# Understanding Intact & Damage Stability of Ships

The only time a vessel can be assumed to be stationary and in upright position is when it is in dry dock or before it is launched to the sea. Once in the sea, the ship has to sustain different environmental conditions, along with effects of various external and internal changes.

A ship is always acted upon by several forces from factors such as seawater, wind, internal mass weight, free surface effect etc. Thus, it is of utmost importance for a vessel to always remain stable and afloat in all conditions.

A Stable Ship

Ship stability can be defined in simple terms as its characteristics or tendency to return to its original state or upright state, when an external force is applied on or removed from the ship.

A ship is at equilibrium when the weight of the ship acting down through centre of gravity is equal to the up thrust force of water acting through centre of buoyancy and when both of these forces are in same vertical line.

B isÂ centerÂ ofÂ buoyancyÂ and G is center of gravity

A ship will come to its upright position or will become stable, when an external force is applied and removed, if the centre of gravity remains in the same position well below metacentric height of the ship. When ship is inclined, centre of buoyancy shifts from B to B1, which creates a movement and the righting lever returns the ship to its original position and makes it stable.

M is metacenter and GZ is righting lever

A ship is seaworthy if itÂ fulfillsÂ two important stability criteria- Intact and Damage stability.

Requirements for Intact and Damage Stability of the Ship

Intact stability

Â Â For a cargo vessel, the intact stability requirements are follows-

• Initial GM or metacentric height should not be less then 0.15 m.
• Righting lever GZ should be at least 0.2 m and angle of heel Ñ² â‰¥ 30ÌŠ.
• Maximum righting lever should occur at heel >30ÌŠ preferably but not less than 25ÌŠ.
• The Area of the GZ curve should be at least:

a)Â Â Â Â Â  0.055 m radian up to Ñ² = 30ÌŠ

b)Â Â Â Â Â  0.090 m radian up to Ñ² = 40ÌŠ

c)Â Â Â Â  0.03 m radian between 30ÌŠ and 40ÌŠ or between 30ÌŠ and angle of down flooding.

The angle of down flooding is an angle at which deck immersion takes place with subsequent water ingress.

Damage Stability

A damage stability criterion varies from ship to ship and the requirement for the same is given in SOLAS chapter II-1. It may be single compartment flooding, multi compartment flooding, engine room flooding etc.

Under all the criteria as applicable, vessel margin line should not be submerged after the damage. Margin line is an imaginary line drawn 75mm below the free board deck.

Intact and damage stability are very important factors that govern the overall stability of the ship.

You may also like to read-What is Anti-Heeling System on Ships?

References:

Book on Naval Architecture by E A Stokoe

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The information contained in this website is for general information purposes only. While we endeavour to keep the information up to date and correct, we make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the website or the information, products, services, or related graphics contained on the website for any purpose. Any reliance you place on such information is therefore strictly at your own risk.

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1. talaat says:

i am glad by this site and i found waht ineed in it ,thanks

2. C Thu Zaw says:

To get free e-book for seafarers.

Many thanks for certain Informations which I got through Marine Insight. I will be around.

4. lau says:

margin line is at least 76MM or 75 mm???

76MM

6. NATASA says:

IF THE DEKCK IS NON CONTINIOUS HOW CAN WE DEFINE THE MARGIN LINE.
FOR EXAMPLE : GENERAL CARGO SHIPS HAVE USUALLY A STEP IN THE FRONT.

7. Mohsen ghahremanpour says:

Tanx due your very useful information and valuable effort
I wish this activities progresd more

8. Aljosa says:

First pic. G is above M, GM negative, unstable ship.

9. Anish says:

@Mohsen: Thank you ????????

10. rhoderick tamayo says:

what is the standard GM fo 172m Length Container Vessel

11. kenny says:

information is very usefully

12. Captain Kunle Olaiwola says:

Good information. Thank you.
Capt.Kunle Laiwola