PG700 and Compass Deviation

[MarMax]

When on the water in a fixed position like tied up on the dock, the PG700 is typically reading 5 degrees less than the Ritchie compass. The Navpilot is set to Magnetic in the Heading Display.

When parked in the garage (which is a pre-fab metal building), the difference is 37 degrees. There is no electrical equipment near the boat, the floor is concrete and is the closest to the PG700 at 3 feet. The building walls and ceiling are 7 feet or greater away from the PG700 and compass. The compass is probably a minimum of 7 feet from everything.

The PG700 and compass readings do not change in the garage when the boat's electronics are powered up or when shore power is connected. The PG700 reads 332 degrees and the Ritchie compass reads 9 degrees pretty much all the time in the garage.

I can see how the metal building would affect the magnetic field but not along the lines of 32 degrees (37-5=32). There is no problem here, just curiosity. Everything works fine on the water. Any ideas, and should I at least adjust the compass offset 5 degrees in the Navpilot?

 

[C-Bass]

Good morning Marmax,

Thank you for your inquiry and observations. I don't really know what to say about the PG700 changing so dramatically when compared to your Ritchie compass when the vessel is in the Garage. But I guess it only matters when you are on the water, that is where we need the results.

When I check compass calibration I will look at a couple of different points of reference:

1.) I usually check the PG700 at a minimum of four different directions. Typically this is North, South, East, West. I also try to look for a calm day, with minimal wind, and if tidal influence is a factor I try to check around a slack tide.
2.) I will usually check the PG700 to Course Over Ground. If the vessel has the ability to get on step or is a planing hull I will check the 4 points with some speed, so if there is tidal influence or side wind that can cause crabbing it will somewhat be negated the faster you go. But ideally at the four directions you are looking for somewhere between 2-5 degrees of difference between your COG and heading. Please keep in mind, if you find a direction that is off more than the others that is what we try to bring back inline. It can be difficult though if you make an adjustment in one direction, it will certainly affect the opposite direction. So when making corrections it is best to start out slowly.

The PG700 compass calibration works pretty darn good. If you find that the compass completely off from COG more than the 2-5 degrees then I would re-calibrate the compass and pay close attention to what is around the compass when you perform the calibration.


Kind Regards,

 

[MarMax]

Thanks for the tips C-Bass. I'll check things out next time on the water and adjust as you have recommended.