what helps to downsize the stabilizer requirements

Discussion in 'Stability' started by expedition, Jan 6, 2008.

  1. expedition
    Joined: Jun 2007
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    expedition Thorwald Westmaas

    Our trawler conversion project may need stabilizers as it really likes to roll (beyond what is comfortable). The GM value is currently 1.05.

    There will be weight taken from the main deck and there are opportunities to remove weight from or near the bottom of the vessel.

    This will make the ship more stable but also make it roll faster and less comfortable.

    But, assuming we employ stabilizers, will a more stable ship help the stabilizers (i.e. allow us to get a smaller size stabilizer or even better, get 2 instead of 4) or not?

    Any feedback is greatly appreciated.

    Thorwald Westmaas
     
  2. Guillermo
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    Guillermo Ingeniero Naval

    In what load condition?
    What is the beam?
     
  3. expedition
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    expedition Thorwald Westmaas

    Hi Guillermo,

    The beam is 9 m. The load condition? Well. The ship weights about 700+ tons so the only thing affecting the load is the amount of fuel we carry on board (up to 95 tons in the central fuel tanks at the bottom). we have space for more fuel in the back but won't use it. We can also carry about 20 tons of ballast water in the forepeak and about 30 tons of fresh water also forward.

    Realistically, we'll rarely take more than 50-60 tons of fuel (we recently crossed the Atlantic starting with about 60 tons). The forepeak will always be full and I think we'll have to add some extra weight there to keep that nose a little down and as far as fresh water is concerned, for the same reason we'll keep that pretty much filled up.

    With that, we had a foreward draft of 3.4 m. and 4.6 after during our last crossing.

    HOpe this helps.

    Thorwald
     
  4. Guillermo
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    Guillermo Ingeniero Naval

    If an inclining test and stability calculations have been not performed, how do you know GM?

    With a GM of 1,05 m and a beam of 9 m, your rolling period should be around 7,8 seconds, so not that bad.

    You should check with designers. They should be the ones answering you.
     
  5. expedition
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    expedition Thorwald Westmaas

    That's the current GM. Now that we will start removing weight and add new one, obviously things will change. A new inclining test will have to be done obviously.

    Remember, my question was: IF we add/remove weight here/there, what will happen with the sizing of the stabilizers ....

    Thorwald
     
  6. Guillermo
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    Guillermo Ingeniero Naval

    Thor,
    The equation of to find out the Fin area in the 'wave slope capacity' method, depends on displacement and GM as:

    A = Disp*GM*sin(theta-wsc)/(density*CL*V^2*arm)

    So in my opinion lowering displacement and GM lowers the required area.

    Cheers.
     

  7. expedition
    Joined: Jun 2007
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    expedition Thorwald Westmaas

    Thanks! Well we're going to take of some items the next few weeks and then do a new inclining experiment. That will give us more info.

    Saludos,

    Thorwald
     
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