Pitch - Roll Order and Axes

Discussion in 'Stability' started by DCockey, Aug 11, 2011.

  1. DCockey
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    DCockey Senior Member

    I'm not overthinking it and I'm not confused. It does appear though that some others are trying to read too much into my question and have confused themselves. JEHardiman understood my question and provided the information I was looking for.

    I'm interested in changes in CG location which result in relatively large changes in both heel and trim. Not a problem most naval architects typically encounter, and frequently "not important/considered". It is of interest to me as part of a comparative study of some historic small open boat types. (Boats where the passengers and "cargo" can weight several times the weight of the boat itself.)

    I know how to do the calculations to determine the exact orientation of the boat with the new CG, even with large heel and trim angles. My question was about what definitions of heel and trim angles to use to describe that orientation, not what heel and trim is.

    I also understand about yaw angle being neutrally stable which is why I didn't ask about it to start.
     
  2. DCockey
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    DCockey Senior Member

    Here is an image of one of the more extreme examples of what I'm currently looking at.
     

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  3. Ad Hoc
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    Ad Hoc Naval Architect

    The CoG does not change, only the CoB. The CoG does not move.

    What you're talking about here is a simple CoG shift...resulting from a large angle of hell/trim...ie like a classic shifting cargo problem. This is a straight forward calculation, which you then apply to your vessel. This is VERY important is is always considered when reviewing stability.

    With respect, i think you don't fully understand your own problem/question.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    No idea what this is...just a 3d shape that is rendered with a waterline.

    As I said you're overthinking this. Just explain in very simple terms what it is you wish to achive. Explain like a 10year kid, so it is crystal clear. I think I know what you're on about...but I want to hear it from you. Since your explanations to date suggest you don't understand your problem. Since in naval architecture, it is a straight forward calculation. But because you're not a naval architect, you are unware of this and seem to think it is a complex situation, it is not.
     
  5. DCockey
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    DCockey Senior Member

    The CoG of the passengers and cargo changes the combined CoG of the boat plus passengers and cargo.
    Correct.

    I'm looking at boats around 4.5m in length. One of the uses was lobstering and the pots were hauled by hand over the rail. We're trying to understand why the design evolved as it did, and a possiblity is the stability.

    With respect, I understand my question, which is what is important.
     
    Last edited: Aug 13, 2011
  6. DCockey
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    DCockey Senior Member

    I'm not having any difficulty doing the calculations. I just wanted to know if there was a standard convention for measuring heel and trim angles when they are large so that I could adopt that convention. JEHardiman provided the answer, though I wonder how universal it is.

    The picture is of a 15' long "peapod", displacing 635 lbs. It is heeled 30 deg and trimmed 12.5 deg. The combined CG of the boat, passengers and cargo is 0.5 ft from centerline, 3.17 ft forward of the center of the boat, 1.0 ft above the outer rabbet.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    The CoG is the CoG. When going to sea, the CoG is of the boats as she goes to sea in the condition of loading at which she departs.

    So if you have passengers, you find the location of the CoG of the boat plus passengers. If the boat also has cargo you find the CoG of the boat + passengers + cargo. The resulting CoG is the final and only CoG, for that condition. You then asses stability based upon THAT loading condition.

    You derive your Intact stability curved based upon your known condition of loading, using hydrostatics of the hull at that displacement. You then ad heeling moments and/or change of CoG to that GZ curve.

    That is it.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    It is called large angle stability.
     
  9. DCockey
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    DCockey Senior Member

    I'm not doing stability studies as such for a ship. The math stays the same though.

    Perhaps I'll post something about the study and methodology in the future.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Well, just replying to your comment:

    Thus can't debate the issue you're refering to without the study of stability.

    Otherwise it is a meaningless issue for a naval archiect. The effects of loading and heel/trim is called stability, it is nothing else.
     
  11. DCockey
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    DCockey Senior Member

    Mistake on part in a previous message #20 which I've changed. Thought I had corrected it earlier. :( Should have said "stability", not "stability standards". There were no standards for the boats I'm studying. The original boats were mostly designed by the builder, generally without plans and usually without half-models. No naval architects around.

    Stability was important to the fisherman who used the boats. Obviously a boat which wasn't sufficiently stable with would be unacceptable. The total displacement of the boats in use (boat weight plus payload weight) could vary be a factor of 3 or more so. There were also considerations such as hauling a lobster pot over the rail. The boats needed to be sufficently stable, but not too stable. And motion may have been a consideration. It's clear that the shape of these boats wass not always chosen to maximize stability.

    So part of the study is stability, but the loading conditions that are being used and presentation of the results will in some respects be different than what is commonly used for ships or yachts.
     
    Last edited: Aug 13, 2011
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Never is in any design. Many factors contribute to the final design, cost, materials, labour, time etc etc.

    For a boat of the size you’re on about, there now exists a “standard” of stability, the ISO 12217. However, you don’t actually need to go through that if you understand the requirements and the objectives.

    You address the stability of the boat only this gives you a baseline. Since if the boat only does not provide sufficient stability from the outset, it just gets worse, so why bother with THAT boat?. Then you address the stability with a full load. You also address the stability of the effect of hauling up a catch from the side (heeling moments). The heeling moments are also straight fwd and easy to calculate.

    You superimpose these heeling moments onto your GZ curve, to establish the stability at a worse case.

    The criteria, you can use ISO or arrive at your own. Such as if you haul up say 100kg of catch from the side, will the boat capsize, yes or no. Not rocket science. You can then establish what is the max catch you can haul up from the side of the boat; coupled with the max number of persons on board.

    Sorry, no, this is done for all boats. I do this every time I design a new boat. As any naval architect would, one looks at all conditions of loading and their effects.
     
  13. DCockey
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    DCockey Senior Member

    Perhaps I haven't been clear. I'm doing a study of historic small, open boats designs from the late 19th century and early 20th century. The goal is a better understanding of why various choices were made by the builders and users of the boats as to the shapes of the boats. Not a design project nor study of boats currently used for commercial fishing.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    You'll only know when you estbalish what the SOR of each boat was at that time. Since a design needs an SOR. And each SOR has many inputs/variables by definition and thus how to satisfy the SOR is different for each boat and in time too.
     

  15. Alik
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    Alik Senior Member

    I believe there were more historical reasons than stability issues in boat shapes. We did a study of Asian fishing boats that are built by axe and without any plans or stability calculations. They used set of frames templates (used for range of lengths) and some canons, say for N feet of extra length add one foot of beam and one foot of depth. Is the boat for squid or fish? Then need to add W tons of ballast. Those guys do not go that far as stability calculations or regulations; probably You can find any dependence or trend but only within one architectural type of boats.
     
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