water ballast hull form

Discussion in 'Stability' started by philSweet, Mar 26, 2012.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    What are the inputs, perimeter for one, and also flood angle? Otherwise the solution would be open-ended I think . . .

    The shapes seem very like traditional shapes where material is heavily constrained by availability like birchbark canoes for example. Whish is to be expected.
     
  2. philSweet
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    philSweet Senior Member

    The heel angle is input. I haven't set that up for dither yet. It would find the absolute max RM shape if that floated as well. Keep in mind this doesn't do an energy calculation, it just optimizes the RM at one particular heel angle and one load condition.


    The perimeter sheer to sheer is calculated and the RM is divided by perimeter cubed (and can add in other constrains as well. I did a run where the perimeter included the deck and got a lot more tumblehome as you would expect). The reference dimension is the hull depth which stays constant at twenty because I have twentyone offsets and some of the calcs are easier if the vertical slices are all equal to one.
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    I'm not getting a mental handle on this: I thought the perimeter was an input not calculated. Are the input data normalized to the hull depth?
     
  4. philSweet
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    philSweet Senior Member

    No, the only input is the heel angle to be considered. The hull depth stays always at twenty. The offsets at each waterline get pushed around until the best shape is found. The charted perimeters of the the 5 degree case and the 17 degree case are obviously very different from each other; but the quantity RM/perimeter^3 is maximised for each case.
    I can scale the results to any value that I have calculated. Theres nothing to stop me from plotting a constant perimeter = 10 hull, it just means a scaling factor gets applied.
    I haven't done that yet. Inputting payloads and some sail moment functions is where I want to go with this.
     
  5. philSweet
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    philSweet Senior Member

    I thought I had posted this two days ago:rolleyes:

    If anyone has noticed an extraneous post in another thread, let me know and I'll remove it:D

    Anyhoo--

    I went ahead and scaled the chart to show any desired perimeter sheer to sheer. I realized I had to do this anyway, since if I want to start adding other weights to the boat, they have to get scaled into the analysis.

    The case shown below is what I get as best RM for a 20 unit perimeter.

    [​IMG]
     

    Attached Files:


  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    I don't know if Phil or anyone else is still monitoring this thread, but I recently got back to it after losing it. Previously I missed the significance of several things like the statement in the first post "ballast is fixed in bottom and has density of one". Of course, the biggest caveat for this work is the constant section due to the 2D nature but it is still an interesting exploratory tool. In its current form it may be most relevant to large ship design, but if further study indicates usefulness for optimizing boat design it should not be too difficult to expand it to simulate a more-or-less normally shaped hull.

    The result in the previous post looks a bit like canoe midsections I've seen - even to the tumblehome, which perhaps dates from the code correction noted in post #13. Tumblehome makes for paddling convenience but it is interesting to note that it results in more righting moment in a design where circumference is constrained - which also constrains material and weight. I would note that canoe designs are almost always weight conscious; mine certainly are. Tumblehome is common in traditional canoe and sailing warship designs and this aspect of it has not, as far as I know, been pointed out before.

    In the last figure the unheeled draft seems rather deep for a typical canoe. Given that the ballast is determined by the heel angle it would revealing to do a case study with some more typical (i.e., greater) flood angles to produce answers representing more lightly loaded hulls. Phil, if you are reading this, I would like to see what hull shapes are produced and how they compare with typical designs of various boat types, not merely canoes. It could be revealing.
     
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