Stability calc in excel.........

Discussion in 'Stability' started by athvas, Aug 21, 2013.

  1. Alik
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    Alik Senior Member

    Many of today's software are using completely different algorithms of calculating volume and its elements. Sections are not used. It works this way: the surface is split into elementary triangles and normal vectors are calculated, and them integrated for entire hull. This is the reason why some software is asking for normal + direction when using 3D models. Thus, it is much easier to write an application to 3D modelling software using internal analysis tools of the package, rather than trying to make Excel and sections exercise.

    Actually I did similar thing years ago by extracting graphical database from CAD package, and converting it into triangles using internal scripting tools, then uploading this file with vertexes into the code and doing the calculations. Now I don't do this any more because a) too busy and b) it is faster to buy such software...
     
  2. TANSL
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    TANSL Senior Member

    Alik, exactly agree with everything you say.
    If we have software that does things very well, why use methods based on spreadsheet, demanding a lot of work, which are not able to calculate with precision expected from today?. In my opinion, a designer must use his time thinking, designing, obtaining ingenious solutions, not add and subtract
    Jehardiman, computers ARE accurate, very accurate, for 1 and 2. But what should be accurate is the calculation method which does not depend on the computer. At last, after all, Excel can not be used without a computer, whether it is accurate or not.The treatment that the current software gives the behavior of water has nothing to do with how to integrate a number of discrete values​​. We must not confuse apples and oranges.
     
  3. jehardiman
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    jehardiman Senior Member

    Yes, apples and oranges. Computers are percise, not necessarly accurate. The belief that those are the same thing killed 18 on the MV Rocknes. As someone who has docked and inclined ships and vehicles for 30 years using D&O's, Bonjeans, spreadsheets, SHCP, GHS, and a couple of custom codes, I have found that there is no real difference in demonstrated accuracy because of all the things you can't control...such as tempature and rain the day before. The best I have ever seen was on the DSV's where we actually measured the displacement in cubic feet and weighed them submerged. We were still uncertain by about 5 lbs out of 73,000 lbs.
     
  4. Alik
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    Alik Senior Member

    Not just rain; whatever we use, the accuracy of measuring waterline is never higher than +/-1cm in the pond. Then, for small craft it gives quite high uncertainty in displacement, see graph for typical deep-V planing boats.

    Thus, usual conditions of classification societies for 2% difference in displacement for sister ships is not feasible here, as it is lower than accuracy of measurements!

    So how do we measure and verify? Do we need to calculate something with accuracy much higher than can be measured? The question!
     

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  5. TANSL
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    TANSL Senior Member

    Jehardiman, greatly respect all the wonders you have done for 30 years. But I'm not talking about that. What I say, and I repeat again here, is that the calculation procedures that can be used in a spreadsheet do not lead to results with the precision required today. Not talking about 30 years ago but today. And besides, they cost so much work that, for neither of these reasons, it is worth realizing them....
    Notes:
    - I do not know if I understand correctly, but an error of 5/73000 is to astonish anyone.
    - As I'm convinced that what you and I are discussing no longer matter to anyone, I will leave, at this time, this exchage of opinions with you.
    Cheers
     
  6. CDBarry
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    CDBarry Senior Member

    I'm working on a light weight limited general purpose stability program right now, (to use for worked examples in a textbook) but it is very tedious. The only reason I'm using Excel is so the process can be understood for learning. It is much more practical to use real stability software.
     
  7. TANSL
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    TANSL Senior Member

    CDBarry, If you use some Visual Basic macros in Excel you can get a much more accurate integrations. You may also create help files that greatly facilitate learning.
     
  8. Alvar1988
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    Alvar1988 New Member

    Ship Stability on exel

    Hi i saw this thread and became curious.

    We did Exel ship stability program in my School of Navigation. And i have it in my laptop. I could send you some pictures and snagit of the program if you want to?
    Fore free of course, Always a pleasure to help a fellow mariner. This program we did is for a small ship tough. About 50 M long.


    PM me for more info.
     
  9. TANSL
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    TANSL Senior Member

    Alvar1988, I would love to discuss your program and compare results from one of your ships. I would like to know exactly what we are talking about and get an opinion with real data.
    Allow me a couple of previous questions:
    • How do you calculate the wetted surface for each displacement?
    • How do you calculate the equilibrium waterplane of a given load condition?
     
  10. Heimfried
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    Heimfried Senior Member

  11. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I am pretty sure you can do this with excel, but you have to look over it's shoulder. Just like in the old days, when doing things with non-electronic spreadsheets, you can't do it blindly. When I finished high school in 1978 I was very lucky to get a summer job in a Naval Architects office and did the hydrostatic calculations for a 40 foot wooden fishing boat. Between station 1 and station 2 there was a discontinuity caused by the stern post. It didn't occur to me until I tried to smooth the curve of areas and realized there had to be a step change at the stern post, which was vertical. It wasn't exactly at a station or even at a half station. The Naval Architect originally said to just smooth curve the damn thing but when I drew in the step change where it belonged he liked it. It does throw everything off though. Simpson's rule is actually very accurate, but not if there are step changes. So with excel you would have to intervene where there are discontinuities like this, or just live with the approximations. In the case of the 40 foot fishing boat I think the stern post was a 10 or 12" wide piece of lumber, and maybe 3 or 4 feet high, so it was significant for a boat of that size. I don't remember doing anything about the keel and stem. I think the stem was angled in but on the sea trials I think I remember it was not so angled in as I thought.

    I didn't do the stability calculations from 0 to 90 degrees and so on. That would complicate things a lot if you had to iteratively determine a new waterplane and table of offsets by doing a nonlinear interpolation from the design plane table of offsets. Could be done. Better with Excel than by hand, and again the discontinuities will make things very interesting. Obviously 3 D modelling is the way to go today, but it would be a fun exercise in Excel. We had no excel or lotus 123 in 1978. We did have calculators though. I dodged the slide rule bullet by a year or two. I think all the calculations were based on initial stability of the design water plane, and then there was some empirical formulas and on the sea trials we ran from side to side and measured the rolling period. I remember doing the stability calculations for different load conditions, crab traps on deck, hold half full of brine, empty, full. Wonderful experience.

    In grade 12 I had wanted to become a Naval Architect, but alas I ended up as a mechanical engineer. I will never forget that summer though, or my high school sweet heart. Oh what fun we had at 16, with Simpson's Rule in the summer of '78. :)

    p.s. Yesterday my 15 year old daughter asked me how many feet there are in a yard. Really? Really? I held out my hands and said, "This is a yard. This is a foot. You tell me." She got frustrated. "I don't know. 3 or 4 or 5. Just tell me." It came up because I got her a Math book from Indigo. They have to text books in school anymore. Crazy. Anyway this is a great book, originally published in 1943 which was enough to sell me. Now that I know it has some old English units I love it even more. How many feet in a yard? Really? I will never make fun of Americans again. At least they still know what a yard is.

    p.p.s. Should I ask her what a stern post is?
     
  12. Heimfried
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    Heimfried Senior Member

    Starting 1965 I used for years slide rules and (mostly) logarithmic tables because I learned my profession in a chemistry lab. Later, studying chemistry and physics, I could afford an electronic calculator with scientific functions.

    It was really hard to do voluminous numerical calculations.

    But Excel is easy. Once put the right formula in a cell (in this case in thousands of cells), it will deliver the results as often as you like. That is numerical power.

    But: as you said, it is necessary to check, if the formulas are leading to the correct results.
     

  13. CDBarry
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    CDBarry Senior Member

    I have messed with it, but it's more of a stunt or a puzzle than a practical process.

    Doing hydrostats for relatively simple hull forms might be a good teaching exercise,though.
     
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