flooded stability?

Discussion in 'Stability' started by kbowen, Aug 16, 2012.

  1. kbowen
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    kbowen Junior Member

    I am building a Mackinaw boat, but don't want to be so "traditional" that the thing will sink or turtle if swamped. What is the best way to get a curve of stability, including after downflooding has occurred? I have designed flotation chambers which seem to keep the gunwales above water when flooded (spreadsheet method / Simpson's rule, etc) but am unclear how to model what will happen when it's at different heel angles, including 90deg.
     
  2. jehardiman
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    jehardiman Senior Member

    Generaly, a downflooded open boat has to depend upon weight stability to remain upright. It does no good to add buoyancy to a swamped boat low, hence the high turtledecks on lifeboats and surfboats.

    As for the actual calculation, you can do it either by figuring out the "ever buoyant" hull properties, or by adding water as in a tank, but this becomes problematic once the gunwale goes under for a open boat.

    Both methods are handled the normal way. Heel the hull to the given angle, sink the hull until displacement = weight, caclulate the transverse CG and CB, add any movable item (i.e. crew and sail loads), subtract free surface (necessary using the "flooding water as a tank method") and then you will have net righting moment.
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    Dunno Mackinaw boats but generally you take into bouyancy calculation only the chambers (and other watertight compartments) and the volume of the materials (hull thickness etc)..
     
  4. kbowen
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    kbowen Junior Member

    I think I understand this in a very crude way, but how does one go about finding the waterline position when heeled &/or flooded? I have a planimeter, but the prospect of guessing a heeled waterline height, doing all the calc's, getting the waterline wrong, and doing it again (and again) is very daunting. At present I am postulating large buoyancy tanks fwd & aft, and with the extreme sheer of the Mackinaw, those will be powerful righting forces at 90 degrees. I am also imagining some buoyancy up under the side decks to help keep it level when swamped, but don't want to create a boat that is as stable as a catamaran is when turtled. I started learning TouchCad 3D until I discovered that it didn't do this job. I have heard that DelftShip doesn't play nice with older existing designs. This is complicated by the fact that Chapelle drew the waterlines parallel to the keel rabbet, so the resting waterline is not even at any design waterline. Please help me get pointed down a road that isn't a dead end.
     
  5. jehardiman
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    jehardiman Senior Member

    That is how it is done, there is no magic other than an experienced eye. Generaly you work it up for several descrete angles and waterlines (placed with an eye for the shape of the hull), draw up a set of curves for each and then interpolate off the curves to get an initial estimate then do the estimate and then correct. Not too much mindless number crunching if you have a planimeter and/or and intergrator. But this is also the reason they can charge money for programs that will do this for you...though sometimes the cure (i.e. getting the lines in the proper format) is worse than the disease.
     
  6. DCockey
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    DCockey Senior Member

    One approach would be to build a 3D model in Rhino including the buoyancy tanks. Use the Hydrostatics and other analysis commands in Rhino to calculate the buoyancy, CoB, etc and plug that information into a spreadsheet. The 3D model can be easily moved to find the equilibrium positions.
     
  7. kbowen
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    kbowen Junior Member


    Thanks for the note, and another recommendation pointing me toward Rhino. I'm resisting because everything else I do is on a Mac and I have heard that Rhino doesn't play well with VM Ware. I may have to buy a whole-nother machine to try it. ouch.
     

  8. DCockey
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    DCockey Senior Member

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