correct birdsmouth mast diameter

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

  1. zvi
    Joined: Sep 2022
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    Location: israel

    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
    Joined: Sep 2022
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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
    Joined: Sep 2022
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    Location: israel

    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
    Joined: Mar 2013
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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
    Joined: Sep 2022
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    Location: israel

    zvi Junior Member

     

  10. Rumars
    Joined: Mar 2013
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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.
     
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