"Gym Sulkinson Fork" wrote in message
news
On Thu, 25 Oct 2018 17:35:48 +0100, Rod Speed
wrote:



"Gym Sulkinson Fork" wrote in message
news
On Thu, 25 Oct 2018 17:11:41 +0100, Nightjar wrote:

On 25/10/2018 16:07, Gym Sulkinson Fork wrote:
On Thu, 25 Oct 2018 16:03:22 +0100, wrote:

On Thu, 25 Oct 2018 15:47:48 +0100, "Gym Sulkinson Fork"
wrote:

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

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

They are just more efficient than a simple hole in the roof with
sufficient cover to keep the rain out. If there was a better mouse
trap, somebody would have made one in the last 200 years.

Yes I know they claim to be more efficient, but why? How can you take
power from the wind to then give it back to make it faster? Isn't
that
breaking the laws of physics?

They are a Savonius rotor, which uses the wind to rotate a shaft. There
is a fan attached to the shaft, which, unlike a simple hole, actively
extracts air from the vehicle. They are also widely used on boats, to
keep them ventilated while sitting at their moorings.

I'd just open the window (with a rain guard of course).

Doesn't work when the boat is unattended and those things are
completely automatic, you don't need to know when it might rain
and when to shut the window.

That's why I said rain guard. I've seen caravans with a roof vent with a
hat n top to stop the rain.

Sure, but those rotating ones work better.