Brian Gaff wrote

Hang on though,

Too radical by far...

to make it spin it has to have drag.

But a tiny part of the drag of the whole van.

If it has drag its using fuel.

Not when stationary and most of them are
stationary much more than they are moving.

Might be moor efficient then to streamline the van

That should be done anyway.

and run the fan from the van electrics which is there already.

Sure, but that wouldn’t work as well when stationary.

"Rod Speed" wrote in message
...
Gym Sulkinson Fork wrote

Why are these fitted instead of just a stationary vent like you see on
caravans?

They act like a pump.

http://www.flettner.co.uk/van-roof-vents/

I don't see the point in using the wind to power a vent, when the air
was already moving.

It isnt from the place you want to move it from.

Surely that's like fitting a generator to your bicycle to power a motor
to make you go faster?

Nope, more like using a wind generator to generate
electricity and using the electricity to drive a fan, but
without the losses involved with the electricity
generation and the electrical fan.