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3/11/2022 7:06 pm  #1


Compass issues

Would appreciate if any of you have suggestions / experience with this.

Problems: Heading is bad

1) We have heading pointing to leeward of COG on both tacks. Current arrows point perpendicular to windward on both tacks and actual current or leeway doesn't seem to be the problem (tested with it on/off and different coefficients). The problem seems to be exaggerated at higher heel angles, but have not tried to measure this systematically.

2) Heading will "blip" 20 degrees off every couple seconds from the main trendline which is on average off as described in 1)

Questions:

1) Has anyone experienced this and resolved the issue?
2) Could this simply be due to a bad compass calibration? Boat was moved from Annapolis to Antigua and no compass cal was completed pre-race, although we had same issue for delivery back to east coast (NC).
3) Is this sort of error consistent with magnetic interference?

Side-question: Is it possible to back-calculate TWA using COG in expedition to possibly salvage polar performance data from this time period?

Thanks

 

3/25/2022 4:55 am  #2


Re: Compass issues

You have moved your boat from one region to another with very different magnetic fields. So, yes, you should definitely re-swing.

You can ask Exp to use COG as heading, however some instrument systems allow you to do this internally.

See https://www.fieldyachting.com/question-and-answer/calibration/compass-re-calibration-only-when-the-boat-moves-locations-or-seasonally?searchTerm=compass+calibration


:::Campbell Field::::::www.fieldyachting.com:::
 

3/31/2022 10:59 am  #3


Re: Compass issues

You write "We have heading pointing to leeward of COG on both tacks". Could it also be that your heading (without leeway) is pointing to leeward of Course (with leeway) as well?

If so, it means your leeway has the wrong, opposite direction. The cause can be that heel has the wrong sign, the instrument system thinks the boat is heeling the wrong way and subsequently leeway also has the wrong direction.

Some instrument systems seem to handle the signs of parameters such as heel (and subsequently leeway) well and others don't.

Bad compass calibration does not affect performance data directly. However, compass calibration affects TWD. When doing the normal wind calibration trying to ashieve constant TWD through tacks, then you might end up with incorrect wind angles in your log files. 

I have seen occasional blips in heading (at least in the visual display) when using multiple sources in a NMEA2000 network. If you use an Actisense, make sure you ignore backup sensors until you actually need them. 

It should be possible to back-calculate TWA if you know your wind cal tables before (when using an incorrect table) and after (what the wind cal table should look like) but it is (A) hard work and (B) possibly still incorrect as correct wind calibration tables will look different depending on the daily conditions.

Last edited by barne (3/31/2022 11:04 am)

 

4/07/2022 9:59 am  #4


Re: Compass issues

Thank you both. I haven't had the ability to trouble shoot the problem further, but I appreciate the added context of your experience.

We partially suspect a new fridge compressor in the vicinity of the compass...

     Thread Starter
 

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