Home-made small fiberglass mast/spars?

Discussion in 'Fiberglass and Composite Boat Building' started by hospadar, Apr 18, 2011.

  1. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    Hello
    I'm planning on building a small mast for myself, and I'm wondering about the possibility of making a short fiberglass mast (and a spar, I plan on using a sprit rig)

    Depending on the specific boat design I settle on, the mast will probably be around 8-9 feet, and the spar will be of similar length. Depending on the boat design, the mast and spar will need to support somewhere between 50-100 sq. feet of spritsail. I'm aiming for a mast 2-3 in. in diameter

    I'm imagining a mast shaped from foam (probably cheap XPS foam), then wrapped in fiberglass (probably mostly unidirectional tape running the length of the mast). Has anyone ever build/heard of/sailed with a mast like this? Anyone have any idea how thick (how many layers*oz of fabric) the fiberglass would need to be - or how to test the strength of the mast?

    Another big question in my mind is how to attach any rigging elements to the mast, the sail will be laced to the mast in all likelihood, so no halyard, but I'd still need to attach a snotter, I'd like a brailing line, etc. I can't imagine drilling a hole or driving screws into such a mast would be very good for it.

    It may turn out that making a fiberglass mast is way too much work or just not feasible, in which case I'll stick to the ripped-2x4s-laminated-together mast, but I'd like to try my hand at building a glass mast if possible.

    Some other thoughts/reasons:
    -I think a wood mast (especially the 2x4 mast) would be much easier to build, but I've got time, and a glass mast seems like an interesting project
    -The glass mast could potentially save some weight over the wooden mast
    -I think it might be a little easier to build a more precise taper into a glass mast
    -I'm under the impression that unidirectional glass laminates are very strong
    -The mast would be very positively buoyant, so I won't turtle when I capsize myself (And I will definitely capsize)
     
  2. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    An XPS plug that size is going to flex like crazy. I'd make a wooden plug, with taper and good covering the finished tube should release without too much trouble.

    Someone else has to chime in on the laminate chedule.

    My 2 cents worth. :)
    Lurvio
     
  3. michael pierzga
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    michael pierzga Senior Member

  4. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    I was doing some more research last night, and I'm thinking about using chromoly (4130) tubing (the same as is used in bicycles and roll cages) as a mast. I'm pretty sure it will be strong enough, and it's fairly cheap. The 9 foot mast will run be about $30 and will only weigh around 9 lbs. Also I'll be able to weld (or braze) fittings directly to the mast.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Do yourself a big favor and preform a weight estimate before anything else. You'll quickly find out why steel and 'glass spars aren't seen on small craft. They just weigh way too much to be reasonably considered.

    Look up "birdsmouth" mast construction. You'll find an easy and very strong method that also can be very light, which is a prime consideration in spar making. I build several a year and most rival aluminum extrusions.

    Carbon is an option, assuming you have lots of money.

    Preventing the boat from going turtle will require more internal volume than a typical mast can provide. You could rig a foam "derby" or just use sound dinghy sailing practice, which is to grab a PFD, just after the capsize, then immediately swim to the end of the stick and tie it on. This will prevent the boat from going turtle, as you gather up the beer cooler and other assorted, hopefully floating items that dumped out during the "event".
     
  6. hospadar
    Joined: Apr 2011
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    hospadar Junior Member

    Thanks for the reply!

    I have seen some birdsmouth masts, and I haven't ruled that out yet, but at least in comparison to a birdsmouth (or laminated 2x4 mast which seems common for the 8-12 foot home-made boats I've seen), is steel really much heavier? An 8' 2x4 weighs 9 or 10 lbs, so I have to imagine a 2-3" diameter solid wood mast would probably weigh about the same. A birdsmouth would certainly be lighter than a solid mast, but for a small mast like this I can't imagine the difference would be more than a couple pounds at most.

    I know when it comes to bike building that steel and aluminum frames tend to be fairly similar in weight and strength at the end of the day since aluminum frames need much bigger tubes. If a sunfish can have a big aluminum mast, why shouldn't one be able to use a smaller (but just as strong and about the same weight) steel mast?
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A solid 3.5" square mast (laminated 2x4's) will weight 18.1 pound if Douglas fir (not counting glue or fasteners). If it had a typical taper, it would be 3.5" by 2.25" at the top, which would weigh 12.6 pounds.

    Now the exact same dimensions as a hollow birdsmouth spar 96" and 3.5" round, no taper will weigh 11.3 pounds (Douglas fir) and about 8.6 pounds if tapered.

    Now, Douglas fir is heavy for spar stock, but in small boats it's not uncommon. The last thing you want is a heavy spar. It just kills performance in small craft, even an 8' dinghy (especially an 8' dinghy).

    Yes, steel is heavy, really heavy at about 500 per cubic foot, while Douglas fir (the heavy spar stock, mind you) is about 34 pounds per cubic foot. Except on very large vessels, steel just isn't an option in spars. The Sunfish mast is really light and so is aluminum, particularly compared to steel.

    The weights I provided above didn't consider how much sail area you had and are likely quite a bit oversize for your needs. A typical 8' mast on a small dinghy would have far less then a 2x4 mast. 2.75", tapering to 1.5" at the top would be much more in line with common dimensions. Again based on several factors, most of which you haven't provided. A Douglas fir mast of these dimensions with a 20% wall thickness will weigh (hollow birdsmouth of course) about 4 pounds! This is a huge difference compared to your 2x4 laminate. This weight could drop down to just above 3 pounds if you used Sitka or white spruce instead of Douglas fir.

    I know it's just a dinghy, but if you want to capsize with every stiff gust, then put an 18 pound mast on it. There's more to these set of equations then meets the eye.
     
  8. cor
    Joined: May 2008
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    Location: Alaska

    cor Senior Member

    I have used steel exhaust tubing for masts in the past. It is not high tech, or fancy, but it gets the job done cheap and easy.

    2" 16 gauge tubing (.065 wall) weighs 1.343 lbs per foot. That would make 10.7 lbs for a 96" spar. Comes in surprisingly close to the birdsmouth.

    Any automotive exhaust shop will have it. It is normally sold in 10' lengths and can easily be spliced to make it longer.

    A wooden birdsmouth mast will look and feel much better, but take a bit more time to put together.

    C.O.
    http://whatsintheshop.blogspot.com/
     
  9. PAR
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    PAR Yacht Designer/Builder

    Exhaust tubing is seamed and not very strong. If it's out of column, it's coming down. If it's dented, it's dramatically weakened and it rusts to all hell in the marine environment. I can think of nothing less suitable for a mast, unless it's to hold a small flag.
     
  10. Ilan Voyager
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    Ilan Voyager Senior Member

    As always you're right Par. Steel for a small mast no...in 8 days it's a broken rusty thing.
    Another technical factor for mast: you must have a good Young modulus/weight ratio, as you need the thickest possible walls, or the biggest local inertia because of the buckling of the walls under compression and bending stresses. Just the Euler column formulae...
    A simple calculation shows that steel does not meet this crucial requirement on small boats...
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    For such a small mast, the easiest and cheapest way to go is with a solid tapered wood mast. I designed some similar masts for the Prema dinghies, and I started with a solid 3.25" dia. section at the lower end, shaped to a kind of elliptical section above the partners, made of solid Cypress. Actually, the bottom end below the partners was hollow, to take out some excess weight, but the upper part above the partners was solid. The half wishbone was originally 2.25" square in section, but that was way too heavy, so we shaped it down to 1.5" dia round in the end which was ideal. Here is a story about that design from my website. Link: http://sponbergyachtdesign.com/WBHMasts.htm.

    I hope that helps.

    Eric
     
  12. PAR
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    PAR Yacht Designer/Builder

    I saw that boat a few years ago at the Antique Boat Show in Tavares. You use a similar half wishbone to a few of my designs, though I use a UHMWPE bearing block and through bolt rather then the very nice cast piece I saw on that boat.
     
  13. Eric Sponberg
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    Eric Sponberg Senior Member

    My client, Wooden Boathouse who builds the Premas, wanted to have all cast bronze hardware to establish a very traditional look to the outfit of the boat. Yes, they have shown at the Antique Boat Show in Tavares the last three years. We went to Bristol Bronze in Tiverton, RI, which owns all of the original Herreshoff Mfg. Co. bronze hardware patterns. They can make anything that Herreshoff made, and their catalog includes diagrams of all the HMC parts. They can also cast any kind of part you like, just give them a master pattern (or an actual part) and they will make a mold of it and cast it. You can specify the alloy and the degree of finish. Of course, the more you specify, the more expensive it is. But that is what Wooden Boathouse wanted.

    Eric
     
  14. PAR
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    PAR Yacht Designer/Builder

    The fittings were lovely, but I wouldn't want the bill. I have a small outfit in Tennessee that I use for bronze, unless the part is especially complex. They're half the price of the "usual suspects" in the marine castings.
     

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    When the item is stamped marine, the price always goes up...Add the magic name Herreshoff and a big thick wallet is needed.

    The lone drawback with bronze castings is that a lot of "yellow alloys" are improperly called by that name. So a lot of casted items are made from melted scrap (bronze and brass) which contains zinc, making it improper for marine use.

    You have to be careful that the alloy used for the "marine" casting is a true bronze (copper and tin without zinc) or the corrosion will be annoying.

    The so called aluminum bronze is even better. For those absolutely wanting to make their hardware; bronze is indecently easy to melt and cast. But the actual prices of copper make it expensive.
     
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