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  1. #1
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    Default Fibreglass Spars on Storerboats

    Another approach to lightweight spars at a more reasonable price than carbon that I'm keen to investigate for future projects is using mandrel wound fibreglass tube. I got some 'off the shelf' 32 mm ID 36 mm OD for less than $10 per metre for drainage tubes. It would have to be greater diameter for a yard or a boom, but even at 50% greater cost for a diameter over 40 mm, it's one fifth the price of carbon, and cheaper than any timber and epoxy solutions (at least at prices I'd pay for spar timber and epoxy to glue it up and coat it)

    Performance wise, the laminate has a modulus of 31 GPa, nearly a third of carbon, but if you do the engineering it's not so bad. Go up in diameter by 20% and up in wall thickness by 30% compared to what you would do in 85-90 GPa carbon, and the spar stiffness is equivalent. Weight will be greater than the carbon by about 56%, so a spar that would go in carbon at 1.3 kg would go in fibreglass tube at about 2 kg.

    A fibreglass tube of 45 mm ID / 2.4 mm wall / 49.8 mm OD would deflect 30 mm with 10 kg hung off the middle of a 3.6 metre length and weigh about 2 kg.

    All this of course depends on you having a manufacturer with the mandrels prepared to do the layup for you, although a yard or boom from off the shelf untapered tube could be quite cheap.

    Ian

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  3. #2
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    Default

    Howdy Ian,

    if we can get numbers that would be completlely wonderful.

    The backyard boatbuilding as well as many layers of professional boatbuilding is full of claims based on little more that hope.

    Some say "glassing the outside of the boat adds minimal weight" and have never measured the difference and what it really means to a 100 metre carry of the boat.

    I talk about this a lot.

    Homebuilt carbon spars that are heavier than well designed wooden ones.

    If you can get some real numbers and set up some sort of system for estimating the extra stiffness for the added weight - that would be a real boon.

    In the end, all we have to count on is real data. Hopeful guesses just lead ppl up blind alleys.

    Like the guys building "lightweight" cedar strip boats by using very thin cedar. I'm sure they go this unrewarding way because of the notion that "glass doesn't add much weight". But glass is 7 times more dense than cedar and has a surface area twice that of the boat and means an equal amount of resin has to be added too - just along for the ride. Need to look at the glass first and the cedar thickness second.

    Whether building a full composite spar or rectifying a too bendy wooden one - we need to know where we are going to know if it is worthwhile starting to add extra glass. And to have a reasonable approximation of the result would be just fantastic!

    MIK

  4. #3
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    Default

    Quote Originally Posted by Boatmik View Post
    Howdy Ian,

    if we can get numbers that would be completlely wonderful.

    Whether building a full composite spar or rectifying a too bendy wooden one - we need to know where we are going to know if it is worthwhile starting to add extra glass. And to have a reasonable approximation of the result would be just fantastic!

    MIK
    The numbers for (professionally) mandrel wound fibreglass and carbon tube are well characterised:

    "Spiraflex" Fibreglass tube: Elastic modulus 31 GPa, density 1.6 g/cc

    "Standard" Carbon tube: Elastic modulus 85-95 GPa, density 1.6 g/cc

    Everything can be figured out from those numbers, and theoretical seems to match actual quite well.

    We know well optimized carbon tube spars can be done at about one third the weight of well optimized wood sections.

    I did some quick calculations on the 31 GPa fibreglass tube, I think savings of around 30% over good wood solutions are possible. For example a yard that weighs 1.3 kg in carbon could be done in fibreglass at about 2 kg and in wood at about 3 kg for similar stiffness values. Price of the fibreglass tube seems comparable/competitive with wood.

    When you go in the direction of adding glass or carbon to wooden spars (whether rectifying a too bendy one or designing that way from scratch), I wouldn't have a lot of confidence in purely theoretical results: I don't know exactly how thick the added laminate is, or its properties, which will be quite variable depending on resin ratios in a hand layup.

    My best estimate is that one of these carbon sleeves people are considering would add about 50% to the stiffness of a typical 40mm dia round wooden spar, but would also add about 500 - 600 g of weight to it.

    Adding glass to a spar: Makes sense locally - to stop the ends splitting, and to add hoop strength/crushing resistance at stress points - for example through the mast partner and at the halyard attachment point on the yard.

    It makes no sense to add glass all the way along a spar to increase the overall stiffness. If you do the calculations, keeping OD constant but reducing the timber component on a hollow wooden spar (either thinner wall or lighter timber such as Paulownia or WRC), and making up the stiffness with glass on the outside, the weight comes out similar or higher than the original standard square or round oregon spar at 20% wall even with optimistic assumptions.

    Back to adding a carbon sleeve to a wooden spar:

    If we had a single data point from an actual application where a spar or shaft had one of these carbon sleeves added to it, fairly reliable calculation of the gains possible at other diameters becomes possible.

    Minimum data needed for figuring out what is going on is:

    * Diameter of the spar/shaft the sleeve was added to.

    * Stiffness gained from this addition.

    This could be in the form of a kNm^2 value; "addition of one of these sleeves to a spar of 'X' mm diameter resulted in an increase of 'Y' kNm^2 in the stiffness."

    I could work with before and after deflection data for a given weight and span. Useful data depends on accurately knowing the span, and ideally the deflection at increments of weight hung of the middle - say 4kg, 8 kg, 12kg, 16kg, 20kg. Done both before the sleeve is added and after.

    Maybe someone can quiz the folk who make or sell these carbon sleeves and see if they have any real data for this stuff. Woodeneye reported a conversation with Andrew Denman that the sleeves can add about 50% to the stiffness of a spar, this would be more useful if it included the diameter and actual stiffness of the original spar.

    Ian

  5. #4
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    Howdy Ian,

    Working out the thickness of laminates can be more accurate than getting a figure from somewhere.

    If you have the resin to cloth ratio and you already know the cloth weight then you can calculate it out.

    The typical resin ratios are very unlikely to be close to the idealish 50%. But the conservative assumption is to use the 50% figure for the engineering as it reduces the laminate thickness.

    But when calculating the weight of the laminate or the epoxy consumption I would be assuming about 60% resin ... so would add 20% to the resin part of both those figures.

    MIK

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