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Old 04-03-2016, 08:52   #1
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Do you suck or blow

Sorry to disappoint, but this is about ventilation and I've heard two opinions:

I'm looking at airflow below deck - I'll build a couple of dorade vent boxes piping - 2 sets....inlet & outlet.... & blowers.

So the question is:which way would you fit the blowers?
A. Position the blowers at the 'inlet' to suck the fresh air from the dorades and blow it through the cabins?

B. Or would you position the blowers at the end of the 'outlet' so as to suck air from the cabins & using that, draw fresh air from the dorades along piping?
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Old 04-03-2016, 09:40   #2
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Re: Do you suck or blow

Well if is a suck system, you may be getting air from places you don't want, like from around that hot engine you just shut down, or out of the smelly bilge, but if you pressurize the boat, then you push air out of those undesirable places.
At least that is my theory anyway.
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Old 04-03-2016, 12:03   #3
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Re: Do you suck or blow

In the navy fresh air was blown in to the ship. Bad air exhausted, fans on both ends.

On my boat there was a fan up forward that blew out, to keep the air fresh in the boat.

Seems that blowing into the fresh pipe may be more effective than pulling air end from the other end.
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Old 04-03-2016, 15:18   #4
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Re: Do you suck or blow

I have a wood boat and in the hull draw air from each the bilge area and the heads. Each bilge area has air drawn at a low rate (about 100 cfm/hour) and the heads use a common bilge blower to draw air as needed for moisture or smells. The replacement air comes from the main deck cabin vents, doors, ports and is drawn in the below deck overhead near the hull sides and flows down between the planking and inside panels in the channels the ribs make. The point being keeping the boat dry and eliminating the usual boat smells.
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Old 04-03-2016, 15:36   #5
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Re: Do you suck or blow

Not sure what your question is, but forward dorados scooping air in and aft ones turned aft to take it out. Or leave the companionway open and all forward facing!
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Old 04-03-2016, 16:12   #6
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Re: Do you suck or blow

Steel boats can get sauna hot without really good insulation. My insulation isn't that great.

When it's that hot & still, like when a thunderstorm's coming, and I'm on a pontoon with shore power I do this:

Temporary velcro around the forward hatch frame holds net bug screen, same with the main hatch. Hate bugs. Pointy little bastards.

Open forward hatch wide & suspend an 18" fan below it blowing upwards through the bug net.
Cord loop from fan's cage hung from hatch handles - quick & dirty fix when I couldn't stand the heat a minute longer.
Used a big golfing umbrella above to keep out the rain last summer - and no, I don't give a flying fox what the neighbours think

Sooner or later I'll make a proper tent for the hatch, maybe incorporating a wind scoop if I'm feeling extra-industrious.

Present arrangement works brilliantly though, keeps it lovely & cool right through the boat.
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Old 05-03-2016, 09:21   #7
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Re: Do you suck or blow

Quote:
Originally Posted by Cheechako View Post
Not sure what your question is, but forward dorados scooping air in and aft ones turned aft to take it out. Or leave the companionway open and all forward facing!
Second that!

Also, most boats have far too little total cross sectional area for ventilation. Esp in the tropics: the more ventialtion the more comfort and less smell and mushroom growth inside. :-)

Our boat is defintely short on the number of dorades so we also have a steel grate (for security) and a large canvas cover/windscoop for our large fore hatch so we can force feed air down below.
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Old 05-03-2016, 11:00   #8
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Re: Do you suck or blow

I have my blower exhausting out the transom vent so it's sucking to bring fresh air through the bilge and engine compartment exhausting hot air and fumes out.


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Old 05-03-2016, 11:15   #9
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Re: Do you suck or blow

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Originally Posted by SVTwilight View Post
Second that!

Also, most boats have far too little total cross sectional area for ventilation. Esp in the tropics: the more ventialtion the more comfort and less smell and mushroom growth inside. :-)

Our boat is defintely short on the number of dorades so we also have a steel grate (for security) and a large canvas cover/windscoop for our large fore hatch so we can force feed air down below.

Yes. We do similar. No fans involved.


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Old 05-03-2016, 12:16   #10
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Re: Do you suck or blow

Whenever there is a cooling breeze to catch, a passive system will be adequate. Otherwise a high exhaust fan getting rid of the warmest air. And a fan blowing directly on you, during the worst conditions, and to also keep the mosquitoes away while you sleep.
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Old 05-03-2016, 12:39   #11
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Re: Do you suck or blow

for moisture control and odor, suck the air out the humidity in the boat will be greater than outside. for temp blow air in through a hatch or some port and directly on you, on no wind days catch wind on windy days. for storage use a solar vent and something like DampRid

Live on the hook or mooring and not at a dock it'll be cooler and dryer. and no bugs too.
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Old 05-03-2016, 12:55   #12
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Re: Do you suck or blow

Quote:
Originally Posted by RajaNagaLaut View Post
Sorry to disappoint, but this is about ventilation and I've heard two opinions:

I'm looking at airflow below deck - I'll build a couple of dorade vent boxes piping - 2 sets....inlet & outlet.... & blowers.

So the question is:which way would you fit the blowers?
A. Position the blowers at the 'inlet' to suck the fresh air from the dorades and blow it through the cabins?

B. Or would you position the blowers at the end of the 'outlet' so as to suck air from the cabins & using that, draw fresh air from the dorades along piping?
Pumps tend to push much more effectively than suck. Fluid mechanics rule number one especially for displacement pumps.

For ventilation we're talking centrifugal pumps not displacement. You need to minimize and balance the inlet and outlet otherwise flow will be reduced.

The bigger picture in terms of comfort is to balance temperature, airflow and humidity. With fans only you can only vary airflow. Shading can help with temperature. AC or heating give you more options to find the balance.

Think in terms of air changes. You want at least 20 air changes an hour in a habitable area. Calculate the volume of the room, multiply by 20 and you have the air volume per hour you need to move. Assume your fan(s) are only 50% efficient and you can size your fan flow rate and number of fans. Size on the slowest speed.

For ducted you need to calculate pressure drop. Typically you need large diameter ducting which wont fit on most boats. Hence needing an inlet and outlet in each room.

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Old 05-03-2016, 14:11   #13
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Re: Do you suck or blow

Can't help myself. Suck blow job is just a figure of speech.
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Old 05-03-2016, 15:16   #14
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Re: Do you suck or blow

Principles of ventilation are quite straightforward :
To get an air stream somewhere, we need to supply the air to the location, but to remove the stagnant/foul air we need to exhaust it.
Having said that, we can, of-course, push it out, as much as we can suck it in.
In simple terms, blow fresh air over where you will be wanting a nice stream of fresh air, exhaust from the many dark back corners of places.

Consider the "in" location and then the "out" location and look at the path between.
Many times we see a toilet exhaust next to an open window and the other side of the room is mouldy and smelly - the air path is in the window, out the fan and to heck with the rest of the space.

Think like a lazy, idle smelly bit of air and consider the easiest way you would move with the through breeze. If this misses various corners, then you need to encourage air into those places.
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Old 05-03-2016, 20:01   #15
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Re: Do you suck or blow

if you put the fan at the discharge end you will have many inlets, displacing the volume more efficiently. if you blow air in at one point the air will create a stream to the point of least resistance. Hence a large percentage of the fresh air coming in will stream right out leaving the old air.

think of the Gulf stream shooting right up through the Florida straights. all that warm water slipping right on up and the relatively cooler water just east of it and north of the Bahamas, barely affected. it is generated from the equatorial currents and forced through the gulf and on up the atlantic coast. if you could suck it from say Virginia it wouldn't be near as warm, as the water would not only be coming from the gulf but the lower half of the north atlantic as well
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