Godzilla

Discussion in 'Software' started by vandoren, Nov 6, 2007.

  1. dallolio
    Joined: Aug 2008
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    Location: Italy

    dallolio Junior Member

    Thank you Leo, thank you Rick,

    I ‘d like to design a sail catamaran, about 3.5 m long. I now understand in order to make the best of Godzilla, hull offsets table is not the way to go but I should rather go with shape functions.

    I’m thinking now to start all again I’ll try to modify the gcatin file in the Godzilla directory, to make the best of your recommendations.

    The question I’d like to spend now is about reading the fsoff files from freeship 2.6, in the import menu I have michlet wave but it’s in shade, not active.

    Andrea.
     
  2. Guest625101138

    Guest625101138 Previous Member

    You can import the offset file as a surface into Freeship.

    If you are making a sailing catamaran then the load distribution on the hulls is not equal unless you have crew weight to balance it out so you will probably find in practice one hull will be more heavily loaded than the other.

    Rick W
     
  3. dallolio
    Joined: Aug 2008
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    Location: Italy

    dallolio Junior Member

    Rick,
    It is good to think it's spring season there .
    The attacked godzilla " in" file provide a hull that looks reasonable to me. You wrote hulls will have different loads, how do I load the hulls, I have not clear the “ship loading type”.
    How do I find hydrostatic trim and trimmed boat longitudinal centre of effort ?
    Can I make it with freeship ?

    Andrea.
     

    Attached Files:

    • in.mlt
      in.mlt
      File size:
      8 KB
      Views:
      400
  4. Guest625101138

    Guest625101138 Previous Member

    A small sailing cat in the limit carries the entire displacement on one hull. This is true for beating and reaching. The other hull acts as windward ballast to keep the boat upright.

    This makes analysis to find the best hull quite simple because you put all the displacement in one hull. The other thing it provides is the maximum allowable sail force moment because this has to be offset with half the displacement plus any movable ballast such as crew sitting on one hull to create the righting moment. You see modern sailing cats have quite high beam to length ratio to improve the allowable driving force.

    So from a Godzilla perspective you place all the weight on one hull and assume level trim. Normally crew can be moved aft to keep the bow up.

    If you have Godzilla unconstrained then it will produce a canoe stern. There is a belief amongst catamaran designers that a flat run aft helps with pitch damping. I have never tested this so do not know for sure. I would reason that the added buoyancy just increases the pitching force.

    Once you get a hull from Godzilla it can be imported into Freeship or Delftship to develop the sections above the waterline.

    Leave the ship loading type as the default it does not have meaning for what you are doing.

    Rick W
     
  5. dallolio
    Joined: Aug 2008
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    Location: Italy

    dallolio Junior Member

    Very interesting !!!, Thanks a lot Rick Willoughby and thanks Leo Lazauskas
    for making available the software
    Andrea.
     
  6. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    Cusped ????

    Hi Everybody,

    I drop in your interesting discussion because somewhere you mentionned the 7 parameters functions of the mathematical hull series.

    I would like to know what means "Cusped",as mentionned in the Michlet's manual and if it is related to the extension of the cross-section parameter from the (0-1) range to the (0-3) range.

    In other words my question is: Which kind of value for the cross-section parameter is likely to provide a thinner section at the waterline than at the chine level, below the water line?

    Thanks in advance to the specalists

    Erwan
     
  7. Guest625101138

    Guest625101138 Previous Member

    Erwan
    This question is better for Leo.

    I believe the answer you seek is '0'. So you need to set both limits to zero and the seed to zero. This produces what I call a hard chine and you get a slab sided hull. This will have less width than the optimum hull where the cross-section function is not constrained. The hull drag will only be slightly less for the optimum but you have a hard chine hull that is easier to build.

    If your purpose solely to get something easy to build you are better off reducing an optimum hull to one having just bottom and sides as Godzilla will always produce vertical sides at the waterline (I think).

    What are you aiming to achieve with the hull?

    Rick
     
  8. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    Thank you for your message Rick,

    I just want to compare different solutions within a fixed framework like A-Cat or F18.

    Try to mimick existing hull shape, like Capricorn, Nacra Infusion, HB Tiger or Flyer put it on Michlet, and then turn around, with Godzilla.

    A long time ago I tried to do something, but I had difficulties with WinEdt so I dropped for almost 2 years, just looking at the forum and did not actually use Michlet, Godzilla & Freeship yet. But I want this to change.

    Sorry for this question again but neither my Harrap's dictionnary, nor Google language tools provided me with a clear answer about "Cusped"
    and also about "Flaired" hull sections, an my English is far from perfect, so could please you enlight me on this point?

    Thanks in advance

    Regards

    Erwan
     
  9. Guest625101138

    Guest625101138 Previous Member

    Erwan
    There is a particular challenge in analysing small sailing cats. The force on the hulls changes depending on the point of sailing so you probably need to determine relative loads on the hulls on a given points of sailing. In the limit beating or reaching they are effectively a monohull with one hull carrying the entire weight.

    So establishing the basis of comparison is most important.

    There is nominally only one optimum hull for a given set of conditions so the starter hull for Godzilla does not matter. The end result will be defined by the constraints. The lowest drag hull will result from the least constrains.

    The meaning of cusp is a point or transition. In the context of the shape constraints I believe it refers to a sharp transition from the hull bottom to the hull sides - what I refer to as a hard chine. You would only constrain this if you wanted a particular shape.

    I am not sure where you got "flaired" from. Do you mean flared or faired. I do not know flaired.

    So for the cat hulls you need to determine the conditions for comparison. You must know displacement, the point of sailing, the likely sail loads (light or at the limit), wind speed for running.

    Remember Michlet gives calm water drag for the hull. There is no allowance for sea state.

    Probably the best way to advance what you want to do is to select two of the boats and do drag analysis using Michlet for say flying a hull reaching. So the analaysis is as a monohull. See how the results compare and assess if this matches practice. If it looks good look at another point of sailing like running with both hulls equally loaded.

    There are other factors that could be significant as well such as keel and rudder.

    So the analysis cannot be done in isolation of some real input to test validity.

    Rick W.
     
  10. dallolio
    Joined: Aug 2008
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    Location: Italy

    dallolio Junior Member

    RicK

    My catamaran godzilla “in” file shape 7 factors are uncostrained, I only set displacement lenth and draft. When I go up with the displacement value Godzilla gives me strange shape along the line of connection of the bottom and sides of the hull. Thanks for any input you might have on this.
    Andrea
     
  11. Guest625101138

    Guest625101138 Previous Member

    Andrea
    Can you post the displacement, length and draft as well as the offset file for the hull so I can see what you mean. (Why are you constraining draft on a little hull???)

    Rick
     
  12. dallolio
    Joined: Aug 2008
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    Location: Italy

    dallolio Junior Member

    I have a race in the Venice lagoon next spring (www.velalonga.it) in some points the lagoon has very low water. But with unconstrained draft I have the same problem, attached file has uncostrained D. The problem is visible importing the file into freeship 2.6
    Thanks. Andrea.
     

    Attached Files:

    • in.mlt
      in.mlt
      File size:
      8 KB
      Views:
      398
  13. Guest625101138

    Guest625101138 Previous Member

    Andrea
    I have attached the lineplan of the hull I get. I have extended the hull above the waterline.

    I can see your need to constrain the draft. This is quite an extreme shape. It is the result of trying to go fast in a short hull. I expect a planing hull might actually have lower drag.

    A cannot see the problem you say about the strange shape. If you reduce the optimised speed then the hull shape will not be quite as extreme.

    Rick
     

    Attached Files:

  14. Leo Lazauskas
    Joined: Jan 2002
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    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    It looks like you are trying to put too much volume into too little length and draft. It is no surprise that you end up with hulls that are almost like blocks.

    If you increase the length, or reduce the volume, or as Rick suggested, reduce the design speed, then the optimal hulls will look more reasonable.

    Good luck,
    Leo.
     

  15. dallolio
    Joined: Aug 2008
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    Location: Italy

    dallolio Junior Member

    Rick, Leo
    I was referring to the shape of the hull near the connection bottom and sides visible in the example and example II. Not an issue but just to make sure I’m not doing something wrong.
    I was last at the velalonga 2008 with my home made mono hull, and I’d like doing better next year, but, I also would like to make something simple to handle for transportation, my car is less than 4 m long….
    Forcing an elliptical cross section, and maintaining L= 3.5 m, with D=0.12 for each hull, forcing T=0.15, Godzilla gives me the attached hull, it looks nice to me. I think I’ll probably need one or two centre board because of the small draft. Adjustable height (draft) centre board.
    Of course any comment would be welcome…
    Thanks again,
    Andrea
     

    Attached Files:

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