Rolling around the Sea!

Rolling around the Sea!

I suspect many of us have sailed through rough seas while cruising, this can result in the vessel rolling uncomfortably from side to side.  Some passengers don’t have the greatest sea legs and can become quite sea sick which is never a great experience, other guests get quite frightened as the ships rolls in the high seas.

What forces are acting on the vessel which bring the vessel back upright?  A cruise vessel floating in a body of water has both a centre of gravity (G) acting downwards, and also a centre of buoyancy (B) acting upwards through the vessel’s centre of buoyancy.  The vessel also has a metacentre (M) which is the point at which the vessel rolls about.  The cruise ship will always be ballasted and loaded to ensure the centre of gravity is below the metacentre, when this is the case we can say the vessel has a positive metacentric height.  Take a look at the below diagram.  When the vessel heels over, the centre of gravity will not change as the weight distributed within the vessel is fixed, but the centre of buoyancy will change as the hull is no longer submerged equally on each side as shown below.

 

 

 

 

Now remember gravity is acting down through the centre of gravity and the centre of buoyancy is acting upwards through the centre of buoyancy.  These forces will result in a righting force bringing the vessel back to its upright position.

Most cruise ships are fitted with stabiliser fins, these are retractable fins which when in the extended position tilt upwards and downwards as required to produce lift and drag, just like an aeroplane wing.

A gyroscopic control system will sense the vessel rolling and send a signal to the stabiliser fins which will tilt accordingly to counteract the roll of the vessel.  This can reduce the roll of the vessel by 90%.  Stabilisers are normally used in pairs, and some cruise ships are fitted with 2 pairs.  Stabilisers can be used individually and will still reduce the roll of the vessel significantly.

The employment of stabilisers will increase the ships drag through the sea and a fuel penalty will incur.  Stabilisers are really only fitted for passenger comfort, a similar size container ship will in most cases not be fitted with stabilisers.  I hope this helps put your mind at ease next time you find yourself rolling in heavy seas.

Ship stabilizer
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