correct birdsmouth mast diameter

Discussion in 'Boatbuilding' started by zvi, Sep 14, 2022.

  1. zvi
    Joined: Sep 2022
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    zvi Junior Member

    Hello all!
    First time posting. I'm building a 13 foot sailing dingy with a beam of 5 feet. The mast will be 18 feet long, and 93 sq feet. I privately emailed an expert in mast design and they told me that based on all scantlings/measurements, righting moments, sail area, etc. that the mast should be 4.5 inches in diameter (and a taper after 5 feet till the tip of the mast). He said this would include a safe margin of error (don't remember if he used this terminology exactly). Regarding the choice of material, I mentioned to this designer that I was constrained to build the mast out of common white pine (I know it's not generally recommended). He recommended this diameter based on the specific properties of white pine. In the meantime, I've found a source of nice douglas fir and I was wondering if anyone knows if the diameter would change because of the higher relative strength of Douglas Fir? I've been trying for a while to reach this generous designer and ask him the question, but he seems unavailable at the moment. Would anyone know how to recalculate the diameter for douglas fir instead of standard white pine
    thanks very much!
     
  2. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    I don't have the calculations for you, but 4.5" over 18' seems plenty for your application. Since you are planning on bird beak construction, your wall thickness will play a big role in stiffness and strength.
     
    Last edited: Sep 14, 2022
  3. zvi
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    zvi Junior Member

    Will,
    thanks very much for your reply. Thanks so much for pointing that out. The designer recommended a 1" wall thickness.
    my question is, could I get away with a smaller diameter? I would love for it to be a bit lighter if it can be.
     
  4. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Calculating without taper and keeping the same diameter the weight difference between white pine (pinus strobus, 400kg/cum) and douglas fir (510kg/cum) is 4.4kg. The taper will reduce it further, the actual difference will probably be in the region of 3kg.
    If I compare the 4.5in white pine mast with a 4in douglas fir one and keep the 1in wall thickness the difference is 1.4kg (again, no taper, so less for the real thing).
    To all this, there is the problem of the real density of your actual wood, wich might differ from my calculations.

    My conclusion, weigh a sample of the actual wood, see where it lands you weightwise and decide if it's really necessary to change the scantlings.
     
  5. zvi
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    zvi Junior Member

    Rumars,
    thanks very much for your reply. I was thinking more along the lines of reducing the wall thickness to something like 3/4" if I could get away with it. I thought that may reduce the weight somewhat significantly. I'm really kind of afraid because when I bought the wood yesterday it weighed quite a bit. BTW, I did weigh each piece and they came out almost exactly the same as an online calculator that I found. I punched in the dimensions and species and it was within a few grams so we can assume that the density about average. I do plan on weighing each piece again in a few days just to make sure all the residual moisture content is gone.

    I bought 3 pieces of wood 5*15 cm 310 cm long (one of the pieces was 280 cm long). the weight was in the ballpark of 85 lbs. ! I calculated that I will be using almost all the wood. I know that I will be doing a lot of resawing, sanding, etc., but I'm looking at least 45lbs+ for the completed mast. If I'm remembering correctly, the designer, told me that the completed pine mast will be around 25 lbs. This is the reason that I'm trying to reduce the weight.
     
  6. Rumars
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    Rumars Senior Member

    85lbs yelds me a density of 570kg/cum wich is in the natural range of douglas fir, but maybe it's just not dry enough.
    Calculating a 4.5in white pine mast (400kg/cum) with taper on 13ft down to 2in I get a weight of 25lbs (11.4kg). The same mast in 510 kg/cum DF is 32lbs (14.5kg). Using 570kg/cum I get 37lbs (16.9kg).

    Reducing the wall thickness is possible, 19mm seems reasonable.
     
  7. wet feet
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    wet feet Senior Member

  8. zvi
    Joined: Sep 2022
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    zvi Junior Member

  9. zvi
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    zvi Junior Member

     
  10. Rumars
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    Rumars Senior Member

    Yes, all calculations are for a 1" (25.4mm) wall thickness. I can calculate for 19mm if you like, asuming I got the taper right and the mast is 2" at the tip.
     
  11. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    PVA woodworking glues, recommends 100 to 150 pounds per square inch (psi) for clamping softwoods and 175–250 psi for hardwoods.
     
  12. Chuck Losness
    Joined: Apr 2008
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    Chuck Losness Senior Member

    Years ago Eric Sponberg shared a paper he had written on the design of round free standing masts. That paper happened to focus on a carbon fiber freedom 40 mast. But the calculation methodology could be used for any size mast made out of different materials. The starting point for the calculations is the righting moment of the boat. The most force on the mast is at the partners at the deck. The force decreases linearly to the top of the mast. The force diagram is a triangle. You calculate the force on the mast at one foot intervals.

    Although not as good as using the righting moment you can also use the heeling moment from the sail. Once you know the heeling moment of the sail you distribute this force along the length of the mast. The most force is again at the partners deceasing to zero at the top of the mast.

    Once you know how strong the mast needs to be at each interval you can then compare it to the strength of your proposed mast. You need a safety factor of usually 3.

    Eric's paper lays all of this out.

    Your mast may not need to be 4.5" in diameter with a 1" wall. It might be smaller or it could be larger. But you will never know until you determine the forces on the mast.
     
  13. Bruce Woods
    Joined: Sep 2007
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    Bruce Woods Senior Member

    4.5 in diameter mast for that dinghy dimensions is crazy talk.

    check out similar sized dinghy’s with unstayed timber rigs for guidance.
    Ie the Finn dinghy, more sail , taller rig, slightly less beam , but I guess you are not hiking like a 100 kg Finn sailer, is using much smaller mast cross section.
    100 mm x 6o mm at the base tapering to approx 55 mm x 30 mm at the tip.
    Google Finn class rules for guidance.
    That 4.5 in tree trunk is just going to make it so much harder to right after capsize.
     
  14. Howlandwoodworks
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    Howlandwoodworks Member


  15. tkk
    Joined: Jul 2009
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    tkk Junior Member

    If you are concerned about weight you should taper not only the diameter of the mast but also the wall thickness. Same diameter/thickness relation along the length of the mast. Otherwise you will end up having a solid trunk at the top. That is how I designed my masts.
     
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