extractor fan for vents

[pcatterall]

I mentioned in my recent 'keeping cool' thread that I was considering installing fans to the existing vents.
I read another thread where solar powered and computor cooling fans were installed.
These fans were really just to help ventillate the cabins. I feel that something more powerfull may be useful in hot climes to quickly exchange hot cabin air for cooler evening air.
My after cabin has volume of 400 cubic feet or 4 cubic meters aproximatly.
I wonder what would be a useful fan capacity?
I see a computor type which works at 56 cu m per hour and uses just 0.2 amps which will fit in or neatly over the vent hole
A 12v bathroom fan which works at 85cu m per hour and would fit quite neatly over the vent hole
And a bilge blower which works at 270 cu m per hour but uses 6 amps ( and is big and needs ducting)
Has anyone any knowledge of what sort of 'exchange rate' would get rid of the warm air in a reasonable time?
If I reversed the polarity would the fan then suck in air?
Thanks for your comments.

 

[pappaecho]

I have PC fans in two vents powered on the lighting circuit.

My daughters cellar was excessively damp, and I fitted a cheapo extractor fan, which as approved for "wet" situations and turned out to be 12 volt(DC) running on a transformer.

It moves 160 cu metres per hour, and was 125 mm..big enough?

 

[oldsaltoz]

We put up the cover that runs from the just ahead of the cabin and right back over the cockpit, when a bit cool air is needed we pull down the aft end and raise the forward end forming a cone that direct the air flow down the 8 hatches.

Thia also means no jumping up if it rains overnight and cool decks for the morning.

Cost to run nil.

Good luck and fair winds.

PS this is on a 42 foot cat by the way.

 

[[2574]]

Noise is an issue...

...don't forget that you need the fan to be quiet so that the cabin occupants can sleep with it running. I reckon our aft cabin is about 5m3 and has a passive ducted vent built in. I made an in-line 12v fan and fitted in the duct to force the air flow. It is a 12v brushless pc fan, probably 60mm dia or thereabouts. At slow speed it is almost silent and makes a surprising difference to cabin freshess. Airflow can be felt on the back of the hand buts its not a "gale", nevertheless I believe it to be very effective. Power consumption is tiny; probably as little as 100ma. Motor life is said to be 50,000 hours. Purchased from Maplins for next to nothing. We leave it on constantly when we're on board. I've wired it to suck air in to the cabin creating postive pressure and thereby exhausting stale here though other exits. I read on here somewhere a while ago that fans should create postive pressure; can't, remember the reasoning.

For those that want to know now to make an in-line fan read on:

I found it impossible to source a housing for an in line PC fan. After much judicious research I established that a round gutter downpipe joint had exactly the right diameter for the duct and also had male and female fittings at either end. I cut the joint in half, fitted the fan in the middle (araldited in place) and hey presto, an inline 12v fan. The male and female fittings then slotted in to the duct with the duct ends secured with a cable tie.

 

[pcatterall]

Thanks guys all 3 responses are very helpful. Old Salt oz's will certainly be employed, we have the awning made and will see how to modify to act as advised. Once again lying on a mooring/anchor will help funnel funnel the wind as required.
Yes the bathroom one I am looking at is 12 volts and can be bought without the transformer.
Fitting the PC fan using a soil pipe is another winner !!
Thanks again