Compas deviation

fred1diver

fred1diver

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Aug 17, 2013
1,309
Pearson P30 202 Ottawa
hey everyone, Quick question, it's,m'y first Time out this year and I've noticed major differences between my main compass and the reading on my fishfinder, some bearings are pretty close but some I'm reading close to 20 if not 30 degrees deviation, can there be that much difference between the two, or is my fishfinder going bananas? I'm questioning the fishfinder because it's having problem Viviane me the right depth, it's giving me readings of 300 ft when there is less than 50ft under me according to my chart
Thanks All in advance
 
Davidasailor26

Davidasailor26

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May 17, 2004
6,150
Beneteau Oceanis 37 Havre de Grace
Keep in mind that the fish finder is likely giving course over ground rather than actual heading (unless it's equipped with its own Flux gate or rate compass). Course over ground will be affected by current and leeway, to a greater extent at low speeds, so that might be part of the difference (although 20-30 degrees is more than I'd expect for that).
 
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Capt jgw

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Feb 8, 2014
1,300
Columbia 36 Muskegon
It's very common for then not to agree most of the time. The GPS heading is averaged over a few seconds and is about a second behind so you have to be holding a very steady course for it to be accurate. Position and bearing to destination are always good, but instant heading and speed are not.
On the other hand, everything electrical, magnetic, or made of iron or steel affect the compass, and it may be way out. Am ready way to check the compass using GPS is to set a waypoint on something you can see, then head directly toward it. The GPS bearing will be accurate and you can determine deviation for that heading. Repeat every 20 degrees or so and write it down.
 
Rich Stidger

Rich Stidger

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Feb 10, 2004
4,233
Hunter 40.5 Warwick, RI
You can have three different direction readings on your boat, none of which are likely to agree. The compass reads magnetic heading subject to any external magnetic influences on your boat and calibration. This magnetic heading is the direction your boat is pointed and may not be the direction you are moving. For instance, your boat could be on a river that is running south while your boat is pointed west toward the shore. The compass would read westerly and your actual movement over ground would be southerly.
The GPS needs to actually be moving to calculate a direction. If you are sitting still the GPS could read any direction at all. The GPS can also be set to read out in True or Magnetic, so you need to be sure you are comparing the same kind of headings. The GPS can only read COG or Course over the bottom. Depending upon wind and current this can be quite different from the compass magnetic reading.
Finally you have the bearing to where you want to go. And this heading may not match either the compass or the GPS.
Welcome to the world of piloting.
 
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rgranger

rgranger

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Jan 19, 2010
12,926
Hobie 16 & Rhodes 22 Skeeter Charleston
With your boat on the trailer, point the bow north, see what the compass says.... then west.... south and east....

That will take out all issues with leeway and currents....
 
JamesG161

JamesG161

SBO Weather and Forecasting Forum Jim & John
Feb 14, 2014
8,019
Hunter 430 Waveland, MS
Rich Stidger said:
The GPS can only read COG or Course over the bottom.
Click to expand
That is why my auto pilot uses a flux compass to maintain a course and NOT GPS.
After you set all to be Magnetic (True if you plan open ocean 300+ mile trips), you should trust you steering compass only (no beer cans nearby, lol) and use GPS cross checks on long over night trips. The GPS will give you two positions, draw a straight line between them for the average magnetic course over passage of say ≈5 nm.
Jim...

PS: My flux compass can be calibrated to match my magnetic steering compass. GPS COG is minor.
 
rgranger

rgranger

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Jan 19, 2010
12,926
Hobie 16 & Rhodes 22 Skeeter Charleston
JamesG161 said:
you should trust you steering compass only (no beer cans nearby, lol)
Click to expand
I assume you are referring to the effects of alcohol on navigation:cowbell: since aluminum is not is not ferromagnetic :wink3:.
 
JamesG161

JamesG161

SBO Weather and Forecasting Forum Jim & John
Feb 14, 2014
8,019
Hunter 430 Waveland, MS
rgranger said:
I assume you are referring to the effects of alcohol on navigation:cowbell: since aluminum is not is not ferromagnetic :wink3:.
Click to expand
Well a bit of both.:)
But...
Check out the full "Beer Can" compass deviation;). Maybe the iron content of beer?

Trivia: There are 3 Common magnetic metals (Plus many Rare Earth metals):wink:
1) Iron
2) Nickel
3) Cobalt
You can move an old Buffalo Nickel with a magnet!

PS: @rgranger , Also don't place your normal diet of canned Spaghetti O's or Pork & Beans cans near you compass.:cowbell:
 
fred1diver

fred1diver

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Aug 17, 2013
1,309
Pearson P30 202 Ottawa
Not sure what the compass reading is (true or mag) cant seem to have access to that setting Will get a hand bearing compass and see what deviation I get
 
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Pat

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Jun 7, 2004
1,250
Oday 272LE Ninnescah Yacht Club, Wichita, Ks.
I have heard that the magnet's on cockpit speakers can affect the compass.....Pat
 
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LeoLambert310

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Nov 18, 2013
171
Catalina 310 Campbell River
Using a GPS unit can be set for True or Mag if you use True then zero deviation, Mag will have the deviation for your location set by the GPS unit. If you want you deviation number look at any current chart and it will be stamped on it by the compass rose
 
Ken Cross

Ken Cross

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Oct 24, 2010
2,405
Hunter 30 Everett, WA
They are right about GPS showing movement not where the nose is pointed but there is one more thing. Magnetic heading from GPS are produced by an algorithm and that algorithm may very well have errors in certain physical locations on the globe actually it's common for it to have errors. Even on large commercial
aircraft those errors can be over 4 degrees.

Ken
 
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