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Old 10-05-2006, 07:37   #1
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High air flow / low battery consumption?

I am looking to start overnighting on the boat. (Hunter 27) I want to be able to add some comfort of air movement through the boat, but at the same time I don't want to kill my battery. (Just upgraded to 2 27's)

Being a computer geek, I have been looking at some of the high flow 120mm fans. They have some that will move over 115 cfm at .5A.

1. Is that enough air flow, or would 2 be a better choice?
2. I am building a new entry door that will secure the hatch from the inside, and was thinking of building the vent system into it.
3. What kind of trouble am I getting myself into thinking about this?

Thanks for your opinions in advance
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Old 10-05-2006, 07:55   #2
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Opeing hatches with bug-screens usually will provide enough airflow when anchored.
In a marina the air is dead and ya need extra fans.

I have been using the Italian fans with "soft" blades for years.

http://www.mikesmarine.com/m-144-CAFRA.aspx

They seem to last forever and don't use too much power.
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Old 10-05-2006, 12:18   #3
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The difference I see is first the price. The fans I am looking at run about $3-$15, just require a wireharness
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Old 10-05-2006, 12:37   #4
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computer fans are a good resource. If you have the space/size/capability, installing one inside a mushroom vent will make a big difference to air flow.

Another alternative is a wind scoop over a hatch.
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Old 10-05-2006, 12:41   #5
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I've got an older Caframo that runs on one "D" cell for something like 24 hours. Well under ten bucks at WalMart if I recall. For a portable fan it has been great.

But the cheap 12v computer fans should work well, they're certainly a low-cost source although I think they are meant for high airflow speed, which isn't really matched to what you need on the boat. Lower speed and higher volume could help more. A scoop on the forward hatch, and a small riding sail to keep you pointed into the wind, might do the job on all but the stillest nights.
If you can find drawings or articles for the old Albin Vega 27, they used a clever idea of pulling air alongside the hull, to cool it and condense the water out of it, as a way to supply cooler drier air in the boat.
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Old 10-05-2006, 13:03   #6
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Yeeup, I second that comment. Low speed high volume. You have to "move" the air. Computer fans won't. They will affect the air imediately around them. Great for low cost long term ventilation, but no good if you want to feel a cooling breeze.
There is a rule of life. "You can't get something for nothing".
If you want to keep cool, it will come at a cost. Both in fan and in power.
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Old 10-05-2006, 13:11   #7
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Alan, speaking of cooling a boat...
I've often thought that if one could use a small mist sprayer (used for some gardening applications and in supermarkets here to keep the greens misted and fresh) on deck, keeping the deck damp and using the sun and evaporation to cool it, that should be a good way to cool down a boat on a hot sunny day. Of course, the raw water in most places would clog most mist nozzles without filtering...but what do you think? Ever seen anyone try that?
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Old 10-05-2006, 22:30   #8
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Funny you should say that. One place I worked in was a gloried Shed. It had a Tin roof and in the middle of summer, it was an oven inside. One day I just couldn't bare it anymore and I strung a soak hose along the ridge. It made an enormouse difference to the heat inside.
As fo a boat deack, I wouldn't worry about misting water, I would simply pump water on deck and soak the deck. The cooling is from two elements. Evaporation and the simple fact that the water is muh cooler than the sun heated deck.
The main issue would be, the power required to pump the water.
Internaly, there is a simple poormans cooling unit. You have a "filter" for want of a better description, made form wood shavings. You pump a small amount of water oer the filter so as the shavings are kept damp. You use a fan to blow air throught the filter and the air on the otherside can be 3 to 5 degrees cooler.
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Old 11-05-2006, 07:44   #9
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Ok, so now you have confused me quite well. I thought CFM was CFM, a volume of air over a period of time. Truth be told, I am having a hard time finding CFM values for other types of fans.

I understand that the values are not always actual due to the High/Low pressure effects on the efficiency of the fans themselves.

Can anyone point me to an example of a fan with a higher volume and lower speed?
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Old 11-05-2006, 08:13   #10
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Wood shavings, huh. I'm told terracotta was used for decorative "screens" in the mideast many years ago, then water was allowed to soak down them, cooling the air as it entered the building. If there's that much sun solar power should be feasible. I was thinking of a mister because less water = less power = less leaks, too.<G> (Yeah sure.<G>)

Doghouse-
CFM *is* CFM. But a 3" box fan pushing 1000CFM is going to be pushing that air at 4x the pressure of a 6" fan, in order to get that much volume moving in 1/4 of the space. So, to get more volume at lower speed, you either use bigger slower fans (which will also often be quieter, although fan makers like Rotron rate their fans in db and charge more for the quiet models, partly because they need more expensive blades) or a larger number of smaller slower fans.
That's why ceiling fans work so well, they are high volume--but usually very low speed/pressure.
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Old 11-05-2006, 11:17   #11
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I may not be on the same page with "mister" as I would have presumed you would need pressure to createt the mist which would require power. So mist may not be the same meaning for both of us. But yes, a mist will absorb into the air and cool it quickly. I doubt cooling the air will help though. Too big an area to cool. Hence why my thought of just wetting down the deck.
When I was a lad back home on the farm, I used to feel a very big temperature drop when the irigators came on. Amazing the volume of air they big quantities of water could cool.

Doghouse, Hellosailor is spot on. It's about moving volume, not speed. Yes it is CFM, but it is like watering the garden using a sprinkler, covering a greater area, or trying to water it with just the hose. You will water it, but it will take ages and use far more water.
ummm, maybe not such a good analogy, but.....
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Old 11-05-2006, 12:15   #12
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Computer fans: If you can afford a few dollars for an experiment, I suggest you just try one and see. Cut a sheet of cardboard to the shape of your hatch board and tape the fan to it. At 150 CFM, you're probably looking at running the fan continuously, so give it time.

If you make a hole for fans to blow through, be sure to make a way to seal it up. It doesn't have to be fancy. In the winter, I use blue masking tape over a ventilation hole in my hatch board.

Misting: I never used a mister, but I sometimes pour a few cups of water on top of my dodger when sailing on hot sunny days. It makes a substantial and immediate difference in the cockpit. Of course, you have to keep doing it as the dodger dries out. I use my tank water because even the cleanest sea water has a lot of gunk that will be left behind when the water evaporates.
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Old 11-05-2006, 12:54   #13
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Alan-
Yes, a mister (mist nozzle) would require some pressure behind it. They are typically rated in a range from 4-20 liters/hour with a specific rating fixed for each type. Assuming that you don't have a local waterfall...<G>...I would think you'd need some kind of pump in order to get water up onto the deck anyhow. By misting it, rather than flooding it, you would indeed need to create some pressure but then again, you wouldn't need to pump anywhere near as much volume. I'd think the mister would win since you *only* need to move as much water as will evaporate, which is way less than just hosing the deck. Some type of bladder or pump sprayer (i.e. a gardener's pump-up sprayer or a surplus alchohol stove pressure tank?) attached to a string of mist nozzles might do well enough to let you pour in a gallon or two, then just pump it up two or three times to get through the 1-4PM heat zone. Or, use an electric pump like the ship's fresh water pump and accumulator tank. Maybe even tee off that.

Just an idea.
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Old 11-05-2006, 14:15   #14
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In my younger days when I sailed in a North Texas lake I just used a bucket to keep the deck wet with lake water. Helped a lot when it was over 100F.

Up here in the Pacific Northwest I'm more concerned with heating the boat. I installed a computer fan so that it circulates air through the plenum area between the hull and the liner on my C&C 34. I blogged about it here and here (including pictures). I'm pretty happy with the setup.

Also, the Caframo fans work well, last a long time, don't use all that much power, and are more efficient in that you can move them to do the best job. Just my experience.
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Old 11-05-2006, 15:21   #15
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Have you looked at the solar powered vent fans. You can get them with batteries installed so they operate overnight and even longer in cloudy weather.

Phil.
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