Replacing my "engine room" blower? - Page 3 - SailNet Community
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post #21 of 26 Old 11-18-2013 Thread Starter
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Re: Replacing my "engine room" blower?

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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.
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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post #22 of 26 Old 11-18-2013
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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.

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post #23 of 26 Old 11-18-2013
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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.
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Re: Replacing my "engine room" blower?

Blower selection guide on page 24...http://www.defender.com/pdf/Jabsco_V...on_Blowers.pdf

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Re: Replacing my "engine room" blower?

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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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I think it is true, but the question was specific to the intake.
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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!

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