Homemade fiberglass mast strength calculations

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

  1. rxcomposite
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    rxcomposite Senior Member

    1. All uni running in the same direction is prone to cracking, like bamboo. A cover such as biax is to hold it together. Biax is also for twisting load.
    2. Right.
    3. I thought you were going to use wood as part of the structure. Post #21.
     
  2. rxcomposite
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    rxcomposite Senior Member

    Order of lamination will be like this. Thicknesses will depend on load requirement as per calculations. Section of mast.png
     
  3. Rumars
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    Rumars Senior Member

    kerosene and rxcomposite like this.
  4. laukejas
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    laukejas Senior Member

    Sorry for the late response - and thanks for your suggestions, guys. Though that last mast build from teamvmg site makes a lot of sense, it is far, FAR too complicated for a one-off project. I was just trying to find a dead simple method of building composite or semi-composite (thin wood core) mast with a taper and joint in the middle. Something that could be done in comparable time to a regular birdsmouth mast build. I guess that's a bit naive! Anyway, thanks a bunch for lots of good info.
     
  5. Rumars
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    Rumars Senior Member

    Can't you go to aluminium? For the tapering just use different size tube with fiberglass bushings.

    For the fiberglass mast, use a pipe of the greatest diameter as the mold and make a tube with a single layer of fiberglass. This tube you taper by cutting wedges, then use as a mold onto wich you wind the reast of the laminate on.
     
  6. laukejas
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    laukejas Senior Member

    Well, I did some math back in the day, comparing aluminum mast to a wooden mast, and it turns out that as far as strength/weight goes, the ratio comes out pretty much the same, with only a few percent towards aluminum's favor. That is, discounting taper, which is difficult to make with aluminum, so I thought that wooden mast is preferable, even if it requires a bit more work. Regardless, as for that first option you suggested, the mast would have these steps along the length, wouldn't they interfere with raising the sail, as the sail or lashings would get caught on these steps of changing diameter?

    I thought of that second option as well! But why not cut the taper into the aluminum mandrel in the first place, and then bend it to close the gap before starting the lamination on top?

    EDIT: I made some calculations comparing weight of a non-tapered aluminum tube versus tapered one made with bushings like you suggested. For a 3 piece, 5.5m long mast with gradual diameter steps from 70mm to 65mm to 60mm with 2mm wall, and 200mm long joint, the resulting weight almost exactly the same than it would be for a non-tapered mast (single piece aluminum tube), due to the extra weight of 200mm inserts.
     
    Last edited: Feb 5, 2021
  7. Rumars
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    Rumars Senior Member

    For the steps you build a ramp out of fiberglass to ease the transition. You can see it on this video of a Core Sound, they put a track over it, if you use lacings you need to do it all around, or at least on 4 sides.

    The fiberglass mast will be at least as heavy as a wooden one, if not more, fiberglass is not very stiff, you need carbon for that. Making an aluminum mandrel is possible, but a wood one is simpler. Aluminum has an advantage only if you cook the mast, the Al expands with the heat and when done releases easily.

    Since this is a dinghy used on a lake, one option would be to go to a stronger Al alloy, like 7075, and decrease wall thickness. This will probably get you into carbon territory in regard to weight. These people seem to say they can supply it, but I have no ideea about price or availability. Aluminum Round Tubes https://www.linoksa.lt/en/products/aluminum/aluminum-round-tubes/
     
  8. laukejas
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    laukejas Senior Member

    Thanks for that link, I will send them an email. But as I wrote in the edit of my previous post, making aluminum mast in steps to create taper comes out pretty much the same in terms of weight, because of the additional length needed for the sections to fit into each other... The center of mass should be slightly lower compared to a full-length single-diameter tube, but I am not sure if it would be really worth it. Of course, at least one joint would have to be made anyway in order to have a collapsible mast that can be stored in small garage. The guy in that video glued his sections. I wonder if there wouldn't be a weak point at the joint if the pieces were not glued together? The guy in the video mentioned some serious play even in such tight a joint that he made, and that sounds pretty scary.
     
  9. Rumars
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    Rumars Senior Member

    For a disassembling joint you use the same process as glueing, but put mold release on the inside of the tube. That creates a perfect fit and there is no play. The ramp prevents the tube to slip further in. No problem with weak spots, the burried part transmits the load.

    I just realised I don't know if the mast is stayed or not. If it's stayed using a stronger alloy is not necessary, the cheaper grades are just fine.

    Another material to consider for a mold is PVC pipe. It's cheaper and easier to taper then Al. But, I do have to say, unless Al is really expensive, making a composite mast is only worth it if you go carbon, the fiberglass one is just a lot of work for not much benefit. Maybe you can find a used windsurf mast and use it as a base to stiffen it up. What are the dimensions of the wooden mast, and how much sail area?
     
  10. laukejas
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    laukejas Senior Member

    That is good info, thank you. I searched around, and I think that it would be really difficult to get 7075. That link you gave is pretty much the only company that sells this grade, but unfortunately they wrote back that they only deal in bulk. I will search around, but I don't think there will be that many options.

    The mast I'm currently designing is 5.5m long, for a sail of 8.6m^2 (92 square feet), unstayed, partial batten Marconi. No more than 68mm diameter. I do have super-stiff windsurfer mast that I can stiffen up further if need be, but that will add a lot of weight. Really I started this topic not for this particular mast, but more towards exploring what alternatives there are for building ultra-light spars easy and cheap, if possible.
     
  11. Rumars
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    Rumars Senior Member

    Unfortunately there are not many options. Easy and cheap is either wood or Al. Carbon from scratch is expensive and complicated. Stiffening up a windsurf mast with carbon works well, and using premade carbon tubes also works, both are easy but not really cheap.
     
  12. boatman1975
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    boatman1975 New Member

    Nice topic and video. Will watch it asap. I am a newbie in boat design so I am interested in any type of information here. Thanks!
     
  13. laukejas
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    laukejas Senior Member

    I spent two days contacting all possible aluminum suppliers, all sorts of maker and aviation enthusiast groups on Facebook, aviation clubs, and dozens of companies dealing with aluminum, and they all said the same thing - there is no, and never has been a single 7075 aluminum tube in Lithuania, nor any comparable grade (such as D16T). I even inquired with companies shipping their aluminum from abroad, and contacted their suppliers directly. There's just nothing. The best I could hope to get is 6061, which is even worse than knotty lumber.

    So I guess it's either wood, or risking it all and doing it with carbon using a regular aluminum tube as a mandrel, therefore forgoing the taper.
     
  14. gonzo
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    gonzo Senior Member

    7000 series aluminum would not work for a tapered mast since you can't taper or weld it. 6000 series alloy is what is typically used. Have you tried flag pole manufacturers?
     

  15. laukejas
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    laukejas Senior Member

    Yeah, I tried, but no one makes flag poles of the diameter I need... Either way too large or way too small. And we don't have that many flag pole manufacturers to begin with... It's a small country :D
     
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