Couple of bilge blower questions

[Murv]

Some of you may remember my thread about my bilge blower (non working)

Long story short, the intention is to use it to blow air into the engine bay to help with engine cooling.

There is current flowing to the blower when it's switched on, but it doesn't run.
However, if I connect it directly to the battery, it does run.
The wire was corroded so I stripped it back a fair way to clean wire, reconnected and it still won't run.

I'm getting a voltage drop (compared to a direct reading from the battery) of nearly a volt through the feed wire, could that be the reason?
Also, the plan was to run it "in reverse" so that it pulls air in rather than blows. However, by reversing the polarity, the air flow is fairly weak from it. Is this down to the design of the blades that it runs far faster when wired with the correct polarity?

If it do invert it and get decent flow from it, is it worth me running it from a relay so that it's fired by the switch but draws power direct from the battery?
The wires that run into it (factory fitted ones) are quite slender so I can't imagine it's drawing vast amounts of power?

 

[BarryH]

I can't see that it will make much difference wether it blowing or sucking. Its moving air. Why reverse the polarity of the unit? Turn it around and connect the ducting to the other end!
When I had the last boat, the "blower" sucked air from the bottom of the engine compartment. Cool air was drawn in at a higher level. You could feel a noticeable draught on the intake side when the blower was running. It made bugger all difference to the engine temp....

 

[Cardo]

You'll probably find the blades are designed to turn in a particular direction. If they turn the other way they'll be massively ineffective.

 

[Murv]

Thank you, that's what I feared, it's a very feeble draught from the end of the hose!
Inverting the unit is obviously ideal, but it's extremely difficult to get to, hence my quick polarity reversal experiment.
Will check the output draught tomorrow though when it's wired "correctly," I'm just wondering if the design of it is more like a squirrrel fan rather than a simple propeller type where the direction of rotation might make a big difference.

 

[Marsupial]

Don't invert it (use it to blow into the compartment), if it blows air into the engine compartment it wont aid cooling as much as it could, which is why engine compartment fans usually extract the hot air. (we tried all the permutations back in the dark ages). You need two pipes one inlet that goes to the bottom of the engine compartment and one outlet from the top of the compartment with the fan in it; extracting. By arranging it that way you are sure the hot air is forcibly removed and the engine is fed with cool fresh air. Because there is an inlet pipe there is no discernible pressure difference between the engine compartment and the outside air. BUT, extracting also discourages fumes and smells from finding their way around the boat which would be the case if the fan pumped air in.

 

[Marsupial]

It made bugger all difference to the engine temp....

It wont that's not what its for, it is to supply cool air for combustion which will aid fuel economy and engine efficiency.

edit

And yours was fitted the wrong way round.

 

[BarryH]

It wont that's not what its for, it is to supply cool air for combustion which will aid fuel economy and engine efficiency.

edit

And yours was fitted the wrong way round.

Not on a petrol driven boat it wasn't..........

 

[Murv]

Fantastic, thanks very much, let's hope it pulls air more enthusiastically than it blows!

Any ideas as to the second part of my question, why it will only run direct from the battery?

 

[rob2]

why it will only run direct from the battery?

Knackered switch, poor contact/join, broken wire, blown fuse...?

Just check for voltage at every available point along the supply wire and you should find the problem. Almost every electrical issue on a boat is better understood with a multimeter.

Rob.

 

[Momac]

Fantastic, thanks very much, let's hope it pulls air more enthusiastically than it blows!

Any ideas as to the second part of my question, why it will only run direct from the battery?

I had a similar issue with a nav light. Thin wires causing a voltage drop. I installed thicker wires and everything works fine.

 

[NormanS]

Why not leave it the way it's meant to be, (and is) pulling hot air out of the engine-room? The engine will be pulling in all the air that it needs, and that air will cool the place. If you do manage to get above atmospheric pressure in the engine-room, by having the fan blowing, you will send fumes all through the boat.

 

[Rossynant]

Yes, + 1 for sucking out. If this was for gasoline (when it's forbidden for safety reasons) - just turn it the other way. Up-side down
But then it should suck the warm, from above the engine.

 

[William_H]

Some of you may remember my thread about my bilge blower (non working)

Long story short, the intention is to use it to blow air into the engine bay to help with engine cooling.

There is current flowing to the blower when it's switched on, but it doesn't run.
However, if I connect it directly to the battery, it does run.
The wire was corroded so I stripped it back a fair way to clean wire, reconnected and it still won't run.

I'm getting a voltage drop (compared to a direct reading from the battery) of nearly a volt through the feed wire, could that be the reason?
?

I imagine you have disconnected the blower and tested at the now not connected wires for voltage (not current). This shows the classic shortcomings of digital volt meter of very high resistance. It runs on (measures) almost no current so that you can read a voltage Ok when as soon as you try to draw current through the wiring it disappears.
If you measure a volt drop through the wire at the current drawn by the meter (typically about 1millinonth of an amp) then that is a really bad connection from corrosion etc.
So you need to measure the voltage to the blower with blower connected.
I expect you will find no voltage as a result of corroded wire or bad switch so track it down that way. Don't forget to check the negative line as well.
An alternative to useful fault finding is to use a decent sized lamp for checking power supply with or without a multimeter connected across it to measure actual voltage. good luck olewill

 

[Murv]

Good stuff, thanks all

I am going to connect it the "right" way around (after reading posts above) but the voltage problem was a little mystifying.
Now cleared up by William H though, thank you.
It's been a long, long time since I studied any electronics, I should've realised that any voltage drop is very bad news, especially one that large!

Cheers,
Chris