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Old 10-04-2012, 17:46   #1
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Compasses

I just had my Ritchie compass serviced by Ritchie and was all excited to mount it and set it based on various other sources ( hand bearing compass...new btw, other vessel compasses on the same heading, and lastly...my smart phone). To my dismay, amazement and horror non of these agreed with eachother.
They say your last hope of salvation if your electrons quit flowing is to rely on the ships compass. In all honesty, I have my doubts. Not only is my compass in question but there was little to no agreement between instruments on a number of vessels right around me. What is an offshore sailor to do... What would Bernard do.

I'm open to ALL kinds of comments but since I started to try and get some sort of consensus on the subject of compass reliability... I have opened a few eyes.

Bring it on...
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Old 10-04-2012, 17:52   #2
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Re: Compasses

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Originally Posted by Cruising Moxy View Post
I just had my Ritchie compass serviced by Ritchie and was all excited to mount it and set it based on various other sources ( hand bearing compass...new btw, other vessel compasses on the same heading, and lastly...my smart phone). To my dismay, amazement and horror non of these agreed with eachother.
They say your last hope of salvation if your electrons quit flowing is to rely on the ships compass. In all honesty, I have my doubts. Not only is my compass in question but there was little to no agreement between instruments on a number of vessels right around me. What is an offshore sailor to do... What would Bernard do.

I'm open to ALL kinds of comments but since I started to try and get some sort of consensus on the subject of compass reliability... I have opened a few eyes.

Bring it on...
read up on compasses...there's lot's to learn before even asking the question...

some can be adjusted...all will require tables to use...fortunately the days of struggling to adjust them are over if you have differential or WAAS gps onboard to make the adjustments.
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Old 10-04-2012, 18:02   #3
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Thanks. I used my fluxgate compass and am still 20 degrees from being able to zero it in when I run out of adjustability.
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Old 10-04-2012, 18:04   #4
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Also...the point is to be able to trust the ships compass if/when the lights go out.
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Old 10-04-2012, 18:36   #5
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Re: Compasses

Oh, the days of cruising up and down the degauzing range to set adjustments aboard a steel ship. Don't even think there are many who recall what that is! Capt Phil
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Old 10-04-2012, 18:50   #6
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Re: Compasses

If it's a good compass, all it needs is adjustment. There are professionals who do this for a living, and they're worth their weight in gold.

Does anyone know a good compass adjuster near Charleston?
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Old 10-04-2012, 18:53   #7
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Re: Compasses

Set a course between bouys on a chart. run that course and see which unit is accurate. Oh... and reverse the course also! It is disconcerting as hell for sure. Then again I dead reckoned all the way to mexico in the 80's with a compass that swung 15-20 degrees when the engine started and tapered back to normal as the alternator gradually backed down! They say you got to learn to trust your compass.... but... hmmm it's tough at times
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Old 10-04-2012, 19:00   #8
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Re: Compasses

geeeez guys..a GPS will give you the head...as long as you can find a place without set/drift and can run around 5 knots.

most new compass directions have how to do it in them.

look up a owners manual on the internet if you can find one.

I did several of our assistance towing boats that way in just minutes and the compaqsses are fine for finding home in an emergency.

most compasses don't need huge accuracy because between them and the helmsman...you can't be that accurate anyhow over very long distances.
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Old 10-04-2012, 19:32   #9
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Re: Compasses

As Cheechako said - use some fixed points and align the boat - you will know how much off the instrument is.

b.
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Old 10-04-2012, 19:51   #10
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Re: Compasses

You cannot calibrate the ship's compass against an uncalibrated fluxgate compass (instrument, iPhone, autopilot, whatever).

If using GPS and a careful helmsperson one must use the heading not the course. I'm not so sure reversing the course between marks is very effective or convenient. A trustworthy (calibrated) heading cannot be obtained with the typical GPS or chartplotter. A sight thru the mast and forestay to a landmark plotted on a known good chart is about the only way. At night one can use Polaris aligned with the mast and forestay. For the EW calibration you will need to choose another star, landmark, or boat feature.

If variation is large one should use a magnetic reference rather than a true one, perhaps.

Don't attempt it in a harbor. Or where the chart indicates any underwater issues like pipelines, powerlines or magnetic anomalies. Or near a bridge.

It's a good thing to fiddle with on long calm nights at sea.
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Old 10-04-2012, 20:00   #11
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Re: Compasses

Sounds like you need to make up a deviation card. If you have any range lights in your area they are a good place to check your compass.
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Old 10-04-2012, 20:02   #12
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Re: Compasses

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Oh, the days of cruising up and down the degauzing range to set adjustments aboard a steel ship. Don't even think there are many who recall what that is! Capt Phil
After every dockyard refit. Steaming in circles in the Bedford Basin for an hour or two.
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Old 10-04-2012, 20:05   #13
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Re: Compasses

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Oh, the days of cruising up and down the degauzing range to set adjustments aboard a steel ship. Don't even think there are many who recall what that is! Capt Phil
I remember it as degaussing.
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Old 11-04-2012, 01:10   #14
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I got into this a bit with one of my ASA instructors who knew a lot about sailing and nothing about compasses. I used to make field maps before
GPS, with just a map and a compass. I've used what was considered to be the gold standard of handheld compasses, and it had a margin of error of half a degree either direction in perfect conditions. My normal compass has a 2 degree margin, and it isn't cheap.

He was teaching that the cheap "look through" bearing compass he had, with 5 degree increments, was accurate to less than a degree. In the cockpit, moving around.

I grin when I see a giant plotter parked above a compass. My little plotter is up in the companionway and I can see the change when it's on. Good luck with that thing, even if it doesn't have magnets holding the chart card doors closed.

Here's something else I found out the hard way. I have a wheel pilot, and it would just randomly get lost sometimes. Took me a while to realize the iPhone in my pocket was jacking up the little flux gate. When I stand behind the helm, the pocket I normally keep the phone in is mere inches from the AP head. I imagine it's deflecting my binnacle compass as well.

JRM

Even my watch has a digital compass in it. Tracking barometer too. For under $100. It even tells time!
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Old 11-04-2012, 05:11   #15
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Re: Compasses

Quote:
Originally Posted by daddle View Post
You cannot calibrate the ship's compass against an uncalibrated fluxgate compass (instrument, iPhone, autopilot, whatever).

If using GPS and a careful helmsperson one must use the heading not the course. I'm not so sure reversing the course between marks is very effective or convenient. A trustworthy (calibrated) heading cannot be obtained with the typical GPS or chartplotter. A sight thru the mast and forestay to a landmark plotted on a known good chart is about the only way. At night one can use Polaris aligned with the mast and forestay. For the EW calibration you will need to choose another star, landmark, or boat feature.

If variation is large one should use a magnetic reference rather than a true one, perhaps.

Don't attempt it in a harbor. Or where the chart indicates any underwater issues like pipelines, powerlines or magnetic anomalies. Or near a bridge.

It's a good thing to fiddle with on long calm nights at sea.
If you don't think that a differential/WAAS GPS can be used (of course correctly is the operative term) and ANY other method is better on a small boat attempted by one or two "unqualified" people then go ahead and spread the word...

On a calm day with little or no current (in comparison to boat speed)...I dare anyone to calibrate the average small boat compass with something other than a GPS and get better results...go ahead and waste your time....the average vsmall boat compass isn't even marked in increments big enough to notice the change...

Here's right off the Ritchie Compass site-

Ritchie Navigation - Compass Compensation

Method 2. (Requires the use of GPS or Loran)
Step one. While at sea, with the compass in its intended position, but not finally secured, obtain the Loran/GPS bearing to a visual buoy or landmark that is within 10o of a North/South line.
Step two. Position your boat along that line and steer your boat directly at that mark. Turn the port/starboard compensator until the compass heading matches the Loran/GPS bearing.
Step three. Check the Southerly course by steering away from the mark, to a bearing 180 from Step 2. The compass heading should be bearing from or bearing to +180 degrees. If any error is present, it is an alignment error. Rotate the compass itself to correct for one half of this error. Repeat Steps 1 & 2 and then recheck this Step 3.
Step four. Simply repeat the procedures of Steps 1, 2 and 3 for the East/West course, using the fore/aft compensator, although, at this time, any alignment error should have been eliminated, and no alignment correction is required.
Step five. Upon completing the procedure, secure the compass in its final position.
Note: When performing this method, always use the bearing "To or From" mode on the Loran/GPS. Do not use the Heading Information because it is inaccurate in real time.

I will add that with a WAAS GPS and a decent boat speed, you CAN just use the heading (in real time).... just make sure that it is staying within one degree or so with the bearing to/from. It's easier and no need for a visual landmark)
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