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The weight of solid versus hollow wood masts

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  • steelsil2
    The question naturally arises, then, of just what are the weights of a solid wood mast vs. a hollow wood mast. The facts needed to address this are: a hollow
    Message 1 of 52 , Dec 2, 2007
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      The question naturally arises, then, of just what are the weights of a
      solid wood mast vs. a hollow wood mast. The facts needed to address
      this are: a hollow mast should have 10% greater diameter than the
      equivalent hollow mast, the formula for the volume of a cylinder is pi
      x the radius squared x the height, and the fact that hollow wood masts
      should have a 20% wall thickness (not to mention solid inserts at the
      masthead and partners (deck or upper point where it is held firmly.)
      I think it is reasonable to use the average diameter to calculate
      this, so since the mast head is 41% of the greatest diameter, so
      (discounting any taper below deck) you want to use say .71 x the
      greatest diameter to calculate the average. The weight of Douglas Fir
      (aka Oregon Pine,) is @32 pounds per cubic foot, and a cubic foot is
      12 x 12 x 12 inches = 1728 cubic inches.

      (I'm not very good at math, so fellow posters, please feel free to
      double check and correct my arithmetic.)

      If your solid mast is 12" in average diameter and 70'long overall, its
      weight would be (3.14 x 36 x 840 / 1728) x 32 = 1,755 pounds.

      An equivalent hollow mast would be 13.2 inches in diameter, and its
      "removed" space would be 11 inches in diameter, rounded for
      convenience. So, you calculate the two, subtract the second from the
      first, and there is your weight. So, 2128 - 1372 = 756 pounds for
      the hollow equivalent.

      That is a heck of a lot lighter. Does it matter?

      If we are comparing junk masts to marconi masts, there are various
      factors to consider. Junk masts are rather short compared to marconi
      masts, and so the heavy masts have less effect on heeling than you
      might expect. A 35' (length above deck) mast weighing a 1000 pounds
      has the same heeling moment, roughly, as a 45' mast weighing 750
      pounds. Also, let us not forget that shrouds, stays, tracks, tangs,
      etc. have considerable weight.

      Based on my experience of two boats 34 and 35' long, one a rather
      tender 12,000 pound marconi sloop, and the other a very stiff 18,000
      pound junk schooner with solid masts, not if you have a stiff boat
      boat with moderate to heavy displacement. The sloop in question
      usually went to windward with the lee rail and stanchion bases under
      water. The schooner never touched its rail to the water. I sailed
      the sloop about 6,000 miles, and the schooner perhaps 14,000 miles.

      So, the stiffness issue aside, obviously, some boats were designed to
      be light in displacement, and the weight of a solid mast (or 2) would
      be undesirable, regardless of any effect on stiffness. Another issue
      is the effect of weight aloft on the motion of the boat. In my
      experience, weight aloft makes a boat more comfortable, as it slows
      the boat's motion (roll, heave, and pitching.) The investigation
      after the Fastnet race disaster also suggested that weight aloft would
      make a boat less likely to capsize.

      My conclusion is that a moderate displacement to heavy displacement
      cruising boat should have sold grown sticks, if feasible, unless the
      boat is tender, and light displacement boats should have hollow ones.

      Tim Dunn, somewhere near Seattle
    • steelsil2
      Hi, Gary Regarding mast height, my example of a junk schooner with a main mast 35 long above deck, and a marconi sloop with a mast 45 long above deck: these
      Message 52 of 52 , Dec 3, 2007
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        Hi, Gary

        Regarding mast height, my example of a junk schooner with a main mast
        35' long above deck, and a marconi sloop with a mast 45' long above
        deck: these were not theoretical boats. They were two boats I
        actually owned, Batwing and the Artful Dodger, an Ericson 35. See
        http://www.boatus.com/jackhornor/sail/Ericson35.asp

        Batwing's rig was designed by Hasler, and its AR was 1.7 for the
        mainsail and 1.8 for the foresail. You can see Batwing in PJR.
        Batwing was 34' long and 18,000 in displacement, and the Artful Dodger
        was 35' long and 12,000 pounds in displacement. -TD
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