Venturi Effect Formula

[capta]

If I were to take a standard cowl 4" (see pic) and desire to change it into a venturi exhaust system, perhaps using a funnel, does anyone have any idea of the ratio between incoming and out going opening's sizes. I know the exhaust must be larger but is it a critical ratio?
When I was a teen, they were standard as exhaust vents on many work boats and some even turned on their standpipe with the wind. They were great; no moving parts, rain and spray proof and no planned obsolescence.

 

[rpwillia]

I used this, but did not do a lot of thinking

I wanted to get a little more active ventilation so I replaced my engine exhaust vent with this.

http://www.campingworld.com/shopping/item/cyclone-sewer-vent/31992

 

[Ross]

A computer cooling fan and a small solar panel work wonders

 

[capta]

A computer cooling fan and a small solar panel work wonders

I kinda doubt that a computer fan will survive on the aft deck for very long, even if it's in a funnel. Most everything I found in the marine market is too small to move enough air through a 4" hole with any power.
We have 5 entry points for air into our aft cabin, but no exits aft of the middle of the bed. I'm going to need to drill holes in the aft bulkhead and exhaust the air through the emergency steering deck plate to get any flow through the cabin. It needs to be very portable, as the deck plate must be in when we are sailing, rain proof and hopefully un-wired.
These old Venturi funnels moved a huge amount of air, were rain proof and portable.
rpwillia; thanks, I've never seen those before. I doubt there'd be enough air flow on a sailboat at anchor for these to work for me.