Calculating Wind Load on a Katamaran

Discussion in 'Stability' started by Heimfried, Dec 19, 2017.

  1. Heimfried
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    Heimfried Senior Member

    What kind of data decribing the shape would be appropriate? I've got a freeship file for one hull or an Excel-csv-table which gives the z values (height above base plane) for all points in a grit of x and y coordinates. Shape data concerning the superstructure at now are just the wind exposed Area is 11.3 m² and its centroid is 0.89 m above WL.
     
    Last edited: Dec 20, 2017
  2. TANSL
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    TANSL Senior Member

    It would be necessary the 3D model of the ship in iges or dxf format, or a body lines plan or a table of offsets.
    But I suppose that the same software that calculated the GZ curve for the displacement of 1050 kg could calculate that curve for any other displacement.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Can you not anchor the boat?
     
  4. Heimfried
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    Heimfried Senior Member

    Yes, but not always.
    Given the situation to pass a harbour entrance or enter a boat box with crosswind. Assume the same amont of windage area and crosswind a boat with with large displacement and draft will heel more and make less leeway, than my boat with small displacement and draft.
     
    Last edited: Dec 21, 2017
  5. Ad Hoc
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    Ad Hoc Naval Architect

    No, can't assume that.

    The amount of heel is related to the metacentric height. Which is governed by the inertia of the waterplane area and the volume BM = I/V and the location of the KG to that metacentric height.

    Think of the QM2...huge boat high windage yet does she heel a lot from a 20 knot wind...not much if at all noticeable.
     
  6. Heimfried
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    Heimfried Senior Member

    You are right, naturally.
     
  7. Heimfried
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    Heimfried Senior Member

    At moment there is no 3D model.
    Yes, the software calculates the GZ curve according to the displacement input (also a given list or trim will be included).
     
  8. TANSL
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    TANSL Senior Member

    I do not understand why you have been able to calculate the list angle produced by the wind, for a certain displacement, and you can not do the same for another displacement. If you know, or can know, the curve of the GZ values for this last displacement, proceed as follows:
    - Calculate the heeling moment due to the wind.
    - in the equilibrium position the heeling moment must be equal to the righting moment of the hull.
    - divide that moment by the displacement, with which you will obtain the righting arm of the hull.
    - check the GZ curve to which heel angle corresponds a value of the GZ equal to the righting arm.
    You can proceed the other way, if what you need is to calculate the force or speed of the wind that produces a certain list:
    - in the curve of the GZ values obtain the righting arm for a certain angle of heel.
    - multiply the righting arm by the displacement. That will give you the righting moment (which at the point of equilibrium is equal to the heeling moment due to the wind).
    - that moment, divided by the distance of the center of pressure of area exposed to the wind in this case, will give you the force of the wind and from there you can obtain the verlocity of the wind by means of the formula that you already know.
    If I have not understood you correctly and all this is very obvious to you, I beg your pardon.
     
  9. Heimfried
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    Heimfried Senior Member

    Thank you TANSL for your explanations.
    But sorry, you misunderstood the question.

    I wrote: The further interesting question is how much of the wind force will cause a heel of a light boat with small draft without outer keel or skeg and how much will cause leeway?

    The idea is, a light boat whith shallow draft in a (constant) crosswind will take easy lateral movement (leeway) and only the "remainig" apparent wind will cause heel.
    crosswind = true wind;
    true wind - lateral speed of the boat = apparent wind

    There are a lot of formulas describing the power or force needed to drive a certain boat with a certain speed - longitudinal.
    Is it - with considerable effort - possible to calculate the force needed to drive a certain boat in the lateral direktion with a certain speed?
     
  10. TANSL
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    TANSL Senior Member

    @Heimfried, you are right, I have totally misunderstood your question. Sorry for having wasted your time reading my post.
     
  11. Heimfried
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    Heimfried Senior Member

    As to the software: yes I can e. g. add ("load") just another 500 kg to increase the displacement and click for the next GZ curve.

    [​IMG]

    [​IMG]
     
    Last edited: Dec 21, 2017
  12. TANSL
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    TANSL Senior Member

    I do not know if, the following could help you. In the case of tugs, a study of the heeling moment that occurs when the vessel moves laterally must be made.

    HM = (C1 * C2 * gamma * V ^ 2 * Sp * (H * cos (tita) + C3 * Tm) /19.6

    where :
    HM = heeling moment
    C1, C2 and C3 = coefficients that are obtained by a series of curves prepared in this regard.
    gamma = density of seawater
    V = ship's transverse speed, m / s
    Sp = projected lateral area of the submerged part
    H = height of the point of application of the force with respect to the flotation
    tita = heeling angle
    Tm = medium draft.
     

  13. Heimfried
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    Heimfried Senior Member

    Thank you, TANSL.
     
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