Why Ships Sink

[Capt_BJ]

actually center of gravity is only one half of the equation

 

ship 'stability' or its proneness to roll is based on the relationship between the center of gravity - pushing the ship down into the water - and the center of buoyancy - pushing the ship UP, out of the water. In most cases, the center of buoyancy is below the center of gravity. How far apart these two positions are determines 'GM' or

in Naval Engineering this is 'metacentric height'. In very simplified terms a big GM means a ship 'wants to stay upright', and when it does roll, it 'snaps back upright quickly' .... like a destroyer. Less GM means the ship has a tendency to roll and return more slowly.

 

In most conditions, if the positions of the centers of buoyancy and gravity change (due to damage) and get closer together it is a bad sign. If the positions get reversed ... this is called capsizing!

 

http://www.navweaps.com/index_tech/tech-009.htm

 

http://www.wisegeek.com/what-is-metacentric-height.htm#didyouknowout :

 

Metacentric height is the distance between a floating body’s metacenter and its center of gravity. It is a measure of a floating body’s stability such that a ship with a large metacentric height is more stable than a ship with a smaller one. A ship that has a large metacentric height also has a shorter rolling, which is less desirable for passenger ships. A passenger ship should therefore have a metacentric height large enough to be stable, but not so large that it is uncomfortable for passengers.

 

In "ship Captain School" a lot of time is spent on this topic - especially for USCG Captains because observing the change in roll characteristics is a very significant indicator of the changes in a ship's stability condition, especially when the vessel is damaged. There is video of a very significant case from a number of years ago when USCG was towing a damaged vessel and no one noticed that over time the roll period was increasing significantly (hours) ... until suddenly the vessel turned turtle and sank taking several souls with it . . . We watch that video over and over now as an intro to the potentially very boring topic: ship stability calculations.

 

And to over stress the point, it is not just in damaged conditions that GM becomes and issue. Burning fuel empties tanks and reduces the weight of a cruise ship.... GM changes with the weight shift. Taking on ballast replaces the weight but can't happen in a flash ... As Captain I received a daily report of the theoretical 'GM' based on calculations factoring in liquid loads .... tracking day to day changes.

[oncruisecontrol]

actually center of gravity is only one half of the equation

 

ship 'stability' or its proneness to roll is based on the relationship between the center of gravity - pushing the ship down into the water - and the center of buoyancy - pushing the ship UP, out of the water. In most cases, the center of buoyancy is below the center of gravity. How far apart these two positions are determines 'GM' or

in Naval Engineering this is 'metacentric height'. In very simplified terms a big GM means a ship 'wants to stay upright', and when it does roll, it 'snaps back upright quickly' .... like a destroyer. Less GM means the ship has a tendency to roll and return more slowly.

 

In most conditions, if the positions of the centers of buoyancy and gravity change (due to damage) and get closer together it is a bad sign. If the positions get reversed ... this is called capsizing!

 

http://www.navweaps.com/index_tech/tech-009.htm

 

http://www.wisegeek.com/what-is-metacentric-height.htm#didyouknowout :

 

Metacentric height is the distance between a floating body’s metacenter and its center of gravity. It is a measure of a floating body’s stability such that a ship with a large metacentric height is more stable than a ship with a smaller one. A ship that has a large metacentric height also has a shorter rolling, which is less desirable for passenger ships. A passenger ship should therefore have a metacentric height large enough to be stable, but not so large that it is uncomfortable for passengers.

 

In "ship Captain School" a lot of time is spent on this topic - especially for USCG Captains because observing the change in roll characteristics is a very significant indicator of the changes in a ship's stability condition, especially when the vessel is damaged. There is video of a very significant case from a number of years ago when USCG was towing a damaged vessel and no one noticed that over time the roll period was increasing significantly (hours) ... until suddenly the vessel turned turtle and sank taking several souls with it . . . We watch that video over and over now as an intro to the potentially very boring topic: ship stability calculations.

 

And to over stress the point, it is not just in damaged conditions that GM becomes and issue. Burning fuel empties tanks and reduces the weight of a cruise ship.... GM changes with the weight shift. Taking on ballast replaces the weight but can't happen in a flash ... As Captain I received a daily report of the theoretical 'GM' based on calculations factoring in liquid loads .... tracking day to day changes.

Thanks for this detailed, yet easy to understand explanation. Very well done. Interesting, too.

[Treven]

Yes it was very interesting.

This thread certainly has brought out more information than I originally anticipated. I, for one, have appreciated the physics lessons from those knowledgeable.