michlet/rocker

Discussion in 'Software' started by ediestel, Jun 11, 2008.

  1. Guest625101138

    Guest625101138 Previous Member

    V6 - note V6, bow is just a cone made from aluminium - shaped like I have for the orange outriggers fitted to V7, the yellow Pahoa. Thin vertical volume does not make perceptible difference. Having it submerged eliminates the bow wave but it is a small saving and only applicable to very calm conditions. Even a ripple and it breaks the surface.

    You should look it my web site it helps understand my boat designations and the development. Godzilla was the greatest find but I was going in the right direction before I started applying it.

    Rick W.
     
  2. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

  3. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

    Somewhat uncertain

    Rick, I am currently trying to find sufficient information reagrding open ocean behavior of possible hulls. It is quite difficult to make a good judgement.

    I found that the added drag of the chines cost almost 4/10th of a knot in speed at 20 N.


    The shown hull in Gaussian coloring is my best so far with 3.91 N at 5 m/s.
    I started with Godzilla, length 6.9 m and cut .2 m off the stern.
     

    Attached Files:

  4. Guest625101138

    Guest625101138 Previous Member

    My view is that you go for the lowest drag underwater section for calm water. Create a deck design that you believe will best cope with the conditions.

    Build a boat and see how it performs in your conditions.

    You can go on for a long time trying to postulate what will be best. I can tell you that no theory copes well with waves and chop so the easiest way is to experience it. There will be cues from other designs so if you can actually sample their merits then they might give pointers.

    Rick
     
  5. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

    Empiric data

    I was out in the open ocean today in about 4 - 6 ft swell; while paddling I thought of boat design ...


    There is no way to fully predict a hulls behavior in these constantly changing conditions.

    Many theoretical considerations seem to have merit. But which one will perform most of the time in most of the conditions at average or better ?

    Example bow design: do you want it very buoyant to keep it out of the water, or do you want it minimally buoyant to keep the boat wet and reduce the amplitudes ? Both thoughts are valid in my opinion.

    I agree that there are many variable that can well be assessed by trial and error.

    I will take your advise and engage a more pragmatic approach by building a hull and changing it fit.

    I am glad that your design has made it to North America with the purpose of breaking the 24 hour distance record; I stumbled on the website of gregsadventures.com.

    Could it be that the density of the water accounts to a degree for the lower average speeds that Greg has achieved so far ? Is it favoring him that he is sitting higher ?

    Thank you very much for your gracious help. I think that I will be building with female molds and stripped foam.
    The LR 2 class A catamaran of John Lindale is an inspiring project, as is yours.

    Aloha

    Eckhart
     
  6. Guest625101138

    Guest625101138 Previous Member

    Eckhart
    Greg has just about got there. He has now used all my developments but one and he is working on that now - he will remove the seals from the gearbox. I have a box that is well run-in and uses light oil. His is new and costs about 6W in losses. Mine is under 3W.

    To get the design speed of 12kph at 150W he needs dead calm conditions. The boat is very close to that now.

    I started a lighter version of my hull this weekend using the old boat as a plug. The old boat still has the original foam plug inside so is about 6kg too heavy.

    If you make sound observations and take an analytical approach you should get a good result.

    Greg has a power meter on his crank that gives highly repeatable input power this helps in quickly analysing performance.

    I have a Garmin GPS that includes heart rate monitor so I can guage my working level from week-to-week. I plot my performance and make notes on weather. You would be able to use the same method to assess performance. If you are racing with others then you should also have a good basis for comparing benefit of changes.

    Will look forward to production of your hull. Sometimes it is quicker to actually get your hands dirty than researching a topic.

    Rick W.
     
  7. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

    Speed

    If it comes to speed I would consider a foil under the outrigger.


    I have built a variety of foils - attached to the rear iaku, about 1 1/2 ft deep.
    They work, the boat starts to glide.
    Pitch and roll are reduced, the glide per stroke is longer, stability is increased and a huli - that i s capsize -more or less excluded.

    My versions did not do well for open ocean: you cannot fly the ama anymore, and there are situations where the foil works as a break.

    For flat water and a record attempt - it would work. It would allow you to set the regular outriggers higher, above the water at all times. The balance gets real easy while moving, almost like a bike.
    One side with a foil should be enough.


    Eckhart

    There is one thing that works for the open ocean actually: it is a foiled rudder.
    Unfortunately I lost my last version after I forgot to tighten the screws. While attached it controlled pitch and roll, - less than with the outrigger-foil -, and prolonged the glide in the swell.
    My interpretation was that the boat being closer to ideal trim and having less pitching amplitude made for the facilitated surfing. That day we had three ft wind chop.
    It was a simple aluminum bar, less than 5 inch span and 0.5 inch chord. Not finished, just for initial tests.
     
  8. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

    Can you improve this basic design

    Rick,

    within my given parameters D, L, B = 0.28 this is my best design with 4.01 at 5 m/s.

    Can this be improved with form parameters ? I tried many, many different options so far, but there may be a culprit.

    How can I judge the stability ?


    Eckhart
     

    Attached Files:

  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The hull is just a Wigley hull before you run the optimisation. Do we have to run the optimisation ourselves? Or did you have a set of results after your optimisations you want us to look at?

    Also, the number of waterlines and stations in your input file is far too low. Use more, e.g. 33 x 33.

    Have fun,
    Leo.
     
  10. Guest625101138

    Guest625101138 Previous Member

    Echkart
    This is better than the Godzilla result. I doubt that I can improve on it.

    Leo makes the point that adding more stations and waterlines might improve the result so this would be worth a try but I doubt that you will get better.

    Leo - The basis of the hull was one produced by Godzilla with 17 stations and 9 waterlines. There has been a little manual fiddling to fair it.

    When I get some time I will try Godzilla with greater resolution to see if does anything better.

    Rick W.
     
  11. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

    Godzilla

    Yes, this is the input file only.

    I use Godzilla this way: I run a specific 'in' file, then look at the output/results.

    If the results are good, I save the input file directly with a certain label that allows me to see what specific constraint I have in the input file.

    During this communication it will allow everybody just to download the file and have it ready to go for a run in Godzilla with equal or new constraints.

    It does not show the entire results; it makes it easier to reproduce/modify the run.

    Thank you for pointing out that more stations should be used.

    The only way I got better results was by allowing a narrower beam, which is not suitable for practical purposes.




    The question regarding the parameters is based on one of Leo's articles, where the optimal a1= 1.0 for the waterline shape for similar hulls, that is parabolic if I remember correctly.

    I wondered how the input parameter f0 relates to a1 for the waterline shape in the article, and if it should get as close to 1 as possible for the forebody of the ship. I do not fully understand the practical interpretation of a given waterline shape.

    I have run quite a few constraint combinations; most results have a similar power requirement of about P = 0.40x for similar hulls.



    Regarding stability: is there a pragmatic way in Godzilla to look at stability by setting parameters ?



    Thank you for your interest.
     
    Last edited: Jun 25, 2008
  12. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

    Metacentric height

    It turns out that the Godzilla run with 33 stations produces a better result for the power requirements at 5 m/s:

    0.403 vs 0.396 with Rt = 7.92504e-002 about 1.76 % better



    General information on metacentric height can be found here:

    http://en.wikipedia.org/wiki/Metacentric_height


    What are longitudinal/transversal target value ranges for the hull with following constraints:
    D= 0.11 t, L 6.7m, B 0.28 m, T =0.115 m, S = 2 sqm, Cb about 0.5, Cp about 0.64


    Center of gravity of the paddler likely ~ 12.5 "; 6'5" height.


    PS: it has been said many times before, but it is still worth to mention: thank you to Leo Lazauskas for providing Godzilla/Michlet to all of us; also thank you to Rick Willoughby for promoting and helping with the practical applications.
     
  13. Guest625101138

    Guest625101138 Previous Member

    Eckhart
    You can control stability by limitting the lower value of GMT for the hull. This value is the height of the metacentric height above the CofG (the height of CofG defaults to waterline unless you change the hull loading parameter). To have a stable hull without outrigers you would need a GMT of around 0.35m.

    Delftship give the stability figure as KMT. This is the metacentric height above the keel. Michlet and Delftship will give the same height allowing for the difference in reference.

    By the way I limit the number of stations and waterlines to values used to make it easier to work with. In practical terms it will be difficult to assess 1% difference in power but it is lower.

    Rick W.
     
  14. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

    Stability

    Rick,

    thank you.

    I understand the reduction of sections and waterlines.

    I also understand the concept of metacentric height.

    There are two situations of seakeeping that I think may be important:


    1. On the crest of a wave in following sea, heave
    2. surfing, mostly done 'quatering' a wave to starboard


    1. While in the open ocean, one of the least stable moments occurs when the paddler/OC1 is to slow to catch a wave, the boat is lifted up unto the crest of the wave and then sinks back. I believe that this motion with reduced gravitational force results in reduced stability.

    2. surfing is mostly done to the starboard side, many resaons for that including race courses, position of the outrigger, better control as the outrigger is not in the way etc.
    Stability in this scenario - on a wave face, 'surfing' with trim minus 10 -15 degree, 4 - 8 ft waves from 3/4 aft, at the beginning of the acceleration - is very important, as regarding roll and pitch.


    The answer to soften the impact to 1) may be to have the LCB aft of the LCG
    regarding 2) I would guess that a 'round' hullshape may be good as the wave impacts the hull from the back right.


    How would or should the metacentric height be adapted for these two crucial scenarios ?

    If it is too high then the roll stability will be irritating while quatering the wave,
    if it is too low the stability during the heave may be a problem.



    Is there a 'rule of thumb' that can be applied here ?
    'Minimum' CG to CMT distance ?


    The values for rowing shells are 0.1 m, your value for a canoe is about 0.34 m. My current design seems about 0.13 m, thus well below CofG.

    Where in this range would you see an open ocean canoe with outrigger with described use characteristics ? I have not found any publications that address the outrigger in this constellation yet.

    Eckhart
     

  15. ediestel
    Joined: Jun 2008
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    ediestel Junior Member

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