Quote:
I'll go ahead and run some tests on my Airmar when I'm on the boat next time and will share the results.
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So, I've made it to the boat the other day and took a few readings from compasses. Here's my approach:
0. Get out of the marina.
1. Pick a landmark or a large aton about a mile away that's marked on the chart and is clearly visible.
2. Use tiller pilot to turn boat to point as close to that landmark/aton as I could.
Record a reading from tiller pilot
screen as TP.
3. Check the reading from a regular
marine compass (the one floating in a ball of liquid installed in a bulkhead).
Record as MC.
4. In
OpenCPN, determine bearing to the said landmark (by pointing cursor at it on the chart and checking the reading in the right bottom of the screen). This one is based on my
GPS position and known location of the landmark, not actual compass, so this is a reference reading. Record as BNG.
5. In
OpenCPN, determine
current heading: hover a cursor over the end of the red heading line (not the COG line!) that's sticking out of the boat icon. The boat is rolling a bit, so the heading line drifts left and right a couple degrees. I picked approximate average (while making sure that the bow of the boat still points at the landmark), check the reading in the right bottom on the
screen and record as AM (Airmar).
6. Repeat 1-5 for each selected landmark/aton.
BNG AM TP MC
294 292 282 279
180 178 161 160
103 103 086 090
316 313 300 300
340 341 324 325
268 269 254 250
Airmar seems to be within 2-3 degrees from the reference, which is close enough that I never noticed the difference.
So, because I was taking this samples from a moving boat, I expected them to be all over the place, and was somewhat surprised by how consistent the difference between the readings turned out.
A bit more context on the setting:
- My home port is Edmonds, WA and that's where I calibrated Airmar after installing it, and that's where I took the above samples.
- For my regular
marine compass I never bothered to make a correction table because I hardly need it here in
Puget Sound.
- My Tiller-pilot is a
Raymarine ST2000 and I did calibration near Point Roberts before I sailed from there about a year ago. Clearly, it needs to be done again. I haven't noticed the problem before because again in familiar waters I navigate by line of sight anyways, and it keeps the direction okay regardless of what the numbers say.
Now that I've seen the deviation though, I might not be able to unsee it again, so you can probably catch me running circles in the
ferry lane sometime next week...