Kmt Rowboat

Discussion in 'Stability' started by leonardo2502, Feb 9, 2017.

  1. leonardo2502
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    leonardo2502 Junior Member

    How to calculate the KMT (transverse metacentric Height) of this boat
     

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  2. TANSL
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    TANSL Senior Member

    Can we assume that for other types of ships you know how to calculate KMT? Because, as you probably know, the calculation procedure is the same for any floating object.
     
  3. leonardo2502
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    leonardo2502 Junior Member

    I do not know calculate KMT, I control the KMT from DELFTship software report, but I know only change existing files and not draw a new boat
     
  4. leonardo2502
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    leonardo2502 Junior Member

    I do not know calculate KMT, I control the KMT from DELFTship software report, but I know only change existing files and not draw a new boat.
     
  5. leonardo2502
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    leonardo2502 Junior Member

    I know how to behave this boat into the sea, I drew (modified) a new boat, I would like to be a little more stable of the boat in * .pdf .

    In these boats the displacement is about 500 kg, (4 rowers, coxswain, rowing and boat).
    Or even if I can not calculate the KMT, I would like to know what should be the minimun KMT of the new boat, cosiderando that the seat bench (rowers) is about 27cm from the keel
     
  6. TANSL
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    TANSL Senior Member

    KMT = Ixx/V + KC

    where :
    KMT = Height of metacenter on the base, for 0 deg. heeling.
    Ixx = Moment of inertia of the waterplane area
    KC = Height of buoyancy center on base

    If you can not change the shapes of the ship and want to increase stability, all you can do is lower the total center of gravity.
     
  7. leonardo2502
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    leonardo2502 Junior Member

    KMT = Ixx / V + KC
    To calculate KMT should know how to calculate Ixx + Kc (I'm pretty ignorant on the subject).
    The KMT of the boat that I drew is 0.734m (report DELFTship), the sitting benches of rowers are at 0.27m from the keel, the gunwale is about 0.3 m (minimum height to be respected, race rules), the vertical center of gravity boat + rowers from keel should be about 0.53m - 0.55m, so the kmt should be greater than 0.55m. In this case is 0.734m, and the boat should have an acceptable primary stability. If everything I've written is right, I would like to know if I can reduce ( how much) the KMT, cosiderato that an intermediate level of rowers (rowing technique).
    Can you please tell me what you think.
     

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  8. leonardo2502
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    leonardo2502 Junior Member

    lines plan new boat
     

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  9. TANSL
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    TANSL Senior Member

    Leonardo, with all due respect I will tell you that if you do not know how to calculate Ixx, you probably will not know how to calculate V nor KC. So either you start studying theory of the ship or you get a program that calculates the value of the KMt.
    To calculate the KMt, you need, of course, to know the weight of the boat and, therefore, the draft for this weight. At the moment the KG does not intervene in the formulas that we are commenting.
     
  10. leonardo2502
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    leonardo2502 Junior Member

    Tansl

    In the first pdf file does not know exactly the draft, in the drawing is written that the volume is 423 cubic decimeter or about 432Kg to the waterline. and this is a mistake because the boat must weigh a minimum of 130 kg minimum helmsman 50kg, 4 rowers + oars about 320 kg, total about 500kg or 0,488 cubic meters.
    So from ingnorante on the subject, I think you could draw the boat in DELFTship is , and read the hydrostatic report.
    Even though I had studied the theory of the ship (as rightly say) you would be a problem to apply any formula, because as I wrote before the LWL and therefore the draft and volume are wrong.
    Perhaps the design does not take into account the burden that you have to put in the boat to get to 130Kg, and in recent years have said that the helmsman (children) must weigh a minimum 50 kg.
    I saw the video where these boats weighed before being ballasted and some have weighed about 90kg.
    40kg from the weight of the boats, and 20 kg by the helmsman and you get about 432kg or 0,423 cubic decimeter there about the project.
    Anyway, as already said I wanted to know the kmt of this boat (not very stable, so for experienced crews) for comparison with the new boat (with average experience crews).
     
  11. TANSL
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    TANSL Senior Member

    I had not opened the pdf file with the hydrostatics values but I see that in it you have all the data you need, moment of inertia of the waterplane and position of the center of buoyancy (CoB). Therefore, you can calculate the KMt for the displacement of the hydrostatics. It would have to be verified, for another displacement of the ship, how they change the Ixx and the CoB to know if the KMt increases or decreases as the displacement changes.
    (I do not have DELFTship and I can not study the ship with that software. I am sorry)
     

  12. leonardo2502
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    leonardo2502 Junior Member

    Ok Tansl , now I try to do what you said, thanks for everything
     
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