Mast Design Q's

Discussion in 'Sailboats' started by minno, Jul 18, 2015.

  1. minno
    Joined: Aug 2014
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    minno Junior Member

    Hi All :)

    I'm working on a 10' wooden sailboat and have decided to go with a spritsail, mostly for the shorter mast, I've got a bad back and don't want to be wrestling with a 16 foot mast, at least until I'm more comfortable on the water.

    to keep the weight down and for ease of construction I've decided to go with a hollow square mast 10'-12' long, and just want to make sure it's heavy enough to carry ~60 sq ft, about 12 sq. ft. more than I'll ever put on it but I'd like a safety factor.

    I'm planning on using 3/8" spruce and making the mast 2 1/4" square overall, it'll have 2"x2" par braces inside at the mast step and 18" from the top, (the plan being to extend the mast with a 2"x2" in the socket if i want to try a sprit boom or cat rig) and I'll put a couple wraps of glass tape at the mast step and at the top.

    Will the mast carry the load?

    minno
     
  2. PAR
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    PAR Yacht Designer/Builder

    A 12' tall mast should be in the 2.5" at the base and taper to 1.75" at the head range, for a 60 sq. ft. sail, it'll weight about 6 pounds after assembly.
     
  3. SukiSolo
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    SukiSolo Senior Member

    ...and make sure you have some 'meat' for any gooseneck fittings, or anything you want to screw/fix to the mast....;)
     
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I think that is a good choice. You could look at an old wooden optimist mast, sprit, and boom and scale them up from 35 square feet to 60 square feet. I think if you just increased all the dimensions of the spar by 30% that would be a good start. If you want it to have some more bend and still be strong you can sand it down some and add a layer of glass epoxy. This will make it easier to depower the sail in stronger winds.
     
  5. Jamie Kennedy
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    Jamie Kennedy Senior Member

    My own choice for a small wooden spar would be to go out to the woods and walk about to find a nice tree of the right size and taper and make a solid spar out of it. Drying it out might be a bit tricky but you could try sanding off just some of the outer material for the first season and giving it just a light coat of stain or varnish and use it heavy for the first season. Then at the end of the season sand off a few growth rings and let it dry out some more, and give it a thicker coat of spar varnish at the start of the next season. Once you get it down enough, and dry enough, if you really like the way the spar turned out maybe sand off one last layer and giving it a layer of glass epoxy and another coat of epoxy before the finishing coats of spar varnish. This should yield a somewhat heavy but strong and bendy spar, and it would be fun to know where the tree grew.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Solid spars just aren't reasonable any more. Not with the building methods and particularly with the materials we now have and especially on small craft, where weight aloft can be critical.

    A square section wooden mast is about as easy a thing to make as you can ask. Birdsmouth has more pieces, but also fairly easy to do. I recently did a traditional "built up" mast (attached), which has the same number of pieces inside as a birdsmouth, except all the cuts and assembly are perpendicular, making ripping stock and assembly simple.

    Aluminum tube sections (6061 T-6 only) aren't very costly, if you know where to look and sure simplify the attachment of stuff too. Lastly spars don't scale up proportionally, so up sizing a smaller rig is a guarantee, it buckle and bonk you on the head.
     

    Attached Files:

  7. minno
    Joined: Aug 2014
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    minno Junior Member

    Thanks All :)

    I considered a solid spar, seemed a bit easier but I'm trying to keep the weight down.

    I grew up in the middle of a forest of "that tree" wasn't much sailing going on anywhere nearby though :)

    I had planned on using a solid plug about a foot long at the partner for a bit of extra support, Hadn't considered the need for extra wood to anchor fittings though, I'll make it a bit longer, a bit more weight, but at least it's at the bottom of the mast.

    that's perfect Par, thanks, I had decided on birdsmouth but this looks much easier, I couldn't see myself cutting a taper in 8 staves, shouldn't be hard at all your way, I'll use your dimensions as well, I think I'll pick up a new ripping blade before I get started :)

    Should I taper it end to end or start the taper 4 or 5 feet up? buckling and bonking doesn't sound like any fun at all :)

    minno
     
  8. PAR
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    PAR Yacht Designer/Builder

    Typically you taper both ends to save as much weight as you can. Internal reinforcements (swallowtails) should be tapered to eliminate any internal stress risers. This simply means you taper the ends of any plugs to a feather edge. You'll want this type of reinforcement at the gooseneck, partners, heel and masthead, plus any spreader locations or other fitting attachment points, like a fractional jib stay for example.

    Tapering birdsmouth staves is the same as tapering the above flat plank staves. Admittedly, skinny birdsmouth staves can be a challenge, because they'll want to squirm around, but a jig setup can lock them down during the deed.
     
  9. minno
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    minno Junior Member

    Thanks Par :)

    I'm not sure what you mean tapering the ends of the swallowtails to a feather edge ( I'm a talkin' like an old salt now :D ) I've run across a picture where the ends of the plug were cut off at about a 60 degree angle, is that it?

    minno
     
  10. PAR
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    PAR Yacht Designer/Builder

    Any internal reinforcements should not just start and stop, but should taper down to met the mast wall with a feather edge (or very nearly so). If you just glue in a hunk of 2x2 (as and example) you'll have a dramatic transition from the mast wall thickness to the reinforcement, which creates a "hard point", a stress riser and these will weaken the mast a lot. If the same reinforcement has tapered ends, so there's no sudden change in wall thickness, but a gradual ramp up in wall thickness, the strains are mitigated and point loading can be avoided.

    This is a masthead plug for a birdsmouth mast:
    [​IMG]

    Being inserted in a mast half (note wiring):
    [​IMG]

    A square mast reinforcement:
    [​IMG]
     
  11. minno
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    minno Junior Member

    Thanks Par :)

    sorry it took so long to reply, bad bout of tendonitis and being on the computer aggravates it.

    I went with a 8' square mast, stood the support up on end and ran it through the table saw to cut the swallowtail, worked quite well.

    minno
     

  12. LP
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    LP Flying Boatman

    Great method, Paul! I've read your mention the swallowtail cuts for mast plugs many times, but your images purvey to absolute simplicity of the necessary cuts for this method. These images are superb: worth many more than a thousand words. Thanks for sharing them. I've been tossing this conundrum around in my head and the solutions where always way more complex. :D
     
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