1. The buoyancy and stability of ships
Sink or swim?
The buoyancy and stability of ships

2. Session outline Archimedes in practice –
The science behind buoyancy (floating)
Why ships need to float! – the value of
shipping
How do ships need to float? – A bit
about moments and stability of ships
This presentation is designed to apply the theory of buoyancy and stability concerning moments practically to the shipping industry, emphasising the importance of an understanding of physics to the real world.
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We start by looking at the science behind buoyancy and its relation to Archimedes principle
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We then go onto looking at why we need to understand these theoretical terms in relation to one of the most valuable industries on the planet – shipping
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Finally we will look how ships need to float in relation to moments and the stability of ships, and undertake a practical exercise to put all this theory into practice

3. Archimedes Principle
“An object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object”
Lets start with buoyancy.
Archimedes, one of the great Greek Philosophers of his time, famously stated that:

4. Archimedes in practice:
Explains why steel ships float!
Ball: displaced water weighs less than the ball - SINKS
Hull: displaced water weight is the same as the
hull weight – FLOATS
Hull
But what does this mean in practice? Lets take a look. I am about to show you two objects falling when I click my mouse. Before they hit the water you need to tell me whether each will sink or float. The two objects are made out of Steel. Exactly the same masses just different volumes. What is going to happen. Why?
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GO!
Why do you think this happened?
To the ball?
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To the hull of a ship?
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Click four
This explains why huge steel ships that weight thousands of tonnes can float on water because they have a huge volume over which to distribute their mass. So they displace water and therefore float!
Ball

5. Archimedes in practice:
Explains why steel ships float!
Forces are balanced
Weight
Weight of ship
Which two forces do you think are acting together on this ship to allow it to float?
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Weight of the ship is pushing down on the water
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Up thrust of the water is pushing back up against the ship
Are these two forces equal or is one larger than the other? (equal)
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What would happen if the weight became greater than the up thrust of the water? (the ship would sink).
Now we know the theory behind floating and sinking on ships in relation the Archimedes, lets take a closer look at the maths behind it…
Upthrust of water
Upthrust

6. How do we work out if something floats? The maths behind the theory:
Which variables do we
need to look at?
weight?
volume?
If we were to provide an equation to work out whether something floats or not, which of these variables do you think we need to use?
Maybe its important if we first understand what each means
(ask pupils for some definitions)
Encourage them to say density, volume and mass.
This might be a good time to clear up any confusion over the meanings of mass and weight with pupils.
density?
mass?

7. We look at its DENSITY! The maths behind the theory:V D
Density = Mass
Volume
Calculating mass:
In g
Calculating volume: (cm3)
Length x Height x Width
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Our mathematical equation is density = mass over volume. The greater the volume the mass of an object is spread over, the more likely an object is to float! This is really important for ship builders and designers to understand as it helps them work out the minimum size and maximum weight of a ship.
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This is our magic triangle which helps us work out the calculation if any one of the variables is missing.
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To calculate the mass of an object, all we have to do is weigh it in g.
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To calculate the volume of an object, all we have to do is times the length of an object by the height and then the width.
What unit do you think density would be in? g/cm3
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We know if an object will float or not based on the outcome of this calculation. The golden rule to remember is that if an object has a density of <1g/cm3 it will float in water! If any greater it will sink.
So, lets have a bit of a practice. If a model tanker has a total mass of 3,000g, and a volume of 4,600cm3 will the ship float on water?
0.6g/cm3 so yes it will float in water. h
Golden Rule: If something has a density
of <1g/cm3 it will float in water! w l

8. The Benefits: Shipping in our everyday lives
Employs 250,000 in the UK
Generates £37 billion to UK economy per year
The fishing industry provides us with a good source of protein for
our diets
Cargo shipping provides us with 95% of the products we use on a
day-to-day basis
Why do scientists and engineers need to design objects that can float? Is it really that important? In relation to shipping this is certainly true:
Shipping is of importance to us all
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In the UK in particular, shipping employs 250,000 people in the UK,
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and is the second biggest earning industry after agriculture generating a massive £37 billion to the UK economy per year, that’s £162 per second (shown in the did you know box).
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As well as economic benefits, the shipping industry also provides social benefits as well, including a source of protein in our diets
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and the delivery of over 95% of the products we use on a day-to-day basis right to our doorsteps!
So understanding how to design ships that float is of key importance to each and every one of us!
British shipping earns the UK economy £162 per second!*
*SeaVision UK
Did you know?

9. But we need ships not just to float… …but to float upright (stability)
But its not just the ability of ships to float that engineers have to worry about! Its whether or not they can stay upright too! i.e. are they stable?
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Is this ship floating?
The answer is yes, it is, just not the right way around! That’s because a moment has been applied that has pushed the ship beyond its centre of buoyancy and so the ship has found a new point of stability – resting on its side!
What do you think a moment is?
Moments are forces applied to levers or pivots. In the case of a ship, if a moment is applied beyond its pivot (centre of buoyancy and centre of gravity) the ship will capsize!
The ability of a ship to come back to an upright position after a moment has been applied tells us how stable the ship is.
Examples of moments acting on ships in the everyday operations include – waves, winds, the movement of cargo on board, and liquids inside the ship itself such as ballast water.
Lets look in a bit more detail at moments -
Is this ship floating???

10. Just a moment… Moment Applied Centres of gravity and buoyancy
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When a ship is stable in the water, the centres of gravity and buoyancy are in a central position.
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However, when a moment is applied (such as a large wave), the centre of buoyancy shifts in the direction of the force being applied.
Centres of gravity and buoyancy
when ship is upright
New centre of buoyancy when
ship rolls

11. All or nothing! OR The ship rights itself, ‘bouncing
Whether a ship rights itself (comes back to a position of stability as in the first diagram) or capsizes (topples over into a new position of stability which is not upright as in the second diagram) depends on the size of the moment and the stability of the ship.
The ship rights itself, ‘bouncing
back’ from the moment applied
The ship capsizes!

12. What influences an object to right or capsize?
Centre of BUOYANCY
Needs to be central and low
Centre of GRAVITY
Needs to be central and medium height
For a ship to be stable, the centre of buoyancy needs to be pretty central and low towards the base of the ship. This reduces the impacts of roll.
The centre of gravity ideally wants to be at the centre of the ship to generate the greatest point of stability.

13. Practical time! Sink or swim?
The Scenario
Captain Eco. can’t work out what to base his new super ship design on!
He has provided you with a range of materials and objects to test for
stability and buoyancy
See if you can give Captain Eco a hand to recommend which material/object
would be the best choice for a perfectly stable and buoyant ship!
Good luck. Captain Eco. is counting on you
Now its time to put all that theory into practice!
(refer to the sheet sink_or_swim in this resource pack).
Here is the scenario:
(read from slide).