines the rigidity. But the "E" is elusive and is defined by the type of CF, the resin/fiber combination, and the type of process.

    There are at least four distinct type of CF that is available. The Low modulus, Standard, High Modulus, and the High Strength. The Elastic modulus is very much different from each and the process used largely dictates the final E.

    If you are going to do it by manual layup, you will be limited to Uni with biaxial sleeve and limits you to about 50 to 54 fiber content. If you go filament winding, You can only use Uni.

    Look at how the E varies with different CF laminate type and process used. You can only make an educated guess. Composite Modulus_process.png
     
  2. rob denney
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    rob denney Senior Member

    The formula is indeed simple, but there are a bunch of ways masts can fail, so there are several things to allow for and adjust.
    It is necessary to analyse all the options if you are building a race boat mast. For a cruiser, not so much. It will be standard modulus carbon with epoxy, either infused or wet laminated and vac bagged. Both have pros and cons, but infusion is my preference as it is cheaper, allows more time to get everything right and does not have the panic factor associated with bagging wet laminates.

    It is easy (and necessary for a first timer) to do a sample piece, weigh it and determine the resin/fibre ratios. With properly infused carbon laminates, 60:40 fibre:resin by volume is not difficult, which is slightly better than 2:1 by weight.

    Biax sleeve for a big mast is expensive, does not properly allow for the taper and cannot be bagged without crimping the fibres. Better to use +/- 45 glass to save money for a bit more weight, although with the price of carbon remaining static vs the price of resin going up, this is becoming less applicable.
    Filament winding a big mast (or even a small one) without some means of laying the fibres lengthwise is near impossible without using prepreg, and bagging it every layer or 2.
     
  3. rxcomposite
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    rxcomposite Senior Member

    Absolutely right. Small mast for small craft, strength is not a problem. It is the stiffness or bending that leads to catastrophic failure. Constant section will not work. See images of preliminary bending analysis. You need to vary the thickness along the length to control bending. For low modulus type of construction, the “I” is the dominant factor. If CF (high modulus), then it is the “E” that dominates. Thus, low modulus means largely varying cross section tapering small at tip. With CF, it is easier to vary the number of layups. Not just the number but the length is segmented. Not all is the same length.

    For small boat, oftentimes certain species of wood exceeds the modulus of hand laid up fiberglass. So, a birdsmouth construction sheathed with biax will work better. Biax has a high modulus of rigidity and resist the shear which is mostly on the surface of the mast diameter.

    Small mast with little to no taper can be built with Uni and Biax sheath. If the taper is dominant, the sheating bunches up, unless you stretch it. Then the 45 degree angle change and the modulus changes. It is sensitive to fiber angle change.

    Infusion like you said seems to be the best choice for a tapered, varying layup method. Filament winding will tend to bunch up the axial tows at the small end of the mast. Usually it is “pinned” from end to end and there is no way you can have varying length of Uni along the length of the mast. See typical layup schedules.

    So to answer the Q posted by Russel W, NO. Even with the best equipment available, you will not be able to build the mast.

    The sail designer designs the sail first before the mast. The mast is designed according to the sail. You need a complete understanding on how it was designed and spec’d.
     

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  4. rob denney
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    rob denney Senior Member

    We have sold several plans for amateurs who have built their own carbon/glass masts up to 60'/18m. I have built a few myself.
    You don't need fancy equipment: a string line, long bench, skilsaw, some 25 x 25 (i" x1") straight edges and some material that will conform to the bend (formica, sheet of fibreglass, thin plastic) will get you a half mould (does not need to be shiny, just fair), which can also be used for an integral sail track and the join. Then a vac pump, bag, tacky tape and some shade cloth and you can infuse it.

    I've also built a couple of prepreg masts in autoclaves I have had constructed. Not cheap (6mm bent steel plate with 8mm high tensile bolts every 100mm), but 105C and 6 atmospheres certainly gives a void free laminate.

    The sailmaker cannot get the correct luff round for a sail on an unstayed mast unless he can measure it from the mast. So, the mast is built first.
     
  5. rxcomposite
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    rxcomposite Senior Member

    Good then. You have developed the method that works.

    Me, I tend to be impractical and tends to look at numbers.

    My best Working experience is on airplane spars, prepreg carbon and Eglass, vacuum molded, high temperature curing. Pure carbon prepreg laminate has a metallic ring when cured. The math is very similar to a cantelevered load on a free standing mast.

    I design the mast after the sail because of pressure distribution. I need that for segmented design as it dictates the bending characteristics of the structure.
     

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  6. rob denney
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    rob denney Senior Member

    Yes. It is a variation of how we build the Harryproa hulls and beams.

    Looking at numbers is critical to success, but it should be tempered with experience.

    A carbon laminator's favourite noise! Also happens with infusion.

    There are a few differences. Wings don't have vangs, booms and extras. They don't rotate and are not usually as thickness (diameter on masts) limited. Masts don't have birds fly into them at 100 knots, although a surprising number of them get dropped or run over by forklifts, which is another good reason for having an unstayed mast: you don't have to take it out to check the rigging each year. It can stay in place until it needs a repaint.

    That is the sail dimensions, which are a necessary first step, although I usually use the righting moment as the load case. But to build the sail, the mast needs to be built first.
     
  7. rxcomposite
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    rxcomposite Senior Member

    You must have good business running.

    Wings are sails as far as pressure distribution is concerned except the spar doe not rotate except when the flaps are down. Sort off as it is located in the center of lift. We use angle of attack AOA instead of RM. Very very similar. And in sails, it is a compound load. It is a twisting load dictated by the sail profile.

    For bangs and booms and mast support, it is concentrated load in compression hence the need for additional hoop winding to prevent buckling.
     
    Last edited: Oct 3, 2022
  8. rob denney
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    rob denney Senior Member

    Sure do. Good enough to take a couple of years off to build and pay for an 80'ter to see if it is possible to do something about climate change and servicing remote villages.
     
    Flotation and rxcomposite like this.

  9. Andrewski
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    Andrewski Junior Member

    can you offer any links, thanks
     
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