Stability question

Discussion in 'Boat Design' started by richardf, Jul 21, 2013.

  1. richardf
    Joined: May 2013
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    Location: Michigan

    richardf Junior Member

    For a 24 foot, flat bottom, 7 foot beam, flat-iron displacement skiff, used for general recreational cruising (not a work boat) what is an acceptable range of righting moment (ton ft per degree) and moment to change trim (ton ft per inch). I am fooling with Hullform D9 software which computes these moments but I don't have a feel for acceptable values. I can intuitively think of a 200 pound person standing on the sheer clamp at maximum beam and only tipping the boat 10 degrees. Assuming no flare in the sides that would yield a righting moment of (.1 ton x 3.5 feet)/10 = .035 ton ft per degree. Am I figuring this correctly (or at least heading the right direction)?
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Are you sure Hullform D9 is computing (using BALANCE HULL) what you think it is? Generaly you need a KG (VCG in Hullform lingo) for a full RM calculation, not just a buoyancy shift based on small angle theory (and 10 degrees is not a small angle for a properly designed flattie, there will be significant pitch linking). If you have a full VCG worked up for the vessel (including mobile weight like people) then if you want to see if you have sufficient RM, fix the HEEL of the boat to your desired limit with the weight on the rail and see if Hullform still gives you a postive moment. Also check the pitch moment, if it is large, fix the PITCH and SINKAGE so you will get a true RM value.

    If it is for your own use, there really isn't a fixed criteria. Generally you want an easy motion (which is only teniously connected to righting moment), and a reasonable angle of vanishing stability/downflooding (which is why most tradtional sailing sharpie/flat irons have flare, lots of ballast low, and/or were decked over with small hatches).

    Otherwise, there are some general criteria if you ever want to sell the boat. Try starting here: http://www.uscg.mil/pvs/SPV.asp or ABYC standard H-5.
     
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