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Replacing my "engine room" blower?

3K views 25 replies 9 participants last post by  downeast450 
#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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#3 ·
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.

 
#11 ·
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.
 
#5 ·
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.
 
#8 ·
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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#15 ·
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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#16 ·
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.
 
#18 ·
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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#17 · (Edited)
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.
 
#21 ·
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?

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#22 ·
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.
 
#23 ·
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.
 
#26 ·
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!

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