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

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. Click one We start by looking at the science behind buoyancy and its relation to Archimedes principle Click two 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 Click three 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

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:

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? Click one GO! Why do you think this happened? To the ball? Click two To the hull of a ship? Click three 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

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? Click one Weight of the ship is pushing down on the water Click two 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) Click three 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

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?

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 Click one 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. Click two This is our magic triangle which helps us work out the calculation if any one of the variables is missing. Click three To calculate the mass of an object, all we have to do is weigh it in g. Click four 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 Click five 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

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 Click one In the UK in particular, shipping employs 250,000 people in the UK, Click two 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). Click three As well as economic benefits, the shipping industry also provides social benefits as well, including a source of protein in our diets Click four 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?

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? Click one 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???

Just a moment… Moment Applied Centres of gravity and buoyancy Click one When a ship is stable in the water, the centres of gravity and buoyancy are in a central position. Click two 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

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!

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.

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).