Why doesn’t the James Cook roll over?


Hello, firstly a collection of pictures from today:
Today is Ian’s Birthday. Happy Birthday Ian 🙂



Matt working in the lab.

Above: Gael and Roger working hard.

Adam in the lab. Me on an engine room tour. I’ll tell you more about that in the next few days.


Nigel asleep in an outside lab. Can you spot him? He’s doing well 🙂

IMG_1086 - Version 2

A sunset photo from Gael who is striving to capture the ‘green flash’.

Juan has very kindly produced another fabulous blog for us 🙂 Thank you Juan. So without further ado over to Juan 🙂



Why doesn’t the James Cook roll over?

Aaah. Difficult question.

Let’s start with the basics. What’s the difference between ‘stable’ and ‘unstable’?


Having one too many to drink?

It comes down to something called stability, which deals with the alignment between an object’s centre of gravity and the line of action of the force acting upon it.

You sort of know this already by another name – moments – if a net moment exists (e.g. you pushing a door) then a rotation happens. On a ship, the net moment is caused by differences in the line of action between the ship’s weight and the buoyancy force. But stability also has another feature – will there be any restoring forces which try to return the object back to its original position?

Have a look at this diagram, which says more than 1000 words could do:


From the diagrams above, we can only conclude that having masslow down’ in a ship – thus bringing centre of gravity down below the centre of buoyancy is necessary for stability. But you all knew that already.


What is the ‘centre of buoyancy’?

Quite simply, it is the centre of the volume of the ship which is underwater. So, if you had a ship like the one in the diagram below, the ‘centre of buoyancy’ is the centre of the underwater bit:

pic boat 1

So what about the ‘centre of gravity?

If you had a metre ruler which you stuck on masses of random weights at random intervals, then the pivot, or balancing point of that ruler would be the one-dimensional ‘centre of gravity’ of the ruler and all the weights you added.

The centre of gravity of a ship is like that, but in two dimensions. It’s the point where all the masses in the ship (hull, machinery, fuel, water, food, furniture) balance along the length (x-axis) and the beam (y-axis). If you want your ship to be upright (not heeled) and not bow-down or stern-down (not trimmed) then the centre of gravity should be exactly below the ship’s centre of buoyancy.


Testing your understanding: centre of gravity

Are you ready to put your ideas into practice? The questions below are similar to the sort of problems the ship’s officers have to solve when balancing the ship, each time new stuff is brought onboard.

Equations you will need:

Moment (Nm) = Force (N) x Perpendicular Distance (m)

Density (kg/m3) = Mass (kg) / Volume (m3)

1 tonne = 1,000 kg

  • The beam (width) of the James Cook is 18.6m. On the aft (back) deck, technicians have placed 5 large items of machinery across the width of the ship. Show that their centre of gravity is about 3m to starboard (assume y is positive port):



  • Six tanks around the ship have been partially or completely filled. The positions of the tanks and details of their current contents are shown in the table below. Show that centre of gravity of these tanks is about 23m aft of amidships and about 2m to starboard.



An added picture from me of the RRS James Cook in dry dock. Look at the size of those propellers!! Thank you Jim (the Captain) for the picture.




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