Mast builder and I had it out!

Discussion in 'Stability' started by Farlander, Mar 24, 2021.

  1. rangebowdrie
    Joined: Nov 2009
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    Location: Oregon

    rangebowdrie Junior Member

    Reading thru this thread has taxed my math.
    But, it did remind me of the story that when Captain Voss was sailing the Tillicum, and encountered a powerful storm, he experimented with hoisting balls of chain up the mast(s) to change the roll period.
    Addendum; would sometime like to know the properties of the mast on my boat.
    It's a true ellipse shape, 5 1/2 x 8 1/2, 3/16ths wall, circumference ~23in.
    It originally was on a 38' Hinkley, I cut it shorter.
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I like the approach, but these, again, are just numbers with no real way to quantify their meaning ito real world feel or condition.

    For example, based on my crude and of questionable accuracy calculations, roll period is barely affected, but you show a 15% change in inertia.

    How does this translate? Does the boat start to roll 15% easier or 15% quicker? If it takes 6 seconds to start to roll on the heavy mast is it gonna start to roll in 5.1 seconds? But once rolling; the roll period is barely affected?

    This business of the heavier mast being 182 times more resistant to roll is bunk.
  3. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    Hi fallguy,
    I didn't find out what you are referring to with "captains formula". May be you used this one: T = 2 * pi() * k / (g * GM)^0.5
    and factored in only the difference of GM (metacentric height). But there is a difference in k also.
    k is the gyration radius k = ( I / m )^0.5
    k (1) = ( I (boat 1) / m (boat 1) )^0.5 = (119.500 kg * m² / 9,977 kg)^0.5 = 3.46 m
    k (2) = ( I (boat 2) / m (boat 2) )^0.5 = (101,100 kg * m² / 9,786 kg)^0.5 = 3,21 m
    If these calculations are correct, the roll period should be about 7 % shorter. The effect on sea sickness may be much harder than this value suggests.

  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

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