EMF interference

[Randygh]

I'm going to install a small 12 v defogging fan on my boat's dash. I'd like to mount it about 9" away from my Garmin 172c internal antenna chartplotter and 12" from my Garmin Fishfinder and magnetic compass. I realize the magnets in the fan's motor may cause deviation of my compass, but will the motor emit electromagnetic waves that would interfere with the GPS or Fishfinder? I know there are many gurus here that can answer this. Thanks for the info.

[heymagic]

It's never bothered any of my boats, I'm going to add acouple on the Chris.

[Seapuppy]

well...emi is a funny thing...it really depends on the amplitude and the freq. that and and the amount of shielding that is installed on the system...for a plastic motor..the mag. compass is verrrrrrry sensative to any magnetic fields within the field of view of the compass...the best thing to do is turn the fan on and move it around the compass....see if it changes directional heading.....then turn the other eqpt on one at a time....as long as you don't run any pwr cables near the exisiting signal wires from the transducers to the head..it shouldn't affect the overall performance of that unit...then shut it off and go to the next one..gps..the antenna shouldn't be affected...but the pwr cable if ran close may cause a deviation or signal interrupt....
imho...these units operate at a low power rating and have minimal shielding for emi....(EMI = Electro magnetic interferance).......
so don't run a pwr cable along with a signal cable....or near an antenna.....just my two cells firing off again......at least that's what we did on subs..... :argh arr

I realize the magnets in the fan's motor may cause deviation of my compass, but will the motor emit electromagnetic waves that would interfere with the GPS or Fishfinder? I know there are many gurus here that can answer this. Thanks for the info.

Nope. You're only talking about DC current. EMI don't happen with direct current. Keep it well away from the compass, but (as usual) Steve and I disagree. The GPS is receiving microwave RF(1.023 , 1227.60, and 1176.45 MHz), and the fishfinder is looking at low frequency RF (50Khz & 200Khz)... so they could care less about DC motors. You're fine.

[Go Aweigh2452]

yeah, what they said.... I had a fan next to the compass and the GPS. I'm thinking of locating it nearer to my fish finder so that it may interfer with it and actually show fish on the scope...

Only kidding. But I did have my fan between the compass and the GPS... never had a problem.

[Stratocaster]

#1: I remember in my Power Squadron course a few years back, the instructor took an aluminum Coke can and put it beside a compass. What do you think happened, since aluminum is 100% non-magnetic?
The compass swung by about 10 degrees. Try it yourself!

#2: On the last boat, I was going to install a Radio Shack remote VHF speaker oriented beside my right shoulder while sitting at the helm. It was a good 4 feet from the compass. But it pulled so hard on the compass that it was unreliably erratic (10-30 degrees) all the time. So I jettisoned that idea.

So I guess my point is, keep all metal and magnets as far away from the compass as possible.

What do you think happened, since aluminum is 100% non-magnetic?


What's important to understand about compasses isn't that they'll be thrown off by "magnetic" material... but they'll also be thrown off by anything that can block or divert a magnetic field (what the compass is looking at to begin with).

Aluminum IS magnetic btw... At least so far as when placed in a magnetic field an aluminum can produces eddy currents... which themselves induce magnetic lines of flux... which bend those of the earth's. Anything that conducts can produce eddy currents.

[Seapuppy]

lets just say that I disagree with mike also.....and we'll let it go at that..... :argh ar

Lemme guess... you're an electronics guy now? I thought you were a submarine electrician type?

Esplain the whole DC RF thing to me Steve? (no change in polarity = no change in electromagnetic field = no electromagnecic radiation) Yes?

[Stratocaster]

SS, I agree that the Coke can thing makes sense...but my point was, I bet 9/10 people would think it would have no effect.

[Randygh]

Good thing I'm not installing a radome!! Don't want to go there again. BTW Doug--at what angle did you install your new radome?

I appreciate the info. I didn't know, or more likely forgot my college physics, that taught me DC current doesn't produce EMI. Do nonmetallic objects bend the earth's magnetic forces? Will a plastic or glass object (coffee cup) located next to magnetic compass induce deviation or instability?

I have not determined deviation for my compass. Is there a simple way of determining compass deviation without sending somewhere? Can I determine deviation by comparing my GPS heading with the compass heading?

...but my point was, I bet 9/10 people would think it would have no effect.

I agree. Most wouldn't. That's a great example.

Anything that can conduct... can induce a magnetic field... and therefore cause magnetic field interference. That's a far cry from EMI that would interefere with a microwave device or an RF device like a GPS or Sonar transducer.

To check your deviation. the easiest way to be dead on is to check it against a chart of the area you're boating in. If you're on a map heading of say, 270 degrees and pointing directly at a known landmark... your compass should match (as should your chartplotter).

The U.S. Geological Survey provides MagVar data for every updated nav chart. Look on there to get the standard deviation for the area you're in. It's a LOT around here, particularily around Whidbey Island... Lots-o-Rocks there.

[3788sam]

Randy you can't send compass some where to be checked. Compass deviation is caused by the equipment on board effecting the magnetic fields around the compass and thus the devation.
To check deviaton you have to be in a marked area with known headings marked from that spot. You just point the boat at the know heading mark an read the difference on your compass + or - the true heading. I don't know of a spot up your way and have only ceck my compasses deviation cards once since I owned our boat. (up in Anacortes, I had Cap Sante show me where they do the Certs.)

[Stratocaster]

Hey I have a question. I'm playing my iPod on the boat stereo using the FM transmitter. When I press one of the buttons on my Snake River tank level sensing system, I get loud static on the radio. But only when the iPod is playing. Regular FM radio is unaffected.

So my question: Isn't this EMI produced by DC?

[Tedster]

Ken the only way to acurately check a compass is on the vessel, that way they can make adjustments from interferences of equiptment. Ted :D

[Go Aweigh2452]

Randy, my radar is sitting flat on the top with no angle adjustment. I find that it really is not that important to make the radar perfectly horizontal to the water. Due to the width of the beam, I can easily see objects (seagulls too) at almost any angle at any speed... even when coming onto a plane, I still see very well with it...

now as far as coffee affecting a compass, if it is true Navy coffee then the answer is a BIG yes... due to the high metal content of all Navy coffee... I think both SS and Seapuppy can agree on that one... OK, only jk...

I was able to adjust my compass using a wide open parking lot (no light poles around) and using my GPS as my heading source. I took two readings... with electrical equipment on and off. I swung the boat around pointing East (90) first, determined the deviation error, (you can start pointing west first if you like but must use east or west first) then using the fore & aft screw at the base of the compass, corrected the deviation. I then swung the boat 180 degrees (West) and again set the fore & aft screw to 1/2 the deviation error. I then swing the boat to North (360 or 000) and note the deviation, removing it with the athwartship screw. I then swung the boat 180 degrees (to 180) and again remove 1/2 the noted deviation. Now I move the the quardant headings (NE, SE, SW, NW in that order) removing 1/2 the deviation each time. I now note the remaining errors but in my case they were so small that I don't require a Napier diagram (curve of deviations graph) or a compass error card. It is within 2 degrees which is better than I can maintain on course anyway... Also, the errors on my boat were almost identical with electrical equipment (mainly GPS) on or off... Remember, you won't need the mag compass unless you lose your GPS! That is why you want to ensure your compass error is noted w/o power from GPS etc..

BTW, I have a big As$ compass (Ritchie super sport 1000) so it is real easy to read and calibrate. One other important note... use a non-magnetic screw driver to set the compensating screws at the bottom of the compass. A cheap plastic knife blade works well... the screws turn easily...

OK, I'm done...

You can do this on the water but it is a pain in the As$...

One other important note... if your compass is not exactly lined up fore and aft with the boat... you're wasting your time doing all these deviation fixes...

BTW, since I now have my radar and new display unit, I will have to recalibrate again to make sure I am OK...

[Randygh]

Doug--Did you have your Admiral drive the truck in the parking lot while you observed the GPS and compass? Seems to me it would be quite tedious getting in/out of the boat moving the truck to get exact headings, 180, 90, 270, 0 and in between. I think those little handy walkie-talkies at Costco would be great to facillitate communication between the captain and Admiral (Admiral in truck, captain in boat). Now I can justify purchasing them, definitely needed for compass adjustment which is a safety feature. :joker:

[Seapuppy]

Strat just suggested a perfect example of emi..emi can be manifested in a varity of ways....thru the radio or thru a adjoining system...whether it produces a magnetic field or a harmonic freq. that comes over the fm ...it still produces an emi effect...mike..I am and have been (like I've tried to tell you before..) an electrical systems integration design engineer...subs are full of stuff both ac and dc that can produce an emi signal into another system.....any system whether ac/dc can produce emi into another system..via radio freq or induction.....we had a program called EMI/EMP requirements for all subs and surface combat ships....I've forgotten the exact mil-spec..but it has all the data that is associated with emi/emp...noone needs to worry bout emp here....but the emi is and can be induced into another circuit via magnetic induction.....coil a wire with dc running thru it and doesn't it produce a magnetic field???....of course it does...run dc thru a wire and it will produce magnetic flux lines all around it....you saw my radio speaker mounted funny on my dash..that's cuz anywhere else on the dash..it caused the mag. compass to swing off mag. north.....all I'm saying with his other systems is that he should be careful as to not run his transducer cables near a wire that is AC that can produce errors of some sort....an ounce of prevention type of thing...a boat with both ac and dc is an emi nightmare....get a coil and an oscope and walk around a boat one time with the systems on and you'll see flux lines everywhere you never thought possible.....end my side of it....you can argue all you want on it..but emi is all over a fiberglass boat..there is no shielding for anything other than the vhf radio antenna.... :argh arrrr

OK Steve... in your world a DC fan can interfere with a 2Ghz Microwave signal from an orbiting GPS satelite. What color is the sky there?

Not to start an argument, but there is a BIG different from an aluminum can blocking a compass reading (not creating EMI btw) and somehow saying a heater fan would interfere with a GPS receiver or a fishfinder.

It's no big deal as long as Randy gets the straight scoop.
Would I mount it on the dash near a compass (like your speaker?) ... No.
Will it interfere with anything else on your boat? No.

[Go Aweigh2452]

Randy, actually, I unhooked the boat from the trailer to minimize the impact (at that time) a GMC pick up could possibly cause. Moving the boat is pretty easy and you really don't have to worry about being exactly at 90/270/000/180. Being w/i a few degrees is just fine. Besides, if you want to be anal, you could mark the parking lot? Real easy to see where 90/180/270 are when you see the first heading correctly lined up... that is the reason I use a parking lot...

[Roel Jansen]

I just wonder about the interference of the Coke can.
Is it a diet coke or a regular one and is the can full or empty? :D

A Compass fault of 2 degrees? That's how Columbus discovered the Americas! :?

[Lazy Flamingo]

Hoi Roel,
It was a regular Coke can, 1/2 full of Heineken distilled in the brewery
in Amsterdam .... :roll: ( see you in the fall )

[Go Aweigh2452]

yeah, well 2 degrees can be a lot in a big boat with lots of range... but in a small boat with maybe a few hours gas, your bound to find something you are pointing the bow at... I doubt many small boaters can actually keep their heading w/i 2 degrees w/o concentrating like no tomorrow... (flat water the exception) now throw in auto pilot and I'd be happy... in any kind of weather... almost... :oops:

Anyone know how accurate an autopilot on a small boat (23-30 feet) really is?

[Randygh]

I have only really used a compass once while on the water.

I was a teenage kid salmon fishing with my dad and brother at Ilwaco. I was piloting dad's 21' Tolly when we crossed the bar and headed out 10 miles to the lightship. Of course dad never purchased a navigation chart in his life. I took note of our westerly heading while traveling out to the lightship and once we started fishing I observed our drift south along the Oregon coast. A fog bank was way out to sea. We started catching fish and didn't realize our peril until it was too late when we were overtaken by pea-soup fog. Kinda scary, couldn't hear the lightship foghorn and had absolutely no sense of direction. I subtracted 180 from our outgoing heading and guessed on the southern drift. After an hour or so of travel we broke through the fog and were heading dead on towards the bar. We were lucky. Dad never had the compass calibrated for deviation and I'd never heard of it. The gist of the story is that several degrees of deviation wouldn't have made much difference but the simple little compass probably saved our butts.

I wonder how the tilt of my radar scanner will affect the compass deviation from a coke can? Does it matter that I prefer Red Hook over Heineken? What if the can is made from electropolished stainless? :lol:

Raul

There's a big difference between something simply "working" and "working to it's best potential"

Tilt it 5 degrees up... drink Millers High Life from a foam cup and mount your magnetic speakers right there on the dash... it's all good...

[... drink Millers High Life ]

Arrrghhhh!!!!

[Lazy Flamingo]

He could have said:
"Old Milwaukee" or "Pabst Blue Ribbon" .... http://www.websmileys.com/sm/drink/trink42.gif

[Go Aweigh2452]

or Red Stripe or Balintine

[Roel Jansen]

I know some one here likes to drink Rum and Coke. (Cuba libre) Is that name today still used for it?

If you have that premixed in a can the deviation will make your boat going into circles only I think. :)

[heymagic]

Very correct Roel, rum and coke in a can causes EMI.....Easily Mistaken Intentions....I thought I wanted to boat in straight line, but I was mistaken.

If you're in Florida a Cuba Libre may cause your boat to head towards Havana.

Raul

PS: In Puerto Rico the drink is still called Cuba Libre.

[Randygh]

I installed the 12v defogging fan on my boat's dash last night. As far as I could tell there was no affect on the compass. The little fan is a real gizmo and will come in handy for helping keep the windows clear.

Has anyone used the RainX product that is applied to the inside surface of cabin windows to prevent fogging? I sure like RainX on the outside surface of the windows. :dontknow:

[Go Aweigh2452]

I use the RainX defogging stuff and it works well. I too use RainX on the outside glass after I clean the windows with a gentle scouring cleanser like Barbers (not comet or ajax)... For the tough stuff, I use oven cleaner, let it set for a few minutes and remove, wash it all down then do the windows with RainX. I'm sure CLR would work just as well to get rid of the hard water stains... but never tried it.

[heymagic]

Somebody..Ajax, Windex??..makes and anti-fog glass cleaner also. I've used the defog wipes on race car windows also, available at fine automotve parts stores everywhere...