Science
 Buoyancy
4x4 propeller
Propeller, impeller and the ace venturi effect
The problem with paddles
Buoyancy
Buoyancy is the upward push created against an object placed in a liquid. It's more a matter of volume than weight. If a car is placed into a lake, it displaces the same amount of water as its volume. The weight of the water displaced equals the buoyant force required to make the car float. If you have a very heavy vehicle, like the Barley Pickers, you need big buoyancy aids so that not much water is displaced. Also, if a car has less density than its surrounding fluid (for example by adding air-filled tanks) it will also float. Basically, anything will float, even a seven-tonne tractor, if it has enough buoyancy.
4x4 propeller
The Barley Pickers cleverly adapted their four-wheel drive transfer box so the front axle prop shaft turned a propeller rather than the front wheels. This left them with only two-wheel drive for getting in and out of the water but the additional power in the water from the high-speed prop certainly helped them.
Propeller, impeller and the ace venturi effect
Propellers, or screws, usually have three or more curved blades. These rotate on a shaft and cut themselves a thread through the dense water. A high-speed propeller under the water can produce some serious speed.
An impeller is similar to a propeller in many ways but is housed within a special cowling. Whereas a normal propeller disperses a lot of energy out of the side of its arc, an impeller channels all of its power more efficiently straight out of the back of the cowling unit. Some impellers also incorporate a tapered chamber at the back of the cowling. This creates what is called the venturi effect. If water is passed through a tube (or cowling) with a narrow exit, it travels faster through the small opening. This sudden rush of water drops the pressure behind and causes more water to be sucked into the front of the impeller. Result: you go even faster.
The problem with paddles
The Green Goddesses and the Megalomaniacs both opted for paddles attached to their driving wheels. For these to work well they have to be correctly mounted in relation to the waterline. If the teams don't get their buoyancy right, the paddles will be too low. This means that, though they will push the crazy cars forward through the water on the down stroke, they will also pull them down into the water on the up stroke.
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