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Powering ships:

Powering ships:. The physics of propulsion. Session outline:. Why shipping is important What physics has to do with it: the theory behind propulsion. Did you know?. British shipping earns the UK economy £162 per second!* *SeaVision UK. Why shipping is important.

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Powering ships:

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  1. Powering ships: The physics of propulsion

  2. Session outline: Why shipping is important What physics has to do with it: the theory behind propulsion

  3. Did you know? British shipping earns the UK economy £162 per second!* *SeaVision UK Why shipping is important 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

  4. Ships need to be able to move! Need to generate thrust to overcome water resistance so that ships can move from A to B – It’s all about forces What provides the energy for this thrust?...

  5. Earliest forms of boats used ‘sculling’ to generate thrust to travel through the water… e.g. rowing boat ores push water backwards to generate forward motion

  6. Fuel Nowadays we use fuel! Heat energy Chemical energy Engine Combustion Oxygen Work done Mechanical energy

  7. The mechanics Friction Propeller shaft Engine Mechanical Energy Thrust

  8. The workings of a propeller:under pressure Works similar to that of an aeroplane wing: Pressure changes drive ships forward! Shaped blade sections Causes different flow speeds Causes pressure changes ‘hydro-foil’ of the surrounding water which generates thrust

  9. Finding the balance Cannot just put your propeller on maximum speed If the low pressure area generated by the propeller turning becomes too low, the boiling point of water decreases

  10. The risk of ‘cavitation’ If this occurs, water surrounding the propeller changes from a liquid phase to a gas phase… This is called ‘cold boiling’ Low pressure Thousands of tiny gas bubbles form in the area of low pressure and on the propeller blades themselves

  11. Why bubbles are a problem As the propeller blades rotate, these bubbles are quickly transferred from an area of low pressure to an area of high pressure where thrust is generated Low pressure Bubble implosion! High pressure Erosion Under such a pressure change, gas bubbles implode in on themselves! This actually erodes propellers, which can dramatically decrease their efficiency

  12. So other factors need considering when designing propellers

  13. Size • Thrust from a ship is generated because of pressure differences across the propeller • If: Force = Pressure Area • The greater the propeller diameter (area) the lower the pressuredifference and the higher the thrust (force) that will be provided per number of revolutions (turns of the propeller) • It is therefore best to have as large a propeller as possible, because pressure differences will remain stable and the chances of cavitation and subsequent propeller erosion are reduced

  14. Number of blades • With an increased number of blades you increase the hydro-foil surface area and therefore have a better chance of generating more thrust. • However, too many blades and you end up increasing drag on the propeller, which can serve to reduce the ships efficiency at optimum speed.

  15. Material • Materials are an important consideration when designing propellers. Normally aluminium, brass or plastics are used: • Aluminium – Light weight so high number of turns for the amount of energy provided by the engine. This provides a higher thrust. However, aluminium erodes quickly under low pressure so often needs replacing. Also corroded by seawater. • Brass – Tough and hard wearing metal. Heavy though and takes a lot more energy to rotate than an aluminium blade. • Plastics – Only suitable for smaller ships, such as leisure crafts and speed boats, because of the size of propeller that can be made from plastics.

  16. Pitch Pitch is the angle of a propeller’s blades Can be changed to suit the task required Pitch lowered – Greater number of RMP and more strength Used when: Ships are setting off to reach optimum speed Pitched increased – Lower number of RMP and less strength Used when: ships simply need to maintain optimum speed because this conserves fuel

  17. Physics in the real world - still a key role to play in how ships are made Newcastle University’s Cavitation Tunnel

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