Heading and track are two different things. Due to wind and current the heading and track can be off by some amount of degrees
I've had to calibrate my fixed compass (the compass in water that sits on the dash). There's some screws on the bottom of it to adjust it. There's a compass app for my iPhone that I use to determine which one is accurate. I also have two heading sensors, the Garmin autopilot and the Garmin 24XD GPS receiver/Heading sensor I installed. When I saw that both heading sensors were within a couple degrees of themselves, but my dash compass was over 20 degrees off... I pulled out my iPhone compass and realized the two heading sensors were correct. Not saying this is your issue. Just say'ing, best to trust but verify. 🙂
Following up on this thread, I re-calibrated earlier this year which eliminated the crab walking I was experiencing. The autopilot didn’t align up to the Richie magnetic compass but that was bearable until two weeks ago when I was anchored in Gig Harbor swinging back and forth in a slight breeze and noticed that delta between the magnetic Richie compass and the garmin autopilot digital compass were wildly different at various points in the swing. In one direction, it would be aligned and swinging the other direction, it would be 20 degrees off. I pulled out my iPhone as recommended here and got yet another compass heading! Clearly everything is off a bit with some interference but what is actually correct?
My goal and desire is to ensure the magnetic compass is as accurate as can be. In case of an emergency, it reassuring to rely on something accurate but how do you know it’s accurate? For context, I believe, I’m the 3rd owner of my Ranger Tug 31S so not sure if the previous owners adjusted the compass any. Additionally, the previous owner installed a Garmin AIS 800 which is about a foot from the magnetic compass as well as the GPS antenna (it’s on my list to move this antenna to the mast).
I read that you can determine locations of interference on the boat by doing circles and continually shooting Azimuth to an object 2+ miles in the distance. If the azimuth is the same across the entire circle, there is no interference. To do this, I brought my military compass from home and went out yesterday to test it out. It was flat calm with no wind so perfect to minimize external influencing factors. I ran at idle speed, set autopilot to a circle for 5 minutes and continually shot azimuths outside of Des Moines to Three Trees point. Sure enough, I had 10 degrees deviation sitting at the helm. It was a bit better (5 Degrees) standing in the middle of the boat, next to the helm. The best location, was on the bow with only 2 degrees of deviation. I’m not sure on a small boat if one could ever get to zero but this was the best I had. When on the bow, I suspect I as getting some interference from the radar.
After several passes at 0, 180, 0, 180, 270, 90, 270, 90, 0, 180, 270, 90 and multiple magnetic compass adjustments to the Richie along the way, I was able to get within one degree between the Richie and my military compass sitting and the bow.
I then re-calibrated my autopilot compass and aligned it to match the magnetic Ritchie compass.
Finally, to validate the adjustments, I conducted a sweep to capture deviations, every 10 degrees from the magnetic compass to the autopilot compass. It was interesting to see how the two differed. In spinning the boat around a couple times the measures were consistent, some 0 deviation and others up to 2 degrees. In all, I was happy the results but it highlights that re-calibration is necessary (annually or bi-annually) as well as magnetic adjustments when new equipment is installed. 2 degrees is fine for coastal cruising IMO up to Alaska and back, not for Ocean bearing vessels. Regardless, I’m interested in testing things out.
As a follow on, I will plot sailing routes based on true north and convert those based on current magnetic offsets and compare those the magnetic bearings on the Richie compass.
Hope this helps others in adjusting their navigation instruments, both the magnetic compass and digital autopilot.
