Mast Dimensions?

Discussion in 'Sailboats' started by MastMonkey, Jun 21, 2012.

  1. MastMonkey
    Joined: Oct 2009
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    Location: Cali

    MastMonkey Junior Member

    I need help determining the scantlings for a solid wooden mast to be used on an outrigger sailing canoe. The main sail is a 100 square foot Chinese Lug and I intend to lightly stay it to better support carrying a head sail. The working head sail adds 30 square feet of sail. The mast is stepped through the hull 1 3/4' and the length above the deck is 16'. The dimensions I have come up with are based on my reading of Practical Junk Rig and my researching online forums. At the partners, I have determined the diameter to be 5 1/2". The diameter is constant for 2' above the deck before tapering linearly to a diameter of 3" at the mast head. The main purpose of the stays is to set the jib more efficiently while providing some bending resistance. For a mast this length PJR recommended against a hollow wooden spar. I can not afford aluminum. Initially, I was going to use Western Red Cedar, but after reading conflicting reports on its strength and durability, I will probably switch to Douglas Fir.

    Do these dimensions seem suitable?
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A solid mast wouldn't be a good idea with that much area or on an outrigger canoe. I can't imagine what is up with the intelligence level of PJR, but it's dubious and obviously in question, suggesting against a hollow wooden spar in this case.

    Sweet God 5 1/2" at the base of a solid spar this length! I have no idea where this came from, but it's not even remotely close. Using your dimensions and assuming a masthead jib, a solid spar with a reasonable safety margin, would need a 3 15/16" heel, continued up about 4', then continuously tapered to 3 1/16" at the head. If white spruce, will weigh about 33 pounds, if Douglas fir, about 44 pounds. If it was a birdsmouth section of the same specifications (area, length, configuration, etc.) and again white spruce, it would be 2 7/8" at the heel, 2 1/4" at the head and would weigh about 12 pounds. If Douglas fir about 15 pounds. This assumes a 20% wall thickness. So, you decide, probably 50+ pounds with everything attached or in the 15 - 17 pound range.
     
  3. MastMonkey
    Joined: Oct 2009
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    MastMonkey Junior Member

    I thought the dimensions seemed a little excessive, but without having any experience, I couldn't be sure. Thanks for the confirmation. I have read before that the calculations given in PJR lead to overly stout mast. I think there is probably a scaling issue too. PJR is primarily focused on large cruising rigs with large free standing mast. I couldn't understand why it recommended against hollow mast for this size sail though or smaller boats in particular. My preference was for birdsmouth. My own calculations of the weight did make me wonder. Your dimensions seem more reasonable and match my own experience and observations of other boats.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There's a simple reason my dimensions and figures are in line with what you've observed. Simply put, the math isn't hard and though I made several assumptions about your boat, these are reasonable and wouldn't skew the figures too badly. Other successful boats, with similar rigs, hull form, etc. would also have preformed the math. I'd take anything from PJR with a grain of salt, maybe the whole salt shaker.
     

  5. frenette
    Joined: May 2011
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    frenette Junior Member

    The Malibu outrigger's have used hollow fiberglass masts that work really well. Having trimmed one the shape of very flexable in terms of sail shapes. With the sizing your using here the point of no return when the boat points is what 30 degrees?

    Solid wood is going to end up being a reason to break. But that's JMHO
     
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