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Go Back   SailNet Community > On Board > Gear & Maintenance > Replacing my "engine room" blower?
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Thread: Replacing my "engine room" blower? Reply to Thread
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Topic Review (Newest First)
11-18-2013 04:37 PM
downeast450
Re: Replacing my "engine room" blower?

Mounting anything on this "little" boat is a challenge. I have fabricated my share of mounts and made modification for almost every "upgrade" we have done, to this and to our other boats. If I didn't truly enjoy the process it might be considered a sign of dementia at my age.

I think I can fit a Jabsco 35760 Vent Blower at the aft end of the 1/4 birth, against (and through) the engine space bulkhead. Its cover will protrude up into the 1/4 birth space as a small "bump". It will be up higher than the one it replaces. Its cover can be insulated to deaden sound. The space it frees up will solve another problem. Another upgrade. Ha! Its like having a puzzle to play with!

Down
11-18-2013 02:08 PM
xymotic
Re: Replacing my "engine room" blower?

Quote:
Originally Posted by downeast450 View Post
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?

Down
I think it is true, but the question was specific to the intake.
11-18-2013 11:50 AM
Delta-T
Re: Replacing my "engine room" blower?

Blower selection guide on page 24...http://www.defender.com/pdf/Jabsco_V...on_Blowers.pdf
11-18-2013 11:44 AM
svHyLyte
Re: Replacing my "engine room" blower?

If you have room at your transom, a good continuous duty flange mount blower, discharging 250 CFM, is the Jabsco 35760 Vent Blower with a rated service life of 5000 hours. Ours operates whenever the engine is running but we also have a separate switch and I run the blower for 20-30 minutes after we've shut the engine down to help keep the engine heat out of the accommodation, except during the winter months of course.
11-18-2013 11:41 AM
Delta-T
Re: Replacing my "engine room" blower?

If you really want to know the actual CFMs, contact an HVAC company. They can measure using a Volumeter made for balancing systems.

You should be exhausting from down low and makeup air should come in high.

Here is an explanation of fan performance... ftp://www.nyb.com/Letters/EL-03.pdf

As far as what should the CFM be... if your engine compartment is 10 cubic feet and you are able to move 10 CFM, you would exchange all the air in that compartment in 1 minute. Running that fan for 5 minutes would exchange that air 5 times.
11-18-2013 11:09 AM
downeast450
Re: Replacing my "engine room" blower?

Quote:
Originally Posted by Delta-T View Post
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.
As I "study" this installation it seems a little shaky in terms of careful engineering specs. No reference to SP in the blower specs seems like an important oversight? It was built in 1976. "Its good enough" doesn't meet my requirements. It has lasted almost 40 years this way?

There are two shell vents on the stern. One connected to the blower discharge and the other one to a piece of duct hose that opens into the engine compartment near the stuffing box. I understand that one is the inlet and should reduce SP. It includes that 90 degree elbow at the vent (that is two) and the wire reinforced plastic hose. It would be interesting to figure out the actual cfm being moved by this system. I am guessing that the blower's capacity is used to establish the "actual" volume of air moved. It must be a fraction of the blower's capacity. Wire reinforced plastic ducting, restrictive shell vents, general configuration of the engine space and connecting lazarette, ??? It looks like a crap shoot to start with?

Down
11-18-2013 11:08 AM
Stumble
Re: Replacing my "engine room" blower?

Quote:
Originally Posted by downeast450 View Post
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?

Down
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...EBW4971-03.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.
11-18-2013 11:02 AM
Delta-T
Re: Replacing my "engine room" blower?

Quote:
Originally Posted by downeast450 View Post
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.

Quote:
Originally Posted by downeast450 View Post
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?



Down
All fans are rated for continuous running.

I edited my last post with more info....
11-18-2013 10:53 AM
downeast450
Re: Replacing my "engine room" blower?

Quote:
Originally Posted by Delta-T View Post
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?

Down
11-18-2013 10:42 AM
Delta-T
Re: Replacing my "engine room" blower?

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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