Cat draft 2

Discussion in 'Boat Design' started by Saqa, Dec 29, 2014.

  1. Saqa
    Joined: Oct 2013
    Posts: 489
    Likes: 9, Points: 18, Legacy Rep: 75
    Location: Hervey Bay

    Saqa Senior Member

    I had a bit of a WTF moment when drawing a boat which I listed here
    http://www.boatdesign.net/forums/boat-design/cat-draft-52170.html
    Thanks to everyone who tried to help me understand the issue, I really appreciate it

    Using Carlsons Hull Designer program

    To remove any chance of discrepancy I drew a new single hull
    Waterline length 300"
    Height 40"
    Waterline beam 20"

    This is a screen shot of the wireframe of the single hull. I have kept the bulkhead locations the same on the paired hull too to get the same fairing on the lines
    [​IMG]single by jonny.toobad, on Flickr

    This is a side view of the single hull at 1050lb dispacement sitting with a draft of 6.7"
    [​IMG]single 1000lb by jonny.toobad, on Flickr

    This the same at 2052lb displacement sitting with a draft of 13.1"
    [​IMG]single 2000lb by jonny.toobad, on Flickr

    The results of the single hull seem reasonable and expected. Next I drew identical hull but two of them paired in a cat formation. As per the original thread I would have expected half the draft at each displacement but the results are WTF material

    This is a screen shot of the wireframe of the paired hulls
    [​IMG]cat by jonny.toobad, on Flickr

    This is side view of the paired hulls at 1048lb sitting at 25.2" draft
    [​IMG]Side 1000lb by jonny.toobad, on Flickr

    This is the same at 2056lb sitting at 26.5"
    [​IMG]Side 2000lb by jonny.toobad, on Flickr

    Is this due to the law mentioned by some of the guys in the original thread? Is this something about cat hull form behavior that I dont understand? Could it be a bug in the program?

    I have also attached a zip file that contains all the hull data in the native .hul format as well as .dxf format

    Thanks, will really appreciate any help trouble shooting this. The hull is not the final shape that I want but just something simple with straightforward dimensions that might help keeping errors out. The more complex model I did originally is showing the same issue
     

    Attached Files:

  2. sharpii2
    Joined: May 2004
    Posts: 2,025
    Likes: 195, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Something has clearly gone amiss here.

    Being that the two hulls are vertical sided, the Horizontal Center of Buoyancy (HCB) and the Horizontal Center of Lateral Area (HCLA) should not change, no matter how deep the hulls are immersed.
     
  3. messabout
    Joined: Jan 2006
    Posts: 3,131
    Likes: 296, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    When you get the displacement stuff sorted out you will want to put some rocker in those hulls. If you don't, it will be a bear to turn the boat. It will also react nervously when quartering a wave or wake.

    If the program shows 1040 pounds AT 6.7 inches draft, then the computer thinks that your prismatic coefficient is 0.716. That is in the ball park for a hull like that when it has no rocker. Each inch of immersion (draft) will displace 155 pounds. Of course that is true of both hulls if they are identical. That means that each inch difference would amount to plus or minus 310 pounds. As soon as you put the obligatory rocker in, the whole deal is off. For a displacement of 1040 you will have a draft of something like 9 inches. That is because the prismatic will have changed on account of the rocker.

    You will still have your 310 pounds per inch of the rockered hulls but only after the upswept ends of the bottom sunk low enough to have touched the water surface. Before that point the pounds per inch will vary and increase with every additional inch of immersion.
     
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