USCG Stability Test for Power Trimaran

Discussion in 'Stability' started by mike w. schultz, Feb 10, 2020.

  1. fallguy
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    fallguy Senior Member

    I am the idiot here. I am just trying to understand how you determine tippiness by the pictures.

    I have a canoe with amas; I get the basic idea, but if the amas are in the water; mine get wide fast so there is really nowhere to go.

    Is it a shape thing?

    sorry if I was offensive; honestly, just naive
     
  2. BlueBell
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    BlueBell Ahhhhh...

    Apology accepted but I'm out.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    The trouble as I see it is that your outriggers do not develop more beam with immersion, too straight walled, it needs to be put into a CAD program and the stability calcs run, as it stands looks to be way too much width to your cabin where your passengers moving around could cause a drama. The narrow-gutted nature of those outriggers is still reminding me of T-Rex's little "arms" :eek:
     
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  4. mike w. schultz
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    mike w. schultz Junior Member

    Observation's noted... Thanks for the input.
     
  5. bajansailor
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    bajansailor Marine Surveyor

    Just a gentle reminder re my questions in post #15 - I am intrigued by how your two outboard motors are arranged, I presume in a well forward of the swim platform? Is there room for them to be tilted up out of the water as well?
    If the underside of the swim platform is immersed while underway, then there could be a fair bit of turbulence (hence drag) in the well?
    And could you tell us what your estimated buoyancy or displacement is for one of the amas / outriggers at it's load draft (I think about 10")?
     
  6. mike w. schultz
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    mike w. schultz Junior Member

    IMG_0589.jpg
     
  7. mike w. schultz
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    mike w. schultz Junior Member

    Sorry this photo is horrible, poor lighting and confusing, in the fore ground you are seeing the transom, light in color photo left to right, then the opening is motor well, in the back ground you are seeing the "plug" for the swim platform, along with the wood utilized in making the plug,( I previously posted earlier photos of the swim platform mold still attached to transom) this will all be removed and the mold will be removed from the hull and plug. We anticipate drag on the swim platform internally and will be hinging the bottom step of swim platform to open and or hinge outward only to permit the buildup of water from props and drag to escape at water level therefore minimizing the amount of drag and water build up the motor well. Ama buoyancy calculations are still in the air as they are based on how deep said Ama will submerge into the water, which is based on how much the final weight loaded she will carry. We have several contingency plans in place in the event the ama's don't perform as desired, compromise is the name of the game when it comes to certain aspects of the build, will cross that bridge when we set her in the water. Can change configurations on Ama in less than a week with the clam shell molds we have constructed. Has rained 9 out of 11 days since we took her out of barn!!!!!!!!!!
     
  8. bajansailor
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    bajansailor Marine Surveyor

    Thanks for this Mike.
    Is it going to be a 'wet' well (ie where the area shown will be flooded) or will you have vertical side pieces to keep the water out?
    Re your buoyancy calculations, surely you must have a rough idea at least as to how much buoyancy you have at say 10" or 12" immersion of the amas?
     
  9. rxcomposite
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    rxcomposite Senior Member

    Must be difficult. I was going to ask you what is the displaced volume (at waterline) of the main hull. Hehe.
     
  10. rxcomposite
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    rxcomposite Senior Member

    I agree. Also, the amas should have a flare where it meets the crossbeam otherwise you get a "notch" or a discontinuity in the GZ curve. The buoyancy of the ama is usually progressive, like a V or hyperbolic plus the inside flare where it meets the crossbeam so that the righting arm is progressively resisting as the degree of heel increases.

    There is also the ratio of the ama buoyancy/main hull buoyancy. Small ama buoyancy tips over easily. Larger ama resist more but at the expense of comfort. It behaves like a cat. On the plus side, a small, thin, low buoyancy ama that you have is very good in powering as it presents low resistance. It is just finding the sweet spot.

    While I do not know about the USCG requirement for stability, the GZ curve can be plotted. You need a hull modeling program or make long calculations with the use of acad. Volume of ama is measured for every degree of heel.
     
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  11. mike w. schultz
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    mike w. schultz Junior Member

    Thanks RX for the input, first power Tri I saw crossed over to my Island in Palau from Manilla, bamboo trunks for out riggers!!
     
  12. rxcomposite
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    rxcomposite Senior Member

    The bamboo float has very little reserve buoyancy and relies only on very wide spacing for stability. Once the boat heels, the float sinks and very little buoyancy is provided by the crossbeam. Yours is the proper design.
     
  13. mike w. schultz
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    mike w. schultz Junior Member

    I my memory is correct they utilize a Toyota trk diesel engine for power!
     
  14. rxcomposite
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    rxcomposite Senior Member

    They marinize used truck diesel engines. It is cheap, abundant, and there are a lot of qualified mechanics around. The bancas was originally intended for fishing with the cargo stowed below which gave it a low center of gravity but when they started using it as a leisure craft/ferryboat, the cargo/passengers are loaded on the deck giving rise to high center of gravity. The government regulating body has to step in recently.
     

  15. bajansailor
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    bajansailor Marine Surveyor

    I just saw on Facebook that Carnival might be looking at stabilised monohulls at some stage in the future (if they don't go bust from Corona in the meantime). :)

    Carnival trimaran.jpg
     
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