Angele
Well-Known Member
Galadriel's thread on his new compass got me thinking. As alluded to in one of my postings on that thread, there is significant error between my compass heading and the heading shown on my autohelm display. Not on every heading, but on some, and the extent of the error varies - the two agree which way is north, at least to within a degree or two but, when heading WSW for instance, they disagree by about 15 degrees.
Up to now, that hasn't really been a problem for me. I ensure that my crew are aware that the two compasses might read differently. If I ask someone to steer a course, they line it up on the compass and then, if they wish to read off the heading from the autopilot display (or, if they want to turn the autopilot on), I ask them to note what the display is actually showing and then they can steer to that.
The compass was swung by an adjuster and I have a deviation card in which the maximum error is tiny. I therefore believe this to be accurate. All of the nav instruments (depth, log and electronic compass) were calibrated by the agent who supplied the boat to me. I have since recalibrated the log as it was underreading, the depth is accurate, but I have not recalibrated the electronic compass - purely laziness on my part.
I cannot be sure that all of the dealer fit options I had supplied with the boat had been fitted at the time the calibration of the electronic compass was done. One of these items is an inverter, which is probably nearer to the fluxgate than would be ideal, but would be difficult to move. Also, since taking delivery of the boat, I have stuffed various things into lockers in and around the location of the fluxgate. Mostly these are non-metallic, but one or two (e.g. an extension lead) could be causing a deviation error.
I am aware that the fluxgate compass reading is used by my chartplotter to derive various data, such as true/apparent wind bearing (as opposed to relative to boat), implied tidal vector, etc. None of these is really that important and I can mentally adjust for them. However, my trip back across the channel at the weekend (overnight and in fog) has highlighted to me that this compass error is affecting my radar display (since I use it in N-up mode, overlaid on the chart) and the resultant MARPA readings. As this is a safety issue
, I now accept that I really must sort it out.
So, I have resolved to recalibrate the electronic compass next time I am on the water. My understanding is that, to do so, you first of all point the boat north and then motor around slowly in circles, pressing various buttons as you go. Easy, peasy. However, before I do, one or two questions to knowledgeable forumites so that I can get a feel as to whether I can expect recalibration to be effective:
1. Is it possible that the cause of the error, which varies depending on the heading but is zero when pointing north, is down to poor calibration originally or is it more likely that it has been introduced subsequently by items placed near the compass, or is it impossible to tell?
2. If the answer to 1 is the former, what might have been the cause of the poor calibration (e.g. if the circle being steered whilst calibrating was not steady, which might have been the case if the bow was being blown off by a strong wind, would this have been likely to cause the problem)?
3. Does recalibration both reset due north and then remove deviation caused? Or, if something like the inverter is the cause of the deviation, is this impossible to correct purely from recalibration?
Your answers/comments/suggestions gratefully received.
Up to now, that hasn't really been a problem for me. I ensure that my crew are aware that the two compasses might read differently. If I ask someone to steer a course, they line it up on the compass and then, if they wish to read off the heading from the autopilot display (or, if they want to turn the autopilot on), I ask them to note what the display is actually showing and then they can steer to that.
The compass was swung by an adjuster and I have a deviation card in which the maximum error is tiny. I therefore believe this to be accurate. All of the nav instruments (depth, log and electronic compass) were calibrated by the agent who supplied the boat to me. I have since recalibrated the log as it was underreading, the depth is accurate, but I have not recalibrated the electronic compass - purely laziness on my part.
I cannot be sure that all of the dealer fit options I had supplied with the boat had been fitted at the time the calibration of the electronic compass was done. One of these items is an inverter, which is probably nearer to the fluxgate than would be ideal, but would be difficult to move. Also, since taking delivery of the boat, I have stuffed various things into lockers in and around the location of the fluxgate. Mostly these are non-metallic, but one or two (e.g. an extension lead) could be causing a deviation error.
I am aware that the fluxgate compass reading is used by my chartplotter to derive various data, such as true/apparent wind bearing (as opposed to relative to boat), implied tidal vector, etc. None of these is really that important and I can mentally adjust for them. However, my trip back across the channel at the weekend (overnight and in fog) has highlighted to me that this compass error is affecting my radar display (since I use it in N-up mode, overlaid on the chart) and the resultant MARPA readings. As this is a safety issue
, I now accept that I really must sort it out.So, I have resolved to recalibrate the electronic compass next time I am on the water. My understanding is that, to do so, you first of all point the boat north and then motor around slowly in circles, pressing various buttons as you go. Easy, peasy. However, before I do, one or two questions to knowledgeable forumites so that I can get a feel as to whether I can expect recalibration to be effective:
1. Is it possible that the cause of the error, which varies depending on the heading but is zero when pointing north, is down to poor calibration originally or is it more likely that it has been introduced subsequently by items placed near the compass, or is it impossible to tell?
2. If the answer to 1 is the former, what might have been the cause of the poor calibration (e.g. if the circle being steered whilst calibrating was not steady, which might have been the case if the bow was being blown off by a strong wind, would this have been likely to cause the problem)?
3. Does recalibration both reset due north and then remove deviation caused? Or, if something like the inverter is the cause of the deviation, is this impossible to correct purely from recalibration?
Your answers/comments/suggestions gratefully received.
