Methods to calculate moment of inertia for the mast

Discussion in 'Sailboats' started by skippeer, Feb 11, 2019.

  1. skippeer
    Joined: Feb 2019
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    Location: Sweden

    skippeer Junior Member

    Hi

    I have read many threads/posts about rig calculation, there are several different methods and my understanding is that dynamic loads are the tricky component to calculate.

    I have used Nordic Boat Standard to calculate a rig but I would also like to use another method to get a "second opinion" so to speak.
    I aimed to use Skene method but I have not been able to find the required factors/constants for:
    * fractional rig
    * double swept spreaders
    * keel stepped mast
    * single lower shroud

    Is that type of mast covered by constant factors in Skene method at all?
     
    Last edited: Feb 14, 2019 at 2:13 PM
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Sectional moment of inertia for strength or mass moment of inertia for pitch/roll dynamics?
     
  3. skippeer
    Joined: Feb 2019
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    skippeer Junior Member

    Hi

    Ix and Iy do I aim to calculate (Transverse and longitudinal moment of inertia).
    The web pages I have found do only cover masthead and 7/8 rig and I have a 19/20 keelstepped rig with 2 swept spreaders and single lower.
     
  4. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    You will probably need to calculate that by hand or with a custom mast manufacturer specific program. In order to optimize for weight and strength something as specific as that you need to optimize the Euler buckling of each segment against weight and load. Fairly straightforward and an interesting education for those who want to get the very best solution.

    I'm going to be out of coms for a few days, let me see if I can round up an example when I get back next week.
     
  5. TANSL
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    TANSL Senior Member

  6. skippeer
    Joined: Feb 2019
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    skippeer Junior Member

    Hi TANSL,

    thanks for the hint, what method is MyR based on?

    MyR is a great software I have tested and it is asking for many indata components, the majority am I confident on but the angle for mast to aft/fore-stay did I estimate by looking at a picture (20/20 degrees) :)

    "MyR Ix" did match my manual NBS based calculation, I felt happy.
    "MYR Iy" did result in a higher figure compare with my manual calculation, when doing the manual calculation for longitudinal moment of inertia did I use "h" that I expected to be length of panel 1 + 2 + 3.
    I am using NBS calculations from "Yacht Design" book (second edition) and wonder if I did some error or if the root cause is due to different methods in the book compared with MyR.

    Maybe my mistake is using the wrong staying factor;
    * for a mast with single lowers k2 = 0,8 (I used this in my manual calculation)
    * for a mast with swept spreaders k2 = 0,95
    When calculating on a mast with single lowers and double swept spreaders which is the right k2 factor to pick?
     
  7. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The attached file explains what MyR does and the coefficients it adopts. It is based on the NBS, but adopts its own values for some coefficients that the NB S does not leave, imo, sufficiently clear. There may be, logically, some discrepancies with your calculations.
     

    Attached Files:

  8. skippeer
    Joined: Feb 2019
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    Location: Sweden

    skippeer Junior Member

    I have used MyR in multiple rig calculations and also Seldens RM Calculator ( RM Calculator : Seldén Mast AB http://www.seldenmast.com/en/services/calculators/rm_calculator.html ).

    I wanted to double check the "empty displacement/load" of our sailboat, have heard figures about 9,5 ton ( 9 500 kg).
    From the rig data sheet do I pick "Heeling test RM @30 degrees" and run the RM Calculator with (fixed draft, wing keel, beam and ) different displacement values until I get a match
    ==>> "Empty displacement" at the heeling test should have been 9 550 kg.

    MyR do not have any indata option for wing keel and when I use the displacement 9 550 kg both as full load and empty load as indata to MyR do I get a too low RM value as output.
    So my workaround has been to tweak the displacement indata until MyR calculation output shows the same RM as at the heeling test.

    By tweaking the displacement until I got the wanted RM did I finally get all calculated data for the rig, is my "tweaking" method giving me fair output data?
     
  9. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Hi Stefan,
    I do not understand what you are doing but what I can assure you is that RyM does not calculate any value of the RM (it does not have any data that allows it to calculate or assume that value). That is a value that the designer MUST enter.
    The effect of the keels, if they exist, must be included in the value of the RM that the designer introduces.
    If you wish, send me the data of your boat and your rig, I will analyze them, and I will perform my calculations with RyM.
    Cheers,
    Ignacio
     
  10. skippeer
    Joined: Feb 2019
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    Location: Sweden

    skippeer Junior Member

    Hi Ignacio,

    I have uploaded my MyR indata + RM outcome from 2 of my MyR calculations which confuse me when comparing RM with heeling test and figures from sistership.
    It would be great to get rig data for a full loaded boat (blue water sailing).

    thx
    Stefan
     

    Attached Files:

  11. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    At the moment I have seen several things:
    - number of people = 6 (not zero)
    - you must calculate the RM at 30º and also the GZ for 1º, both for the full load.
    - if the longitudinal rig is Single lower & swept spreaders, the "gamma" angle does not have to be given, but it is necessary to give a value for the "delta" angle.
    -If the semi-manga is 1,855, why do not you put a very similar value for "b"?
     
  12. skippeer
    Joined: Feb 2019
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    Location: Sweden

    skippeer Junior Member

    Thanks for helping me to use MyR in the expected way.

    The last "bullet" do I not follow, what is "semi-manga" ? I tried to locate value 1,855 but failed.
    -If the semi-manga is 1,855, why do not you put a very similar value for "b"?

    # # #

    The 3 first "bullets" can I change in line with your guidelines.

    # # #
    Regarding the RM I was refering to is part of the output from MyR

    Here is one example with GZ1 being blank/empty

    RM30º = 6670 kg.m
    LOAD CASE 1

    T1 = 397 kg 3894.57 N
    F3 = 397 kg 3894.57 N

    #
    Here is a second example when I populated GZ1

    30*RM1º = 6671 kg.m
    LOAD CASE 1
    T1 = 397 kg 3894.57 N
    F3 = 397 kg 3894.57 N
     
    Last edited: Feb 18, 2019 at 2:10 PM

  13. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Sorry for my translation error, "semi-manga" means half breadth of the ship.
    The program calculates the values that are deducted from the GZ at 30º and also those deduced from 30 times the GZ to 1º and adopts the most unfavorable values. If one of those GZ is zero, it only calculates what corresponds to the other GZ.
     
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