Wharram 5.5"x32' Mast Desgin questions

Discussion in 'Boat Design' started by abosely, May 3, 2015.

  1. abosely
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    abosely Senior Member

    The design Wharram uses for the mast is box of 1" Fir and 4 triangular pieces in corners then cut to octagon then to round to 5.5"x32' hollow except for a 12" or so solid block at top and foot.

    The sail is his Wingsail, with a sleeve around mast and a short gaff about 3' from top, halyards on outside of mast in sails sleeve.

    I want to build as lite of mast as reasonably able.
    Will be building birdsmouth type construction with 8 staves.

    Can a tapered mast be used with the wingsail and short gaff?

    If so, what information or data is needed to how much taper and where the taper starts.

    If wingsail & gaff won't work with a tapered mast. I would like to taper the top of mast above the gaff at least.

    Is there anything that can be done to lighten the mast?

    Thin/taper thickness of the staves towards the top of mast?

    How difficult is it to check and determine if thinner staves would be strong enough or that I do need 1" thick staves when building a birdsmouth spar?

    I understand that glassing the outside of mast will add more weight and not work and that using Carbon fiber cloth around a wood spar doesn't work because the wood and carbon fiber have very different elasticity.

    Any other options?
    The mast will be lowered and re-stepped often if part of the desire to work at lighten the mast.

    Cheers, Allen

    If using aluminum masts the recommended 5.5"x1/8" aluminum poles are difficult to get and expensive here on Big Island.
     
  2. gonzo
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    gonzo Senior Member

    The best you can do is to contact Wharram. He designed the boat, so he has all the data pertinent to rigging size.
     
  3. abosely
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    abosely Senior Member

    I'm just wanting to duplicate the strength of a mast built with the box & corner pieces using birdsmouth construction and hopefully save weight.

    I figured there was a fairly straight forward way to figure stress on the mast going towards mast head. Then using that to decide where to start taper and how much. Though that's beyond my abilities and skill! lol

    Haven't seen let alone used a gaff sail before, but just guessing a 2" or so taper over 32' wouldn't be a problem. Even reefed the gaff would be up on mast a good ways, so the change in taper would be less then 2'.

    Not able to get much for JWD.

    Cheers, Allen
     
  4. philSweet
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    philSweet Senior Member

    A 32' homemade wooden stick is going to be a handful no matter what. I think you'd be better off concentrating on designing a suitable assist device. Will it always be on a trailer when you raise and lower it, or do you need to do it afloat? Is the sail going to be slid on before the mast is raised or attached afterwards?
     
  5. messabout
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    messabout Senior Member

    Phil has it right. It is more practical to dream up an erecting device than to sweat a miniscule reduction in mast weight. Unless you are Superman you won't want to be erecting or taking down a 32 foot mast by yourself without some kind of mechanical assistance.

    Wharram is an odd guy but he has done this stuff for long enough to know what works. Follow the plans.
     
  6. abosely
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    abosely Senior Member

    OK, different approach.

    Since the 1" thick box with corner pieces cut down to round works.

    Build 8 sided birdsmouth mast with 1" staves, not tapered (no issues with gaff fitting properly) and making inside flats round.

    Using Duckworks' birdsmouth spar calculator for a hollow spar 5.5" OD, starting with 8 - 1" thick staves and rounded on outside and on inside.
    If I did it correctly, will be 5.5" OD x 3.8" ID and .85" thick walls, and Douglas Fir is 31.96 lb/cf.

    A 32' hollow mast except with approx 12" at head & foot solid.

    About how much would this weigh?

    Cheers, Allen
     
  7. PAR
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    PAR Yacht Designer/Builder

    Assuming all is equal, such as wall thickness, species used, dimensions, taper, etc. There's no weight savings associated with birdsmouth construction, just assembly convenience. Without full dimensions for the spar, an accurate weight isn't possible. Lastly the DuckWorks calculator is a very crude thing (no offense Chuck), using some heuristics from Skene's (my assumption) and shouldn't be trusted on much more than a daysailor's stick.
     
  8. abosely
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    abosely Senior Member

    OK thanks, Paul. The wingsail drawings have the specs on the mast used with the wingsail on the Tanenui, I'll post back with actual data and get some thoughts about the mast then.

    Cheers, Allen
     
  9. Petros
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    Petros Senior Member

    you can make it lighter by making it larger diameter, and than thinning the walls. The stress level in the outer fibers are proportional to the square of the outside diameter.

    You can also thin the section down if you use significantly stronger wood that as designed, but the effect of this is not as much as the diameter.

    Of course there is a practical limit since a thin walled mast will more likely get damaged when handling it (raise and lowering and during transport and stowage).

    Taper is a more complex issue, I am not familiar with your mast configuration, shrouds, stays, spreaders if any, etc. A pure cantilever mast (and by the size of your mast I would say you do not likely have fully cantilevered masts), the max stress is at the base or deck level, and drops off as you go up, a straight taper works very well. On a shroud braced mast the maximum stress occurs about half way from each point of lateral support, unless you have a boom, which usually causes the max stress point. The compression in the mast from the side stays (as you load a mast sideways, the shrouds or side stays will put a lot of compression on the mast), and with the side load from a leech attached sail plan, you are buckling critical likely at a point a little below the half way point between points of lateral support (usually deck and spreader). So you can optimize the construction by thinning the wall, or having the dia change with the max bending or combination bending and compression load.

    For a wood mast, this is not just complex, but I suspect too much work to build it for the benefit of saving 15 or so percent of the weight. Particularly since if you damage or crush the optimized thin walled mast during transport or storage, it will fail anyway.

    Typically you design for the location of the worst loading condition, and just allow it to be stronger than it needs to be everywhere else.

    I suppose if you can work out the loading conditions along the length of the mast in your configuration, you can build up a mast out of thin strips of wood and epoxy over a mandrel. Adding extra layers in the area where the stress is higher. than it should get bagged and a vacuum put on it to squeeze all the air bubble out from between the layers. I have been meaning to try that sometime with an all wood mast, but cutting out a solid lumber mast is so much faster and easier for the small boats I usually build is so much less work.

    do you have a diagram of the installed mast, stays and rigging configuration included, that you can post? Some areas to save weight might become obvious once I see your configuration.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Allen, with typical tapers for a 32' mast (Bermudian main, masthead jib, stayed, etc.) with a 5.5" base diameter, Douglas fir will be in the 100 pound range, given a 20% wall, regardless of building method. If you use sitka, it's about 80 pounds.
     
  11. abosely
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    abosely Senior Member

    Paul thanks, about 100 lbs then. That's basically what to find out at this point. I had no idea of the weight or even general weight range.

    I don't know if Sitka Spruce is available here or not. Tomorrow I'll check and see if it is. And then probably have to be sitting down for the price! Lol

    Cheers, Allen
     

  12. PAR
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    PAR Yacht Designer/Builder

    If you live on the big island, then you already know everything you guys get is "imported". White spruce would be another good species, which is slightly lighter than sitka too.
     
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