# Stability of a small ship

Discussion in 'Stability' started by issac82, Jan 31, 2009.

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### issac82Junior Member

Hi all guys!

I am a student.I want to ask that I have been given the task of finding out the stability of a small ship designed several years before.I am not having good idea of stabilty calculations.Can somebody guide me that what else I need to do to calculate the stability and by which method or software.I shall be v thankful.I am going through several books to fid out the right way.Please help me out of this problem.

Thanx to all guys.

Regards

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### GuillermoIngeniero Naval

Let me try to understand your needings:
1.- Have you been asked to perform a stability test to an existing vessel and then perform all load & stability calculations and find out the compliance with some specific requirements?
2.- Or then, have you just been asked to calculate the hydrostatics, KN curves, etc, for an specific design?
3.- Or.....what?

Cheers.

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### issac82Junior Member

Hi,

I have been asked to do the stability calculations for an existing vessel.I want to know what all I require to do.

thanx

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### GuillermoIngeniero Naval

Sorry, but I still do not understand what 'stability calculations' have you been asked for and what information have you been provided with. Please clarify.

Cheers.

P.S. Just in case.....

KB = 1/3 (5T/2 - V/Afl)
BM = 0.08*B^2/T

B: beam
T: draught
V: displacement volume
Afl: floatation area

If you now estimate KG, you can get an idea on what the initial GM is.

GM = KB + BM - KG

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### TcubedBoat Designer

To create a graph of righting moments you need the lines plans and you need to know the vertical center of gravity.

You will first find the longitudinal static center of buoyancy by taking the longitudinal moments of areas of each of the statically immersed sections about a fixed arbitrary point , summing them all and dividing this by total area of all immersed sections.

You will then incline the boat by increments of your choosing and determine the new waterlines by iteratively adjusting it until the longitudinal center of buoyancy is the same as at rest and the displacement is similarly unchanged. Then you multiply the transversal distance between the Center of gravity and the center of buoyancy by the total displacement and that gives you the transversal righting moment for that particular angle of heel .

Then repeat this process for each and every increment of heel you want.

Joining the points gives you the curve of transversal righting moments against angle of heel.

Done by hand you'll take about two full days of calculating as long as you don't use more than a reasonable amount of sections , like say ten. This is why computers are very handy for these tedious calculations.

Displacement is calculated by multiplying the area of each immersed section by the distance between it and the next one. There exist certain shortcut methods too , such as Simpson's rule ; http://en.wikipedia.org/wiki/Simpson's_rule

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### GuillermoIngeniero Naval

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### issac82Junior Member

I wanted to ask what all data I need to ask them and what all calculations i need to do to check the stability of the small ship.Also the next part is of adding weight to the ship.I need help in this regard.

Regards.

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### chabrenasMike K-H

Isaac82: I'm curious as to who, or what institution, set you such a task without giving you any suggestions about how to set about it.

Knowing who set the question would give us all a better idea of the level of complexity you should use in your answer.

Last edited: Feb 6, 2009
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### Sunny.KSunny.K

Hydrostatic table...& Gz Vs Heeling Angle Curve!!

hey guys,

I am also a student and im doing my practical now!! and I am given a task of Calculating stability of like an intermediate vessel like 39.0m..

1) Does anyone know where I could find a hydrostatic table to make things easier for the calculation?

2) Also, the Gz Vs Heeling Angle Curve, Can anyone please explain the Max. Gz on the Curve, and why does it goes up then come back down?? Please!!!

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### Olavnaval architect

Err... since I'm a naval architecture student myself (for far too long, but it's getting close to final... ) I too used to have some hydrostatics/stability lecture at uni - and I can't imagine that my prof and his didactics are so different from his colleagues all around the world. Therefore I'm always puzzled (to say the least) when there are people who tell us they "have to calculate the stability" of a certain vessel but have absolutely no idea even about the most basic aspects of hydrostatics, not even what a GZ curve is... Those things should have been explained at uni right after Archimedes' law has been mentioned!

However, Guillermo has posted some great links (especially the first one with the pdf) where all these things are dealt with. For example on page 12 of this pdf there's a beautiful figure which explains the movement of the centre of boyancy relative to the centre of gravity at an increasing heeling angle and how this is plotted on a GZ curve (GZ = lever arm = distance between weight vector and boyancy vector). But you have to read this however...

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Olav

I too wonder when i read Qs saying "I'm a student i need help with...." It does, as you have raised, begs the Q a student of what, not naval architecture since such Qs are meaningless.

It is just the same on other posts, "I'm a designer and i need help with..." Upon asking further clarifications, the reply is just like above, the fundamentals don't seem to be understood.....what are they designing then??

Giving help is so much easier of one just says what they want and what they do not know and have not been taught and not hide their lack of knowledge or experience. This is the reason for such forums to help!

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### Sunny.KSunny.K

Student...

I am 23 and graduated as Petroleum Engineer but I ended up working part-time for some shipping company because I love boats and cruising in it and now I am currently trying to study the basic of Marine Engineering and also working together with Naval Arch. and I got given these stability calculations to check it out and make a report...I wanna understand it fully and prove myself, I can do the stability calculation all right but understanding the Curve and sorts fully is a different story ...so please if there somewhere to get good explanations or anything to help me with would be much appreciated!!

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### Kay91600T Master

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### Sunny.KSunny.K

Thanks heaps Kay9...

One reason why I joined this site to get help and know more info about the water world in terms of boats, ships etc or any vessels..Oil & Gas Industry have f*cken heaps of money in it, i was a junior drilling engineer in oil rigs and sometimes offshore but probably love boats more than money..that's why im working in this shippin company so that I could study and shift to a marine engineer...love the ocean love boats and this site is freak'n helpful!!

cheers!!

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### Sunny.KSunny.K

My Own stupidity....

Just realized Guillermo's pdf file...errrrr!!!

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