Pedestal Compass, True North?
- Thread starter BAD ORCA
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Heya, heya, heya step right up.A spinny wheelie turny gizmo? Where's the pointer?!?
Spin the pointer, land on the ✮ and win a kewpie doll.
Of course I have paper charts, why I even plot my course on paper charts sometimes.
Use the right tool for the right job, sometimes it is much easier and clearer to use paper than electronic charts.
Will, the inner rose is a magnetic rose, the outer a true rose. For those of us too lazy to do the math we can plot magnetic courses off the inner magnetic rose.
In B.C. you always plot your long distance courses on a paper chart. A common course for us is from Vancouver (lower ⦿) to Port Hardy (upper ⦿) a distance of about 300 NM as the fish swim. It could be done on a GPS but I'm far more comfortable when I see it full size, spread out over a table following a pencil line. We then transfer the chart coordinates to the GPS.
I think your being a bit unfair there Stu. You've been very helpful in answering my questions in the past so i'll cut you some slack and chalk it up to a bad day.
I understand completely that compasses dont point to true north. Most good quality hiking compasses do have an adjustment though to set declination to true north. I've never had to use it and admitedly dont understand the nuances of compasses. I simply hav'nt had to yet.
Knowing very basically that there is a mechanical way to offset for decliation it got me curious and was wondering if sailboat compass makers attempt to adjust for this. From the responses it became clear declination depends on where you are.
Dont forget Stu, you were not born all knowing and i hope your not so full of yourself or arrogant to think your so smart you never asked overly simple questions.
Im immensly absorbed in self learning. Im rebuilding every system on my sailboat on my own with no other previous knowledge of sailboats. I've learned in the past few years what many men take a very, very long time to learn, and to my credit, i'm doing one hell of a job.
I'm a master craftsman at my trade and near the very top of my profession. I've worked harder than most men do at their craft. You dont get that good by being a simpleton asking simple questions, or have you forgotten? I hope your not such a bitter old man you cant remember that we only have so much time and that we have to devote our time wisely. I'm currently tackling some major issues on my sialboat and have way more important things i need to worry about, and an incredible amount on my plate between my day job, and my boat restoration. I'm sorry i offended you that i dont have hours of spare time to devote to learning the granularity and nuances of compasses. I'm sure that will come when im ready to do some long distance sailing. Thats something for another day.
Sometimes we're in a hurry and just want a quick answer. Internet forums were intended to 'chat' lest you've forgotten? To answer your question...i am using the internet. There are no rules on the complexity or depth of the questions that can be or should be asked in any forum. It gets annoying sometimes when people like you seem to determine on your own what the rules should be....but i digress...
Anyways....i'll have a beer tonight after a hard days work and toast one to you...Maybe we'll cross paths some day when your not so cranky and we can put our feet up and drink one together.
Wait!!! You can actually get from Vancouver to Port Hardy by boat?!?! That chart shows too many "things" in the way.
In all fairness, your original statement (post #1) could have been better written. You gave the impression to both Stu and myself (and maybe others) that you did not know whether a magnetic compass points to mag. north or true north. Like it, lump it, take it or leave it, that's the way it came across. Not too reassuring from the owner of a 36 ft. sailboat.
Should have inserted the above statement (post #27) in your first post and avoided the misunderstanding. Same as carefully reviewing an e-mail to your boss before sending.
I agree, maybe your right, my bad. I was in a hurry when i asked. I will be more careful next time , but I thought the words 'set to' implied that i knew about the existence of declination and it was an adjustment to true north. In my defense, I didnt think it was an improbability that a compass on a ship, which seems reasonably more sophisticated than my cheapo hiking compass, would'nt have that ability to be adjusted internally. /shrug...we cant know everything.
And no problem taking my lumps and bumps, but im not one to stand there and be humiliated either. It seems to be a trend in online sailing forums these days. Seems many threads these days, quickly devolve into shit slinging fest for some reason...?
Anyways dont want to beat a dead horse, i'll relieve myself from this forum. Thank you for your input. I always appreciate it.
I'm sorry that some people on here were harsh with you. Please don't leave the forum because of that.
Re-reading your original post as a result of the kerfuffle, I see more precisely what you mean. Yes, I think that when a compass is swung and calibrated by a pro they align things so the deviation is 0º at North. I can't recall exactly, and my compass book is packed away somewhere, but that's my vague recollection. Then there would be deviations marked for other headings on the compass deviation card.
Also, the compass deviation card only applies when everything on board is in the place it was when the compass was calibrated, your hand bearing compass included. It doesn't take much to throw off the steering compass. I've had issues like that with the compass in my binoculars, steel scuba tanks in the lazarette, the handheld VHF, and so on.
I put this all to good work when I compared my autopilot to my pedestal compass. With the autopilot on course 000* magnetic the compass was identical. I continued getting within a degree at all the major directions. We’ve found our way home again so it must be good enough.
Autopilot Heading Does Not Match GPS Heading (COG) | Garmin Customer Support
"At low speeds, the GPS Heading (COG) is not a reliable heading indicator. At high speeds, the GPS Heading may be accurate, but it indicates the absolute motion of the boat including the effects of winds, tide, and currents. The magnetic heading from the Garmin Autopilot only indicates which direction the bow is pointing.
If heading from the autopilot appears to be off, check the compass' (Reactor) mounting location for magnetic interference with a handheld compass, ensure that it is securely mounted, then perform the Compass Calibration and Set North procedures again."
-Will
"At low speeds, the GPS Heading (COG) is not a reliable heading indicator. At high speeds, the GPS Heading may be accurate, but it indicates the absolute motion of the boat including the effects of winds, tide, and currents. The magnetic heading from the Garmin Autopilot only indicates which direction the bow is pointing.
If heading from the autopilot appears to be off, check the compass' (Reactor) mounting location for magnetic interference with a handheld compass, ensure that it is securely mounted, then perform the Compass Calibration and Set North procedures again."
-Will
In my experience my GPS COG matches my electronic compass very closely even at slow speeds. The only real exception is at slow speed when turning - the COG shows the movement of the stern, where the GPS is located. My pedestal compass matches the electronic compass on some headings, but is a few degrees off on others. Beneteau mounted the chart plotter with its magnetic SD card door closer to the compass than spec, so that probably throws things off a little.
For our uses on the water, compass precision is not as important as compass accuracy. It really doesn't matter if your compass is off a degree or two, +/- 5° is close enough, that's about the precision our pedestal compasses.
Accuracy and precision are related but different. Compass precision is the amount of error in the reading, if the true heading is 000° M how far off that heading is precision, the more precise the compass the smaller the difference between the displayed heading and the true heading.
Accuracy means does the compass point more or less north when headed north, east when going east, etc.
Trying to steer a precise compass course will make for a very poor helmsman. In any kind of seaway the boat will naturally swing through an arc of as much as 20 or 30° left or right due to the sea state, winds, and surfing down waves. The goal should be the average course. Does it really matter if a more precise compass shows us swing 19° to 29° in stead of 20 to 30? I think not.
The other factor to consider is distance and mark 1 eyeballs. The closer we get to our waypoint, the more clearly that we should see it, so we rely less on our compass and more on our eyeballs. In the case of a waypoint that is mid-ocean our gps's are quite accurate at finding those to within 6' if that level of precision is necessary, which mostly it isn't.
Let's relax about compass and keel-haul anyone who stores large iron objects near our expensive and precise electronic compasses.
And how accurate are gps? And how accurate are charts? Take a look at Second Star's track down a portion the ICW, The dotted yellow line is where we "motored." Looks like we turned into a swamp buggy.
Accuracy and precision are related but different. Compass precision is the amount of error in the reading, if the true heading is 000° M how far off that heading is precision, the more precise the compass the smaller the difference between the displayed heading and the true heading.
Accuracy means does the compass point more or less north when headed north, east when going east, etc.
Trying to steer a precise compass course will make for a very poor helmsman. In any kind of seaway the boat will naturally swing through an arc of as much as 20 or 30° left or right due to the sea state, winds, and surfing down waves. The goal should be the average course. Does it really matter if a more precise compass shows us swing 19° to 29° in stead of 20 to 30? I think not.
The other factor to consider is distance and mark 1 eyeballs. The closer we get to our waypoint, the more clearly that we should see it, so we rely less on our compass and more on our eyeballs. In the case of a waypoint that is mid-ocean our gps's are quite accurate at finding those to within 6' if that level of precision is necessary, which mostly it isn't.
Let's relax about compass and keel-haul anyone who stores large iron objects near our expensive and precise electronic compasses.
And how accurate are gps? And how accurate are charts? Take a look at Second Star's track down a portion the ICW, The dotted yellow line is where we "motored." Looks like we turned into a swamp buggy.
@BAD ORCA I had no problem with your question or understanding your question. This is a forum where friends get together and discuss these issues, typically without getting brow-beaten over the research that may or may not have been done before asking the question. I've done my share of criticism on these forums but usually not until it's deserved. By no means did you deserve to be treated as a simpleton for the act of asking this question. Your question was logical, after all, and there were several relevant answers.
By no means did you deserve to be treated as a simpleton for the act of asking this question. Your question was logical, after all, and there were several relevant answers.
An old engineering saying is that "Precision is repeatability, accuracy is truth!" So, your compass can be off by 3º on a particular heading, but is it always off by the same 3º on that heading? The former is accuracy, the latter precision. Precision is perhaps more important that accuracy, as long as you know the accuracy limitations of the instrument.
Oh no i meant that particular t
Not permenantly...just to go enjoy a Bourbon and a cigar.
Interesting that we here around 90degrees long, 30 lat, have had the luxury of having magnetic and true be exactly the same. Today it is off by a little bit but not enough to worry about when steering in waves..
Lots of compass stuff was drilled into my head when studying for my pilot's license in 1963.. I remember that the written part of the test asked for magnetic headings on a long trip where the variation changed a good bit. The compass can overshoot or undershoot when turning or when accelerating or decelerating. factors when flying, but not so much in boating at 6 kts. Good Stuff
Lots of compass stuff was drilled into my head when studying for my pilot's license in 1963.. I remember that the written part of the test asked for magnetic headings on a long trip where the variation changed a good bit. The compass can overshoot or undershoot when turning or when accelerating or decelerating. factors when flying, but not so much in boating at 6 kts. Good Stuff