# Large Stability Calculation help!!

Discussion in 'Stability' started by Crodriguez, Mar 25, 2012.

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### CrodriguezJunior Member

Hi
I´m a naval architecture student

My doubt it´s about large stability
whe the ship is inclined 20, 40, 60 degrees
the vertical center of buoyancy (vcb)changes??
Do I have to calculated at those degrees??

2. Joined: Oct 2008
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HI
Welcome to the forum.

The VCG, or KG does not change, that remains constant, unless you have shifting cargo.
But the VCB does change as does its location athwartships. This alters the waterplane area, at that draft/trim, which affects the location of KM. See the two diagrams below for ref:

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### CrodriguezJunior Member

thanks for the help!!
so if I wanna calculate large stability
I´ve to calculate km and VCB
at those degrees??
ok it´s a long job
but interesting

4. Joined: Oct 2008
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Yes, for large angle stability.

This is where a software program is helpful. But since you're a student, you must be able to do this by hand calculations. Nominally use increments of 10 degrees for each. You'll end up with what is called cross curves of stability. Very time consuming to do...but essential.

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### Pascal WarinJunior Member

Additionaly for large angles change of trim may be important so you need to calculate with free trim method.

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### TANSLSenior Member

Do not complicate the live of our student. When using the computer, with a modern software, one can calculate the cross curves of stability considering trimming that takes the ship on each heel. When work is done by hand, which is what our student should do now, to carry out calculations with the trim could cost him some years.

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### Pascal WarinJunior Member

OK but it is important that he knows that it exists.

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### TANSLSenior Member

There are so many things (about how ships float) that exist and that we do not know!!!!. And despite of that, some of us continue desiging boats.

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### Tim BSenior Member

>> to carry out calculations with the trim could cost him some years

Rubbish. It's a volume integration and a 2nd order ODE. You don't need to handle obscure shapes for a proof of concept, so that makes the volume integral easy. This is where you need to understand what's going on, and how you use the computer to help. I.E. you understand the algorithm behind the interface.

Come on guys, let's get the simple stuff nailed first!

Tim B.

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### TANSLSenior Member

I never said that calculations were complicated or difficult to understand concepts. All I meant was that, because calculate the equilibrium position (the trim) to each heel is laborious, calculations made ​​by hand, can last a long time. Otherwise, I agree with you, it's stupid to pretend that it is difficult what is simple and common sense. The term "rubbish" was not necessary to use it, does not contribute anything to this thread. Regards

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### CrodriguezJunior Member

I finished my calculations, but I´ve a question
what stability criteria do you consider the best'???

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There is no "best" as such.

You look at the technical Specification of the vessel, what is she being designed to satisfy?...that tells you what stability criteria is appropriate. For example, you would not use the HSC code on a container ship!

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### zeozeovtJunior Member

it depend on the loading conditions and the shape of hull, as u said that is really a long job to do. . .

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### klaasJunior Member

The criteria depends on the type of ship.

For commercial ships the 2008 Intact stability code will give you a good start.

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### TANSLSenior Member

I am attaching an Excel spreadsheet with a summary of the stability criteria that, depending on the type of boat, requires the Spanish Administration and another sheet with IMO criteria. As it is in Spanish, you will understand it perfectly