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Destroyer science
 The free-surface effect
Pressure and density
Propulsion
Pros and cons |
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| The free-surface effect makes a boat very unstable once it takes on even a small amount of water |
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The free-surface effect
To succeed in this challenge, the teams have to devise a way of destroying an unmanned boat. Both teams decide to try to hole the sides of their target so that they fill up with water.
A vessel becomes very unstable if it takes on water. There's no problem if it remains on an absolutely even keel. But as soon as it pitches or rolls, even slightly, all the water rushes to the lowest point, there is a sudden increase in weight on one side or at one end, and the vessel is likely to capsize. This is known as the 'free-surface effect'.
As the target boats are wooden, they will not sink completely, because wood is not very dense and so has a certain amount of inherent buoyancy. Extra buoyancy is provided by pockets of air that tend to become trapped in a capsizing vessel. For more about buoyancy, see Tugs of War science.
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| Take two identical boats ... |
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| ... the one holed further beneath the water line will fill up more quickly |
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Pressure and density
It seems obvious that a holed boat will fill with water. But why should this be so? The answer is to do with pressure and density.
Pressure is equal to force divided by the area over which the force is applied. It is measured in newtons per metre squared (N/m2), also known as pascals (Pa). Pressure from a liquid or a gas exerts itself equally in all directions.
On the surface of the earth we are affected by atmospheric pressure – the pressure exerted by the weight of the air above us. The air goes up a long way, so even though air has a low density – in other words, a large volume of air contains a comparatively small amount of matter – it exerts a lot of pressure. However, we don't normally notice atmospheric pressure because there is generally air both on the inside and on the outside of objects, so the forces balance each other out.
However, in the case of the hull of a boat, the water on the outside is much more dense than the air on the inside – it has greater mass and so greater weight (mass in the force of gravity), and so it exerts greater force and pressure. If a boat is holed, this difference in pressure forces water through the breach. The flow will continue until the pressure on the inside equals that on the outside – in other words, until the boat is full of water.
The deeper you go in water, the greater the weight of the water above, so the greater the pressure. The teams therefore want to hole the target boat at as great a depth as they can – the deeper the hole, the faster the target will sink.
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| The Boat Buoys use a powerful impeller |
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Propulsion
To propel a boat you need a way of moving water in the opposite direction to that in which you want to travel. Newton's third law states that to every action there is an equal an opposite reaction, so pushing or displacing water backwards will move a boat forwards.
The Boat Buoys go for the commonest form of aquatic propulsion: a propeller. A propeller is a series of rotating blades (usually three) that are carefully shaped and positioned so that as they rotate they displace water in a particular direction.
The Barley Pickers find a jet pump or impeller drive. An impeller is a rotor-like device with curved blades. It sucks water in at one end, spins it around at a high speed and spits it out at the other end, pushing the boat forward.
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| The Barley Pickers' hydraulic prongs are very effective |
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| The Boat Buoys' weaponry is varied, but crude |
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Pros and cons
The Barley Pickers
The Barley pickers find a huge, powerful impeller to power HMS Proper Job.
Their hydraulic scissors work very well (see Digger science for more about hydraulics).
On the other hand ...
The Barley Pickers have a primitive steering system, which proves very difficult to operate. This problem is compounded by the fact that the helmsman can't see where he is going and has to be directed by Captain Andy!
The Boat Buoys
The Boat Buoys have a whole armoury of different weapons for different jobs.
Their craft has the size and weight to do some serious battering, yet it is surprisingly fast and manoeuvrable.
On the other hand ...
Though the Boat Buoys' weaponry is varied, it is also crude.
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