Mast builder and I had it out!

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

  1. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Yep, mast weight only makes the boat roll deeper and slower. The underwater profile has the biggest effect on dampening, since water is denser then air, but the sails also contribute.
     
  2. Scuff
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    Scuff Senior Member

    This is also what I'm thinking. Even with a double spreader mast you aren't going to remove that much weight. The bare extrusion is just a fraction of the total weight of the rig.
     
  3. Farlander
    Joined: Mar 2021
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    Farlander Junior Member

    I thought the roll period was similar to the pendulum effect and is primarily based on the length of the mast, not the mass?

    How do we calculate the change in roll acceleration, or in post #40, the 'roll dampening'?

    Thanks,
     
  4. Will Gilmore
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    Will Gilmore Senior Member

    I am not that familiar with the physics, but thinking about this problem, I realize there is a difference between the rolling motion of a sailboat as a driven harmonic system and the classic physics problem of a gravity driven pendulum.

    With a classic pendulum, the mass factors out when gravity is the motivator. The more mass, the more inertia at the same time, the more mass, the more force from gravity to move that inertia. Therefore, mass is factored out of the equation.

    When the motivating force is not mass dependent, such as the waves lifting a 14 foot wide hull and keel, with a mast as a part of the system. Would that be the same problem? Greater inertia to overcome requires more force to overcome it. Would greater force to over come that inertia be availible if the mass of the mast was increased? I assume, the buoyant force compensates by the hull sitting deeper in the water, but a heavier boat would not respond to small surface changes like a lighter boat. Therefore, any resistance to movement, the mast represents, would require more force acting upon the hull to move the mast. The force of gravity would then be a factor that helps drive the system as the mast leans away from center. The farther it leans, the more gravity affects it. Then, the inertia is overcome by both the force of gravity, which would factor out, and by the buoyant force, which wouldn't be mass dependent, so that part of the equation wouldn't factor out the mass of the mast.

    Just my simple musings.

    -Will (Dragonfly)
     
  5. gonzo
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    gonzo Senior Member

    I believe that people that state a lighter mast makes little difference on the rolling and pitching behavior of a boat have not been at sea on a dismasted boat. The accelerations are increased enormously.
    That works for a simple undamped pendulum over a small arc. The mass has a large effect on the acceleration rate.
     
  6. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Nothing is true nor is it a lie. It all depends on the color of the glass you look at. Applying the saying to this case, it all depends on the weight of the mast, compared to that of the boat, and on the Cog height of the mast and the equipment attached to it.
    Fortunately, for those of us who have never been shipwrecked or broken a mast, physics gives us more than enough reasons to comment on the phenomena governed by the laws of physics.
     
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  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    just add weight like radar etc to top of mast. A little weight at very top should equal much more weight throughout the length of the mast. 1/8" 6061 on a 9x6" oval shaft seems about right. 1/16" 6061 seems like you could stumble and fall into it and put a big dent on one side and then it would want to suddenly fold up.
     
  8. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    Farland,
    Charles Morgan Out Island 41's full keel, center cockpit, master cabin aft, spacious engine room all in 41 ft.
    We bareboat a OI 41 out of Islamorada Fl Keys in the 90's. Comfy enough to sail over to the Bahamas and back with the wife and kids.
    Squalls can come up on you out of nowhere there in August and the masts always seemed like a weak point to me on any sailboat.
    So there were a thousand of them built and sold. huh. I would go with the original designers design. Charles Morgan was a great designer he must have had his reasons.
    Its broke now and someone wants you want to replace it with something lighter weight, seems counterintuitive to me.
     
  9. fallguy
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    fallguy Senior Member

    Does anyone know if one can even purchase a 55' mast at 720#? My hunch is you gotta buy an 80 footer and lop it off. That might get REAL pricey to recycle 25 feet or so of an extrusion. But I am truly just asking.

    Sometimes the answer desired is simply also impractical..
     
  10. gonzo
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    gonzo Senior Member

    I think that the first step is to strip the old mast of all hardware and weigh it.
     
  11. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    As David Cockey said correctly in #14, and Robert Biegler underscored it in #30, there are two completely different "moments of inertia". The "area moment of inertia", which is of relevance in relation to the bending of the mast and is only dependent of the shape of the cross section, and the "mass moment of inertia" which is determining the inertial resistance against a rolling motion. (Mass moment of inertia is not to be confused with dampening - it is always present also without any resistance caused by air or water.)

    The linked calculations from kastenmarine are wrong like as your calculations about this, Farlander (#31). The fourth power of lenght or mass is not part of the mass moment of inertia, the former belongs to the area moment of inertia.

    Some ballpark numbers: the hull of the boat could be roughly approximated as a lying cylinder with its rotational axis through CoB parallel to the waterplane: m = disp = (22,000 lbs - 720 lbs) = 21,280 lbs = 9,650 kg, r = beam 14' = 4.27 m,
    I (hull) = 1/2 * m * r² = 1/2 * 9,650 kg * (4.27 m)² = 88,000 kg*m²

    The mast is a stick rotating around almost one of its ends, m = mass = 720 lbs = 327 kg, l = lenght = 55'4" = 17 m;
    I (mast) = 1/3 * m * l² = 1/3 * 327 kg * (17 m)² = 31,500 kg*m²

    I (boat1) = 88,000 kg*m² + 31,500 kg*m² = 119,500 kg*m²

    The proposed mast I (new) = 1/3 * 136 kg * (17 m)² = 13,100 kg*m²

    I (boat2) = 88,000 kg*m² + 13,100 kg*m² = 101,100 kg*m²

    This comparison shows that the mast represents a relevant part of the overall mass moment of inertia of the boat and the loss of about 20.000 kg*m² of it is not "nearly nothing".
     
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  12. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    The original design should have had moment specs .. can't you just match to that? If not you could contact rig rite they could probably tell you. I'm skeptical that the bare extrusion is 720 lbs. On their site as Fallguy pointed out the isomat ng80 does list the morgan 41. That mast is 5.38 lb/ft for a bare weight of around 295 lbs @ 55ft
    Isomat NG80 Mast https://www.rigrite.com/Spars/Isomat_Spars/Isomat_Masts/NG80_mast.php
     
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  13. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    I'm looking forward to the court case results next month!
    But in the meantime, this thread continues to entertain, thank you.

    I agree, strip the mast and weigh it.
    Build a new mast that is strong and weighs no more than the original.
    If it can be done lighter, fantastic, the OP can always hang sand bags aloft to compensate.
     
  14. bajansailor
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    bajansailor Marine Surveyor

    Farlander, please be very careful when dealing with lawyers - my general impression of cases like this where lawyers are involved are that the lawyers end up with all of the $$'s.
    If you are suing the previous owner for non-disclosure of the defect when you bought the boat, I would think that a good lawyer could argue that he honestly didn't know about it.
     
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  15. hoytedow
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    hoytedow wood butcher

    I would rather walk barefoot through tall grass in snake country than deal with lawyers.
     
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