creating hull cross-sections from bonjean curves?

Discussion in 'Boat Design' started by hmcs, Feb 26, 2009.

  1. hmcs
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    hmcs Junior Member

    Hey Guys,..

    working on a RC model of a modern navy frigate,.. I have drawings from the shipyard with one of the pages devoted to bonjean curves for the hull,..attached is a little screen capture of some of the measurements for one of the "stations"

    since no one seems to have cross sections of this particular hull for me to build up the hull from scratch materials,.. I am trying to generate cross section templates so my hull can be accurate,.. can any one help me figure out where each "anchor point" would be located on each waterline level? based on these supplied measurements,..

    this drawing only shows the transom
     

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  2. hmcs
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    hmcs Junior Member

    is this a better way of asking?

    working on a RC model of a modern navy frigate,.. I have drawings from the shipyard with one of the pages devoted to bonjean curves for the hull,...

    it has all the stations for the entire hull ,... several waterline measurements and area measurements (squared) per waterline for each of the 20 stations

    how the heck can I generate "cross-sections" so I can accurately build the hull in a scale model size?
     
  3. jehardiman
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    jehardiman Senior Member

    Yes, there is a way to get an approximate section from a Bonjean.

    Start with a section grid for the hull.

    The CL vertical offset from baseline will be at the point where the Bonjean is zero, in your example 4.63 m

    Now we go to the first WL above that, 5.0 m. The area of the section below that WL would be given by the Bonjean. We can now imagine that area as the area of a triangle. We know the distance between the profile at CL and the WL so we should be able to work out the base of the triangle 3.910 *2/.37 = 21.135 span or a HB of 10.56 m....However.....

    For you case, I assume from the shape of the Bonjean, profile, and the station number that this is station is actually in the rake of a square transom. In this case, the area is not a triangle but a trapezoid where we don't have any tumblehome data. So the 3.910 m^2 with a .37 m height gives a span of 10.56 m average for a HB of 5.28m.

    Continuing in the normal fashion...

    Then we take the area of next WL (6m) and subtract the area below the 5m WL from that 15.186-3.910 = 11.276. Now we can only assume the area between WL 5 and WL 6 is a trapezoid which has a formula of H*(span1 + span2)/2. We know that H is 1.0m and we have span 1 from WL 5.0 so we can calculate span 2 (11.276*2/1)-10.56 = 11.992 m for a HB of 5.996m (it is obvious from the snippet of plan given that I actually expect these three HB's ( at4.63, 5.0, & 6.0) to be almost linear so we could work that out, but that really doesn't apply to the overall solution to the problem as it is a quirk of this hull form and station).

    We continue in this fashion, using the area differences between WLs to develop trapezoids to find the HB's for the WLs. Then once we have a good set of HBs, we draw in station sections and then work up a set of lines and Bonjeans for as a check. Cycle and repeat until you are happy with the results.

    BTW are you a retired Senior Chief Corpsman? And which DDG?
     
    Last edited: Feb 28, 2009
  4. hmcs
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    hmcs Junior Member

    jehardiman,... you are a genius!!!

    you are right on with the transom shape! the ship is actually a halifax class canadian frigate and no I am not in the navy, just lucky enough to have good friends that are and that have supplied me with great drawings,..

    I will have to read,.. read,.. and read your response again and again to understand it,.. I am 1/2 way there in understanding and may need to ask you remedial questions along the way LOL

    I attached some pics that I found on the internet to further show your brilliance,..

    thank you for your answer you have opened up a whole new world to me!!!
     

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  5. jehardiman
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    jehardiman Senior Member

    No, I am a Naval Architect...;)

    Ok, HMCS, as in Her Majesty's Canadian Ship...

    There are a lot of things you can do to better the approximation from the Bonjeans if you know something about the hull form (such as assuming an elliptic section at the bottom instead of a triangle or fitting to a known maximum beam). A fair number of the points may be off so you will have to let your knowledge of the hull shape guide you. For a RC model though, if it is fair and of the right proportions, no one is going get down with a micrometer and measure it unless you are going for awards in the "scale" catagory.
     
    Last edited: Feb 28, 2009
  6. hmcs
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    hmcs Junior Member

    LOL!
    true,.. "close enough for grenades" is what I am going for,... is there a computer program that I can use that will generate the curves? by imputing the calculations?,..
    you may cringe,.. BUT
    I have been using a graphic design software called adobe illustrator and I have set up a a grid such as you said with my unit increments in metric (because the plans are in metric) and then my hope is to get the points close enough on the grid and then basically connect the dots,.. but with smooth rounded lines,.. I just have to master the math,..LOL
    - I just get sick of hearing "just buy a fibreglass hull,.." ,.. I just want to scratch build the whole thing,..
    thank you again for your time it is a privilege to get your advice!
    My Best Regards,
    Mark
    (I am originally from Comox BC with friends from CFB Esquimalt) I am sure that if you have travelled with the navy you may have been there!
     
  7. hmcs
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    hmcs Junior Member

    jehardiman,.. check out this lovely present that just got emailed to me!

    thank you again for our help just thought I would share my good fortune with you!
    My Best Regards
    Mark
     

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  8. jehardiman
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    jehardiman Senior Member

    Yes, a good body plan cannot be beat...

    Just one thing about that hull form though. I would bet the real ship has active stablizers with that deadrise. In the model you will need to get the weight very low so she doesnt roll out in a turn at speed.
     
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