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Discussion Starter #1
I have a squirrel cage blower mounted in the space under the 1/4 birth separated from the engine space by a plywood bulkhead. It uses two holes in that bulkhead for the hoses that take in air from the engine space and discharge it into a hose that reaches all the way to the stern rail vent.

I would like to change this setup. I have a better use for the space the current blower occupies. I would prefer an inline blower mounted aft of the engine space.

Does anyone have any experience with inline blowers? Any reasons for not making this change?

Thanks,

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I have one of these fans from westmarine in the engine compartment of my boat...it is switched separately from the engine ignition so I can let it run for a while after shutting off the engine. Works fine for me and it was pretty cheap.

 

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Was at Radio Shack this afternoon and saw they had 4" brushless (no sparks!) DC fans that might work well in your setup. Theyre not "marinized", but for $20...
12VDC Brushless Fan : Computer Fans | RadioShack.com
Yikes! This strikes me as one of those penny saved pound foolish things.


I can't even imagine the liability if you had a fire and burnt down a marina full of boats or hurt someone. And I mean the 'right' one shown above is only $30 at WM. I'm sure you could get it even cheaper somewhere.

But I'd go back to is it gas or diesel? If it's diesel just lose the fan.
 

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As to the original post, yes the reason would be that the squirrel ones move a LOT more air, and IMO are more reliable than the cheaper inline ones.

But as you know already, take up a lot more space :)
 

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Tundra Down
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Discussion Starter #8
As to the original post, yes the reason would be that the squirrel ones move a LOT more air, and IMO are more reliable than the cheaper inline ones.

But as you know already, take up a lot more space :)
Volume and reliability are the best reasons I can think of! I might be able to relocate it but it would require a much longer intake hose and a shorter discharge. Now the intake hose is 10" or a little less. It reaches through the bulkhead and angles down beside the engine. The total length of hose doesn't change if I relocate the squirrel cage blower. Does having a longer intake hose diminish the blower's capacity?

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Yes a longer hose reduces volume, but how much depends is variable. The longer and rougher the hose the more it matters.
 

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I have one of these fans from westmarine in the engine compartment of my boat...it is switched separately from the engine ignition so I can let it run for a while after shutting off the engine. Works fine for me and it was pretty cheap.

I hope you are also using the switch to run the blower for 5 min. before you start the engine. clearing the engine compartment of gas fumes before you start the engine is the main reason for the blower.
 

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Tundra Down
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Discussion Starter #13
Does shortening the length of the blower's discharge hose increase the blowers capacity? Is resistance to pulling the air in (longer hose) different in effect on cfm than pushing the air out (shorter hose). It seems that the total resistance = the total length of the hose the blower has to move air through. Not where the blower is located along the hose. Why isn't this true?

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If the overall hose length is the same then there is little to no loss of discharged volume. Should also be a bit quieter too.
 

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Discussion Starter #15
If the overall hose length is the same then there is little to no loss of discharged volume. Should also be a bit quieter too.
That makes sense to me. It will be in a better location for noise reduction. It occupies the forward inboard corner of the space below the 1/4 birth now. That is "valuable" space. The blower doesn't need the immediate access it enjoys in that spot. I guess it was the most convenient place to install it when they were building the boat. I will take a close look at options for relocating it. The cfm of the inline blower are comparable to the squirrel cage blowers.

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Assuming you are using the same exhaust diffuser, you might measure the discharge velocity before then after making changes with an air volumeter. A hand held wind speed indicator would work. And if there is a reduction then just run the fan for a longer time than 5 minutes.
 

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A squirrel cage fan is much more efficient than a prop fan for moving air. With small hoses a high static pressure is needed to move air. Hard to achieve with a prop fan. The fan you remove should have specs on it or you should be able to look them up online. Specs with have a rating of CFM (cubic feet per minute) at like 0.25" of static pressure. If you are going to use a different fan make sure the specs match.

Each 90 deg bend in the hose is like adding 6' of straight hose in back pressure (static pressure). Try not to add any additional bends to the new setup. And the wire reinforced plastic hose is the worst hose for air flow. I know there are little to no alternatives to use and it is used in almost every boat. And with out equalization there can not be air movement. Meaning you need a way for the air to enter into the compartment to be able to remove air. So the fans being used for moving said air needs to have a good static pressure rating. I just looked up a fan for this use and there was no SP rating, only CFM. Be careful a 250 CFM rated fan means with no load on it (no static pressure), unless otherwise stated.
 

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Discussion Starter #18
Assuming you are using the same exhaust diffuser, you might measure the discharge velocity before then after making changes with an air volumeter. A hand held wind speed indicator would work. And if there is a reduction then just run the fan for a longer time than 5 minutes.
Thanks for this suggestion. As I look at the actual ss shell vents mounted on the stern coaming they sure look restrictive! While looking into this I found a table that equates a 90 degree elbow with 30' of straight duct for purposes of calculating air flow resistance. I am taking this a bit too far, I suppose.

Our procedures for starting the day always include opening the engine space. Doing a personal fume check and then running the blower before starting the A-4. This means opening the lazarette and often the companion way steps for other reasons as well. Some folks just keep the blower running whenever their engine is running. I don't. I think the squirrel cage is more appropriate for continuous duty than some of the inline blowers on the market. Any information on running an inline blower continuously?

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Thanks for this suggestion. As I look at the actual ss shell vents mounted on the stern coaming they sure look restrictive! While looking into this I found a table that equates a 90 degree elbow with 30' of straight duct for purposes of calculating air flow resistance. I am taking this a bit too far, I suppose.
You may just need a fan with a higher SP. I am sure you can find one to do the job.

Some folks just keep the blower running whenever their engine is running. I don't. I think the squirrel cage is more appropriate for continuous duty than some of the inline blowers on the market. Any information on running an inline blower continuously?



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All fans are rated for continuous running.

I edited my last post with more info....
 

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Does shortening the length of the blower's discharge hose increase the blowers capacity? Is resistance to pulling the air in (longer hose) different in effect on cfm than pushing the air out (shorter hose). It seems that the total resistance = the total length of the hose the blower has to move air through. Not where the blower is located along the hose. Why isn't this true?

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Switching from a pulling fan to a pushing fan does have some consequences, mostly to do with the pressure inside the engine room. For a gas engine you may need both, but you absolutely need at least one fan blowing out of the engine room.

A proper engine room ventilation system on a boat should always leave a slight partial vacume in the engine room. This is to ensure that heat and fumes are not pushed into the living quarters. With a gas engine it is even more critical because the gas fumes may collect in the living quarters.

See http://pdf.cat.com/cda/files/3375314/7/Engine+Room+Ventilation+LEBW4971-03.pdf for a detailed discussion about engine room ventilation design. Note that this was written for diesels not gas engines, so it doesn't discuss fume elimination.

On a related note the USCG requires every permanently installed gas engine to have a dedicated blower fan with the intake routed at least halfway to the bottom of its engine room. Deeper is better to collect fumes better.
 
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