Mast loads for freestanding masts

Discussion in 'Boat Design' started by dustman, May 10, 2024.

  1. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    OK, now I've finally drank enough rhum...

    Oh! dustman! you are soooo...insightful! Alexander Graham Bell and Guglielmo Marconi where soooo stupid in that they invented the telephone and radio respectively, but just didn't go straight to inventing the the cell phone....what could they have been thinking. Why didn't they just go down to the store and buy one?
    Just like all those stupid people in the iron age...why did they invent the stayed mast. Why didn't they just go out into the forest a cut down a tree with the necessary length, same light weight, and same sectional modulus distribution as a modern carbon fibre mast. How stupid could they be?
    I am soooo glad you came and enlightened me...

    Now you can report me....
     
    gonzo and bajansailor like this.
  2. dustman
    Joined: Jun 2019
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    dustman Senior Member

    Now that I've had a few beers...

    I enjoy a little sarcasm now on this forum. I used to be offended but I've gotten over that.

    You are totally right that at one point it was progress and maybe even a necessity, but no longer. Pretty much as soon as decent aluminum came along. I should have phrased my criticism more precisely.

    Back when I first started daydreaming of sailing I watched hundreds of sailing videos and pretty much the first thing that struck me was that people were always worrying about(and spending lots of money on) their rigging. And how much they had to risk death to mess with their rigs. "Couldn't get the sail down because it was pressed on the stays." And there spawned my interest in junk rigs. Don't get me started on whisker poles...

    Anyways, I'm glad you understand now how barbaric stayed masts are. ;)

    PS: Is just a little more weight up top really that bad of a thing? Based on the findings in the Sydney to Hobart disaster..
     
  3. rxcomposite
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    rxcomposite Senior Member

    This is not a simple cantelever load. It is a compound load. You have bending load and rotational (torsion or shear) load. The math is standard and is available in most engineering books or explained in you tube or google.

    As suggested, the sail is triangular, so the maximum bending effort will be about 1/3 of the sail length and the rotational force will be about 1/3 of the boom length (sail centroid of area). Being triangular, this will be a diminishing load towards the tip which suggest a tapered mast or a diminishing wall thickness if the section is constant. The wind force is slightly higher at the tip so the apparent sail profile is more elliptical. Not much of a concern at this point.

    The stiffness/ strength is usually expressed in section modulus so if it is tapered or with diminishing wall thickness, the computation gets a little longer as you have to subdivide the length into sections to get the proper "bend".
     

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    Last edited: May 16, 2024
  4. dustman
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    dustman Senior Member



    At 35:17, Tom Cunliffe explaining how to get a sail down while running.
     
  5. PhilippeCE
    Joined: Apr 2022
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    PhilippeCE Junior Member

    @dustman I think what @gonzo mean is that there is no point calculating a wind load higher than what your boat can tolerate in terms of righting moment since this wind force would overwhelm your boat righting moment and make your boat roll over, therefore the righting moment is what your mast should be able to withstand. Your mast simply cannot face a higher load. Your mast will have to be heavily reinforced and/or thick at the mast collar, since fatigue will take its toll. No Visible Means of Support – Freestanding Rigs | Good Old Boat https://goodoldboat.com/freestanding-rigs/
     
  6. TANSL
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    TANSL Senior Member

    I think concepts are being mixed, such as loads and who supports them and moments produced by those loads and who supports them.
    There is a force exerted on the mast that gives rise to a distribution of varying bending moments along its length. The mast must be sized to support these bending elements, which vary along its length.
    On the other hand, the force of the wind on the sail generates a global heeling moment, which, assuming that the mast was rigid, should be counteracted by the righting moment generated by the shapes of the hull.
    To calculate the mast, since calculating the force of the wind is very difficult, simplified methods assume that, considering the mast rigid, the boat heels, for example, 30º. The righting moment at that heel, which is easier to calculate, must be equal to the global heeling moment exerted by the wind. Knowing, therefore, the moment exerted by the force of the wind, and assuming that the force is applied in the center of gravity of the sail, we obtain something similar to the total force exerted by the wind.
     
  7. dustman
    Joined: Jun 2019
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    dustman Senior Member

    Thank you for your further inputs.

    I'm going to go with what Sponberg recommends for "expedient" mast design. That being: "The heeling moment at the design condition is considered to be the maximum working load of the masts, and it should be equal to 1/3 or 1/4 of the boats maximum righting moment." "gives a safety factor of 3 or 4 against unknown loads.... [and] ....fatigue".
     

  8. rxcomposite
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    rxcomposite Senior Member

    Dustman_ Are you building a tapered composite mast?
     
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