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Old 01-03-2012, 06:19 PM   #9 (permalink)
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Originally Posted by Peter7307 View Post
Driving along the freeway recently I was passed by a Toyota Hilux towing a small boat with an outboard motor attached to the transom of the boat.

The bottom of the boat was well clear of the road and the outboard propellor was spinning around fairly rapidly.

The combination went past me a pulled in ahead of me and behind another vehicle in the same lane.

After a few seconds the propellor slowed to a complete stop.

I was wondering about the effects of (aerodynamically speaking) clean air vs dirty air and the flow under vehicle being disturbed by following another car.

If you have access to Hucho's book,he has a section on stock car racing.
In a NASCAR two-car draft,the lead car will see a 30% drag reduction,and the trailing car will see a 37% drag reduction.
If the boat behind the Toyota has a small enough frontal area such that it is 'hiding' behind the frontal area of the pickup,the boat can see a zero,even a negative pressure at it's front,with only it's base pressure from the turbulent wake behind the boat/trailer.
Getting behind a really 'dirty' vehicle could have you driving fully embedded within it's wake,effectively taking all the air off you,i.e. the dead prop.
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