Ghs Spgr

Discussion in 'Hydrodynamics and Aerodynamics' started by Fnv90, Apr 2, 2014.

  1. Fnv90
    Joined: Aug 2013
    Posts: 11
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    Location: VA beach

    Fnv90 Junior Member

    I'm using GHS to do some CC calcs and for the life of me i can not figure out how to change SPGR to that of freshwater! I have used all the command such as normal, USCG and standard but they all give me a value for SPGR of 1.025-1.027. I want SPGR to be 1. I have read all of the literature on GHSport but still no help. I have imported my model from RHINO, is there something i need to change in Rhino??
  2. Olav
    Joined: Dec 2003
    Posts: 296
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    Location: Filia pulchra Lubecæ

    Olav arch. nav.

    When you use the AttachGHSData command in Rhino (not just the export function), there will be the following window where you can select liquid densities in the options submenu:


    Maybe this helps...

    Attached Files:

  3. DavidJ
    Joined: Jun 2004
    Posts: 222
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    Location: Canada

    DavidJ Senior Member

    Sorry Olav that is not correct. What you are setting in your example is the densities within the tank. You can set the density for your hull but that only means what your hull holds, not what your hull is floating in. Remember in Rhino you are designing your vessel, you aren't operating it. By contrast GHS is simulating an operating condition. Yes, most vessels are designed for operation in a certain type of conditions and those conditions can include the water density. However, the water density is not a function of the vessel. If you take your little fishing boat out of your local lake and drive it to the ocean the properties of the hull do not change. It's the same boat. It's the operating condition that has changed.

    The density of the water you are operating in is changed in your GHS run file. If you want to see how the stability of vessel changes when it is operating in fresh water versus salt water you could make separate run files. (You could also make one long run file or use macros, but usually I like to keep it simple and do one thing at a time).

    In your run file you just type water followed by a number for the specific gravity you want. I usually put it right at the beginning.

    Here's a sample run file I'd run to get my cross curves:


    read boatname.GF

    water 1.0

    draft "USK" 0@20f,0@20a

    vcg = 2.5
    ghs displacement= 10,12,...,20

    vcg 0
    angles 5,10,15,20,25,30,40,50
    cc displacement= 10,12,...,20

    report off


    Normally you can output your GHS file as a text file simply by changing the filename extension to .txt but that doesn't work when you are generating cross curves.

    draft "USK" 0@20f,0@20a <- adding an underside of keel draft makes the program show you trim in a distance instead of as an angle. In this example the keel is dead level and the fore and aft distances are the same. Can you tell I took it from a double ended ferry?

    water 1.0 <- is what we are talking about here. If I entered 1.025 I'd have standard density salt water, which I believe is also what you get if you leave it blank. If you are using draft/freeboard numbers from a real life inclining experiment or lightship survey you need to remember to bring along a hydrometer to measure water density and you'd use that number here.

    ghs displacement= 10,12,...,20 <- this is the range of displacements. In this example this is starting at 10 and going up by 2 tons until it his 20. You could go up by drafts if you preferred and you don't need to use nice even numbers.

    page is like putting a page break in Microsoft Word.

    report off means that it would automatically attempt to print the file.

    qu is short for quit
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