Homemade fiberglass mast strength calculations

Discussion in 'Boat Design' started by laukejas, Jan 24, 2021.

  1. laukejas
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    laukejas Senior Member

    Hey guys,

    I am wondering how feasible would it be to build a hollow tapered fiberglass mast by rolling a typical 4" or wider 6 oz. fiberglass tape over a wooden mast template, filling with epoxy, shrinking with packing tape, and then pulling it off the template. Template being covered with packing tape to prevent epoxy adhesion, of course. I found several videos on YouTube of people making fiberglass tubes and even masts, such as this one -> , which is pretty much what I want to make.

    But I have no idea how to calculate what strength I can expect from building a fiberglass mast with a 4" wide, 6 oz. tape laid in spirals in alternating directions on each layer. Fiberglass mechanical properties are known, but how do they change when fiberglass is reinforced with epoxy, and what happens when you stack multiple layers? Since the tape is laid in spirals, the weave is laying at an angle, so how does that affect the strength and stiffness?

    More specifically, my questions are:

    1. What kind of Elastic Modulus, Tensile Strength and Yield Strength I can expect from a single layer of fiberglass tape reinforced with epoxy? In other words, how much does epoxy reinforcement modify the base properties of fiberglass?
    2. How does these properties change when the fiberglass is loaded at an angle to the weave? Is a simple multiplication by cosine of the angle accurate enough?
    3. Does the strength and stiffness change non-linearly when fiberglass is layered, like, for example, wrapping that fiberglass tape in alternating spirals? How do these layers interact together?
    4. How can I calculate the resulting wall thickness of the mast with a specific number of layers? For example, I found somewhere that a 6oz fiberglass tape is around 0.2mm thick, but how does this change when epoxy is applied, and how much does shrinking with packing tape reduce that thickness?

    All of these questions boil down to - how many layers of a specific fiberglass tape do I need to have a sufficiently strong and stiff (yet not overbuilt!) mast with known loading conditions.

    Worst case scenario, I could make some smaller fiberglass tubes first and then load them to a breaking point, and then try to reverse-engineer the mechanical properties, but there are just so many variables, and doing such testing in home conditions is not very accurate. Perhaps some of you can shed some light on this? I know I could try and find similar sized fiberglass masts to the one I need and try to build something similar, but I would rather work out the math so I can apply it to any size mast and any fiberglass weight :)
     
  2. bajansailor
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    bajansailor Marine Surveyor

    How long is the mast, and what sail area will it have to be able to carry?
    And what kind of boat will it be used on?
    Will it be like a typical windsurfer mast, with an articulated foot, or will the bottom end be an unstayed cantilever, built in to the hull, or will it have stays to support it?
     
  3. laukejas
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    laukejas Senior Member

    Sailboat with a cat rig, partial battens, around 90 square feet area, mast length 18 feet, free standing, with around 16 inches of the mast inside the hull (between step and partner). Mast diameter around 2.75". But again, I didn't include these numbers on purpose in my first post, because I'm trying to work out formulas for any size mast, rather than just getting some specific advice for this particular mast :)
     
  4. gonzo
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    gonzo Senior Member

    Are you set on learning how to make a fiberglass tube, or are you looking for an economic way of building a mast?
     
  5. laukejas
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    laukejas Senior Member

    Learning how to make a fiberglass tube.
     
  6. gonzo
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    gonzo Senior Member

    The usual method is to use unidirectional tape or filament over a mandrel. I think that infusion or hand wetting/vacuum bagging may be the easiest since it gives you the time to lay the fibers and adjust them as necessary. Are you planning on using fiberglass? In that case polyester would be adequate. If you want to use epoxy, then carbon or aramid would make more sense.
     
  7. laukejas
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    laukejas Senior Member

    It's pretty difficult to get unidirectional tape where I live for affordable prices, but I can get regular E-glass tape pretty cheap, like this one. Epoxy is also very cheap, and I have plenty of it in storage. I don't have facilities and supplies for infusion or vacuum bagging (they are also difficult to get here), but with slow-setting epoxy, I thought perhaps there would be enough time for hand-laying and compressing with packing tape? The guy in that YouTube video seemed to get good results this way.
    Carbon or aramid would be preferable, but also way too expensive by several orders of magnitude. Fiberglass seems to be just in the right spot as far as the weight/strength/price is concerned.
     
  8. gonzo
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    gonzo Senior Member

    Packing tape won't stick to uncured epoxy. As you wrap it over the laminate, the excess epoxy will run over it and make a huge mess. If you use regular tape, the mast will end up being quite heavy since half of the fibers will contribute little to the strength compared to unidirectional. A vacuum pump is not too expensive. A vacuum pump to use for either infusion or vacuum bagging is not too expensive: Vacuum pump VP115, 51l/min (1.8CFM) https://vacuumchambers.eu/en_US/p/Vacuum-pump-VP115%2C-51lmin-1.8CFM/35
     
  9. laukejas
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    laukejas Senior Member

    Well, if I wrap regular tape at around 45° angle, doesn't that mean that both warp and fill would also run at a 45° angle, resulting in the same strength as I would get by laying unidirectional tape at 45°? I mean, it's still 45° either way, right?

    If not, are there any unidirectional tapes available for comparable prices to the regular tape in my link available with international shipping? I haven't found any on Aliexpress, Ebay and similar sites.

    Well, packing tape not sticking to epoxy is the whole idea... I have seen people use it instead of vacuum bagging on many different videos, but I don't really have any such experience myself. Is that run over such a big problem? I haven't seen anyone struggling with it.
     
  10. gonzo
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    gonzo Senior Member

    Half of the fibers on regular tape will be very short instead of full length. It is not the same even though they are at 45.
    You are in the EU, so international shipping should not be necessary. Home https://www.gurit.com/ Otherwise: [Hot Item] Hot Sale Fiberglass Unidirectional Cloth for FRP Rod https://utekcomposites.en.made-in-china.com/product/ojSQmZwbnxkJ/China-Hot-Sale-Fiberglass-Unidirectional-Cloth-for-FRP-Rod.html
    If you try to use packing tape over fiberglass wetted with resin, it will run over the packing tape creating a huge mess. Also, the tape won't adhere to itself once it has resin on it. However, once the resin cures, it will make removing the tape very hard; will probably require grinding. Further, the packing tape will not have even pressure over the laminate to be able to control thickness and resin/fiber ratio. Polyethylene sheet and a cheap vacuum pump will do a much better job. However, if you are in a tight budget, a commercial tube is cheaper.
     
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  11. KeithO
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    KeithO Senior Member

    If you follow the idea of a removable core, don't underestimate how difficult it will be to get the core out. For this idea to succeed, the core will usually have to be segmented with a removable core in the center and once the epoxy has cured you remove the core and after that the pressure on the outer segments is relieved so you can remove them one by one. No simple process. In some cases a different method is for the core to be made out of tube (relatively thin wall) and during molding hydraulic pressure is applied and maintained on the inside of the tube. After curing, the pressure is removed, the tube shrinks slightly, allowing for it to be broken free of the inner surface much more easily.
     
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  12. laukejas
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    laukejas Senior Member

    I couldn't find any prices on that first link, but the second link seems to provide a very cheap option to get that unidirectional cloth. Thank you. It only seems to deal in wholesale, but the minimum purchase quantities are not that bad. Perhaps I will order some of that cloth. Perhaps it would make sense to invest into a vacuum machine as well. I thought the consumables (bag) would be prohibitively expensive, but it seems it can actually be quite affordable.

    Yeah, I hear you. Perhaps I will make some small unit tests first and see just how difficult it is. Since I plan to build tapered masts, it should be somewhat easier, right? In my case, I don't imagine I would have any other way of making a template other than shaving a slightly smaller wooden mast. The double core/inflatable option sounds nice, but I don't have any facilities or tools to make something like that...


    This is very good information so far. Thank you, guys. But back to the original questions in my first post - can you help me with figuring out the strength and wall thickness calculations? Even if I use the right materials and techniques, I still need to know just how many layers of a specific cloth weight I need to achieve desired stiffness and strength, and what the resulting wall thickness I will get.
     
  13. KeithO
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    KeithO Senior Member

    Even if tapered, you have to know that if you have a localized area in the plug that is "hollow" by 1mm (and it is very easy to get many such areas on a plug) then you start with 1mm of mechanical interference when trying to seperate. If you use packing tape as a release agent or something similar, every wrinkle and overlap of the tape is creating a mechanical interference. Most people who try this fail because the force needed to get the core out is greater than what it takes to break the item being produced.

    A long time ago I worked on a machine that wound up rolls of wire as they came off an extrusion line. They were not wound on a reel, they were wound on a former which was removed when the roll was full because when the wire was used, it was pulled from the coil from the inside and just by pulling the wire straight out it came out without any twisting and kinking. It was used on a robotic line making vehicle wiring harnesses. Anyway, the problem was that when the company ran their cheap grades of wire with harder and less flexible insulation, the system worked OK. When they ran their higher quality, softer insulated wire that was more flexible then the system would fault because the former would not be able to extract from the coil of wire because the still warm insulation had too much adhesion to the conical former. The only solution that ultimately worked was to add a 3 segmented cone to the outside of the plastic cone with the segments being able to slide up the cone perhaps 1/4" which of course resulted in the diameter of the steel segments getting smaller in diameter and taking all the pressure off of the surface. It was a fairly complex solution but it was highly reliable and enabled the company to shift to a more lucrative part of the market which demanded the higher quality wire. It was not a very nice place to have to work and I was glad I didnt have to go back again after I got the issue licked.

    Some years later I ended up working for a company that made resistance welding equipment. We made all our own pneumatic cylinders. We were not able to get hollow bar or pipe in the sizes needed so they always had to be machined from bar stock. Time consuming and expensive. Volumes were too low for an extruder to work with us. So ultimately the barrels of the cylinders were made from fiberglass laminate. The inner surface was a wear surface and had to be very smooth with no fibers sticking out. The stainless steel tube and hydraulic pressure method was used for those and they turned out very well. But the tubes were quite short (4-6") and it was easy enough to grind the outside of the tube and then give it a high polish and combined with the internal pressure we had no problems.

    A mast is much longer and the taper would be very shallow and the interior would hold a whole lot of hydraulic fluid and would tend to bend. Getting a taper tube would be tough in and of itself. If you eliminated the interior taper, you could still taper the thickness of the layup on the outside to match the bending moment so as not to be needlessly heavy and would make the job of developing a collapsing core much simpler. But this means that you would have to manually finish the outside of the mast. In my opinion, making a mast, especially a tapered one, is quite an advanced composites project and it is doubtful it is worth it as a "one off". If you did something with a solid core, like using an aluminum tube with a properly shaped foam core on the fore and aft sides and wrapped the outside of that with your laminate, that would be something considerably more feasible. You then leave the core in place when done and the smaller aluminum tube gives a place for rope and wires to be run.
     
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  14. Ad Hoc
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    Ad Hoc Naval Architect

    Simple.
    Do what everyone else does... coupon testing.

    It is your only way...and it also accounts for any irregularities that you introduced in your method of build/layup. So it it is directly applicable to you...rather than some theoretical sales values.
     
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  15. Ilan Voyager
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    Ilan Voyager Senior Member

    I do not understand the wish to enter in so many efforts for making just a 18 feet free standing mast with just 90 sq feet sail.
    That is pretty small so it can be done in spruce or a nice Nordic pine in a few days with a few tools and epoxy glue. Eventually a few strips of UD carbon strategically glued will solve any problem of bending. On a 18 feet mast the difference of weight with a GRP mast will be rather small.
    The great advantage of the wooden mast is that can be easily modified, so you can change the characteristics of the bending.
    In a very old fashion way a good bamboo pole of the good size would do the job, the problem is to find one in Lithuania...
    It's far from easy to make a good functional GRP mast.
    Let's forget the UD winding over a mandril, it's an industrial process needing heavy tooling.
    Let's forget the mold system, the expense of a mold is ridiculous for just one mast.
    For a composite mast remains the hand made UD, glass and carbon, over a foam core like the custom windsurfers. It's perfectly feasible but you need to be comfortable with composites unless you want to mess with some pounds of fiber and epoxy.
    Another method is to start from a windsurfer mast of about the good size and reinforce it in the needed points as you have a strong good core base.
     
    Last edited: Jan 25, 2021
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