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Old 08-21-2018, 06:25 PM   #1
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Engine room blowers

Our boat has four blowers in the engine room. The two forward ones blow air in, and the two aft ones suck air out.

To me it would seem more efficient to have all of them sucking air out and just let the air find its own way in. But what do I know.

I rarely use them since they make a horrendous amount of noise.

Is there a good reason to leave them in their current configuration, or should I change them so all four exhaust air from the engine room?
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Old 08-21-2018, 06:37 PM   #2
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You probably will get more airflow leaving them as is than changing them so all four exhaust. But I have to ask: why do you need to run the blowers at all? Three possibilities:

1. You have a gasoline engine and you need to exhaust the air in the engine room before you start.

2. Your engine room runs too hot in normal operation and you need the blowers to keep it down to reasonable temps. Most engine manufacturers are ok with engine room air at 120 deg F. FWIW a well designed engine intake vent system is all you need to keep a diesel down to 120.

And your blower installation may block off intake air from the diesel when the blowers aren't running and when they are running they are taking air away from the diesel's air intake.

3. After shutting down your engines, you want to cool down the engine room quickly to keep it from heating up adjacent people areas.

So the only reason that is valid for a diesel engine is the last one.

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Old 08-21-2018, 06:40 PM   #3
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Diesels use a tremendous amount of air for efficient combustion. It would not be a good idea to have them sucking air out while running. However if you are just using them to scavenge hot air out after shutdown then it would be ok.
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Old 08-21-2018, 06:44 PM   #4
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I have two intake without blower and on the other side one blower intake and one blower blowing out (totally ridiculous in my sense). I want to change this to 2 free flow intake and 2 blower exhaust just to keep engine compartment (for willy it is compartment not room) relatively cool and provide fresh air.

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Old 08-21-2018, 06:48 PM   #5
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Greetings,
Mr. ww. I don't understand what you're asking. What sort of improved efficiency are you hoping to achieve? Cooler ER?



I just did a very rough calculation of the 2 "blowers" (Ford Lehman 380 cubic inch) in our ER. At 1750 RPM they are consuming about 790 cu ft of air per minute (total) that must come in through the air intakes.
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Old 08-21-2018, 07:05 PM   #6
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I use a large squirrel cage exhaust and dedicated large intakes. The cooler the engineroom, the more efficient your diesels run. Excessive heat also can shorten the life on many things in the usual engineroom.
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Old 08-21-2018, 07:07 PM   #7
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Quote:
Originally Posted by wwestman View Post
Our boat has four blowers in the engine room. The two forward ones blow air in, and the two aft ones suck air out.

To me it would seem more efficient to have all of them sucking air out and just let the air find its own way in. But what do I know.

I rarely use them since they make a horrendous amount of noise.

Is there a good reason to leave them in their current configuration, or should I change them so all four exhaust air from the engine room?
No suggestions here pertaining to how many blowers nor if they should all pull in or blow out. The blowers on our boat were noisemakers until I removed them from their mounted positions and let them float, relying on the hoses for support. I found that the noise radiated throughout the entire boat when they were physically attached. Something that you may want to consider.
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Old 08-21-2018, 07:44 PM   #8
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Before you change things around, see if you can find a manufacturer's requirement for square inches of air inlet. Most engine manufacturers list requirements for free air openings. If you're going to run an exhaust fan (as I do) while underway. You need to provide additional free air openings for this also. An easy mistake to make is assuming that the opening for the 4" fan is going to allow the same amount of air in as the fan blowing it in. While I don't have the number, you will probably need to increase the 13 sqin. opening of the fan to 100 sqin. for free air. Ask any engine manufacturer and they will tell you the engine can't compete with the exhaust blowers, you need to supply enough free air for both.

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Old 08-21-2018, 07:51 PM   #9
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To answer some of the posts:
Diesel engines, so blowers not required.
The blowers were installed by the builder.
The only time I have used them was trying to cool things down after running engines before going into the engine room.
Have never used them while underway, since we are in the PNW heat does not seem to be a big issue.

The source of the question was thinking about evacuating smell/smoke/anything else out of the ER the fastest, and which would work best. 2 pushing/2 pulling or 4 pulling and doubling the volume of air being moved.

I may try the unmounting idea as a way to reduce noise. Maybe hang them from bungee cords to keep them in the same relative place.
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Old 08-21-2018, 08:03 PM   #10
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Quote:
Originally Posted by Lepke View Post
I use a large squirrel cage exhaust and dedicated large intakes. The cooler the engineroom, the more efficient your diesels run. Excessive heat also can shorten the life on many things in the usual engineroom.


That makes sense... except that the engines are already dragging a huge amount of air into the ER just as it is running. Iím not sure how much cooler the exhaust fan will keep the ER while it is running. Iíd be really interested in knowing if you have any actual measurements of the ER temps with and without that blower running?
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Old 08-21-2018, 08:05 PM   #11
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My guess is that the mfg designed the system so it didn't steal combustion. 2 in - 2 out would seem to provide balance for the blower system.
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Old 08-21-2018, 08:21 PM   #12
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Quote:
Originally Posted by wwestman View Post

The source of the question was thinking about evacuating smell/smoke/anything else out of the ER the fastest, and which would work best. 2 pushing/2 pulling or 4 pulling and doubling the volume of air being moved.

I certainly donít know the answer, but I can guess...

The first question, unless I missed it, do you have open air vents besides the 4 blower vents? If you do, how big are the vents? If the vents are of an equal area to the 4 blower vents that you have then I would think that if all 4 exhausted, you would likely increase your air exchange when the engine is not running.

OTOH, if your blower vents are 4Ē, then if your open vents are less than 50Ē sq.in. I think that switching all to exhaust would actually reduce your air exchange. The reason is that if your trying to exhaust through 50Ē sq.in yet have a smaller intake than that, your blowers will be less effective. Now, if your passive vents are around 50Ē sq.in then you might get better exchange if you reverse one of the flans blowing into the space, so you would have 3 in and 1 out.

I have one blower fan. I would like to run it for 20-30 minutes after engine shut down, but it is really loud and annoying. My ideal situation would be to replace it with a quieter fan and put it on a timer.
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Old 08-21-2018, 08:30 PM   #13
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I have a blower that exhausts and draws from up high in the ER. So it is there for cooling after a run. I donít bother since the ER doesnít really get that hot. I donít need it to evacuate smells either.
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Old 08-21-2018, 09:55 PM   #14
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I agree that, unless you have a sealed positive-pressure engine room (and that brings its own problems), there should be absolutely no need for blowing air IN to the ER. I have hi-volume (not squirrel-type) blowers x2 OUT that draw from the top of the ER (where obviously the hottest air accumulates). I run my blowers underway (why not?) and when I get back to the dock and on shore power, I leave them on while we tie up, wash off and pack up. On my long list of 'get to it one day' projects is an idea for a simple timer so the blowers can be left on for a longer period and/or controlled by a t/stat. Unfortunately, my batteries & electronic regulators are in the ER so I like to get temps down as soon as possible. At anchor, I'll generally let the blower on the lee side continue to run for around 40min after the pick hits bottom. Not sure this does much other than make me feel I'm doing something !
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Old 08-21-2018, 10:36 PM   #15
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We have 8 x 6 inch PVC tubes on foredeck inside vented lockers forcing cool air in.
We have 2 x 24v Davies Craig radiator fans about 1ft in dia. sucking warm air out, up two decks and out the funnel.
We have ducting from the funnel to a box with air cleaner above the engine for the turbo to suck cool clean air.
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Old 08-22-2018, 06:52 AM   #16
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John Deere lists the minimum ventilation area at 93 sqin. for my 135 HP 4045 engine. The maximum air intake is 346 cfm. The ventilation blower pushes 200 cfm. I have around 200 sqin. of ventilation area feeding the engine room. With the blower running underway, the near ceiling temperature is a little over 100 degrees. Without the blower during the summer, it can exceed 120 degrees. Even though the engine room ceiling is well insulated, running the blower while underway and for a half hour after shutdown, helps reduce the temperature in the saloon above.

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Old 08-22-2018, 10:43 AM   #17
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Electronics, battery charger, solar controller, hydraulics, batteries all are happier cooler. Engine room vents are to keep the temperature in there manageable and therefore extend the life of these components. Obviously, as long as there is sufficient combustion air the engine doesn't care. Cooling it off faster after use is probably pointless but you need to get air out as well as in so your current system is most efficient. Unless the onlt thing in your engine compartment is the engine, keep the blowers and use them as long as they are running. Most blowers are noisy so spend your time making them quieter if you are concerned.
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Old 08-22-2018, 11:18 AM   #18
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Seems to me that the amount of air needed to supply the engines can be measured by the flow of exhaust out the hoses at the stern. If you want excess air in the ER for cooling, your area of intake vent must exceed the area of exhaust by some significant factor.
Your present setup of 2 in and 2 out, if that is all you have, ie no other openings into or out of the ER, will deprive the engine of its necessary air supply, so I would use the fans only for cooling the ER after shutdown.
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Old 08-22-2018, 12:02 PM   #19
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I run the blower for about a half hour after turning off the engines to "cool" down the ER for the reasons stated. I wonder how much it helps, but figured it can't hurt.
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Old 08-22-2018, 01:44 PM   #20
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Twin Perkins 4.236 naturals here with no ER fans, just passive ventilation through 2 vents forward and 2 vents aft. The ER is roughly 12'x20'. I monitor outside and ER temps at the lower helm and the delta normally runs 20*F to 25*F. The most delta I've ever noted is 29*F.

This boat/engine pairing has been operated successfully for 41 years so I guess it's OK.
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