Can anyone tell me what I am doing wrong with my magnetic compass adjustment procedure?

Here is the pre work to calibrate the auto pilot:

1Executed the compass calibration procedure for my Raymarine auto pilot. EV core sensor (3-axis digital accelerometer | 3-axis digital compass | 3-axis gyro digital angular rate sensor)
2Adjusted the compass reading on the chart plotter to compensate for a one degree error in heading due to a core sensor installation alignment error.
3Use my laptop based Navigation Planner software to prepare a route file between two points on Lake King exactly aligned with Magnetic North. (True North plus 12.9 degree declination)
4Set preferences in chart plotter to display magnetic compass direction.
5On a calm day at slack tide, set the autopilot to follow the track to confirm alignment and calibration steps. Rock solid on 000 degrees M for several minutes.

I've tried a couple time to adjust out the errors in my backup Ritchie magnetic compasses mounted at each helm. I am not happy with the result. The deviation charts don't look normal. I am not sure if it's a fault in the logic of my procedure or error caused by the need to remove the compasses from their mounting position and re fit every time I adjust the N/S or E/W adjuster. Here is the procedure:

1Use the auto pilot to follow the magnetic North heading route. Adjust all error out using a brass screwdriver in the N/S adjuster.
2Use the auto pilot to follow the magnetic East heading route. Adjust all error out using a brass screwdriver in the E/W adjuster.
3Use the auto pilot to follow the magnetic South heading route. Adjust out half the error using a brass screwdriver in the N/S adjuster.
4Use the auto pilot to follow the magnetic West heading route. Adjust out half the error using a brass screwdriver in the E/W adjuster.
5Swing the compasses to produce the deviation charts.

Ritchie Compass............. 00 30 60 90 120 150 180 210 240 270 300 330
Raymarine Magnetic .... 354 24 54 85 117 149 179 209 237 265 294 324

It is possible/likely that your location has its own magnetic variation. When we use to compass swing our Navy aircraft we used a known magnetic neutral area. It wasn't uncommon to have a degree or two error on other parts of the airfield.
IT is also possible/likely that you boat's head is not pointing directly along the chart plotters track. Under motor there will be some torque effect causing the boat to be pointing slightly off the boat's track through the water.
Your compass won't be a military grade item. If it is within 5 degrees the manufacturer would reckon it is great.
Add all these error up and 6 degrees is pretty good.
An accurate compass is most important when manually steering or using a vane steering to it offshore. If you can steer within 10 degrees over time you are doing well.
Your compass is fine.

Rob S said..

Morning Bird

I think you are meaning 'very local variation' Hydro office says the variation is 12.9 degrees East but you are saying something like a bit of wreckage in my path or maybe an iron deposit under the lake. I didn't think about that. Should try somewhere different.

And I didn't think about motor torque effects either but suspect it's not an issue with the two 9.9 hp high thrust Yamaha outboards at slow speed.

Yep, as you note there can be different variations on the earths magnetic field. A submarine cable is another.
You also might not have taken into account compass Deviation, the inherent error in the compass due to manufacture or siting on the vessel. Your compass card might be a degree or two misaligned in manufacture, it may be mounted on a bulkhead a s****teenth out causing the needle to be slightly off position, the compass may be affected by metal in the boat. This latter affect on the compass might vary depending on the boats heading.
All these little errors add up, 6 degrees is pretty good.
I am way out of date on the subject, there are people who specialise in yacht compass swings who might be able to help further.

Kankama said..
I don't even look at my compass. Hell I haven't had one on the binnacle for years (I do have one a combination handheld with light that can mount there when needed - but it hasn't been needed.) The GPS is all I want nowadays. That gives me the track. Then I adjust the autopilot left or right depending on where we are on the chart. Sometimes I do not want to keep down the centre of the track.

For instance, when coming south from Mackay to Curlew I let the boat get pulled to leeward with the incoming tide. The boats with the GPS tied to the autopilot fought to stay down the rhumb line. Then the tide turned and we got pushed back to the rhumb line when all the boats that had stayed on track had to sail deep. We just sailed at the best wind angle, beat em by hours.

Sailing boats don't go straight. Mine doesn't. So I work out where I am, what the wind will let me do to sail the best VMG and then work out gybing or tacking angles later. I rarely sail down the straight line on the chart, but I do have a cat that likes it when I sail good angles.

So, don't worry about getting the compasses super accurate unless the GPS goes down. Then a compass within 10 degrees will do fine as long as you don't trust it too much and check against other nav aids.

Many sailors rely on chartplotters/GPS. So do I in the right situations. However, it can get even experienced sailors in real danger.
Probably worth doing the Yachtmaster Offshore theory course. It addresses the shortcomings of this approach so at least the risks are known.

I tried swinging my mag compass without success - it resembled something like Jonny Depps in Pirates Of The Carribbean.
I ended up calling a professional to do it, Andrew Laslett, he is SA based, but I think he does jobs in Victoria also.
Did a great job that took a fair while to get it calibrated, but I was very happy with the outcome when he finished. He had to account for electrical field interference on my helm position. I never realised how difficult a job it was.
Here's his link:

www.compassadjuster.com.au/Adelaide.htm

I hope this helps.

Kankama said..
Actually I don't use a chartplotter - just the old GPS 128 and a computer plus, and this a big plus a big paper chart.

The paper chart is not really for navigation per se, more for getting a good grip on the trip in my head. I can pore over it before we go and get to know the hazards, plot them out, get my safety zones set, look out for shoals, like the John Young banks off Jervis Bay. I leave it out on the bridgedeck table and have the gps going too. You get a great sense of where you are on the chart.

There have been quite a few people who have missed a hazard, and run into reefs, using plotters. I feel that I could do that too so still get the paper charts to get the "vibe". For plotting distances and courses I tend to use OpenCPN as this is a bit easier. Still I like to keep my hand in. in case the batteries fail.

I had a friend who told me about learning running fixes and cocked hats et al on his yachmaster course. He is only 10 years younger than me and that is enough for him to always have had GPS on his boats. I did a few trips the old way first. But I love my GPS nowadays, just knowing where I am is fabulous and I still remember wishing I could work out my position to within a few miles in the dark years ago. GPS - ooh ahh, we used to dream about GPS - luxury.

cheers

Phil

Love to know where you get the charts for Open CPN
Al

Yeah, I get the idea of danger zones and letting yourself drift onto reefs. On the trip to Curlew I plotted my course and kept a lookout. As I said before, my boat doesn't always like going straight so I sometimes have trips where I have to up the navigation skills. For instance, in lightish winds an alteration of only 15 degrees can almost double the boat speed under reacher by generating more apparent. When the wind comes up I often go straighter but sometimes veer around for comfort. I wouldn't do it where there are nasty shoals - but the vast majority of the coast is clear.

Still, you have to be careful to know what is around. I think paper charts let me get this situational awareness better than the computer. Otherwise this could happen

www.yachtingworld.com/blogs/elaine-bunting/comment-how-the-team-vestas-wind-crash-really-happened-and-the-surprisingly-simple-things-we-can-learn-from-it-62634

As for OpenCPN charts - mine are not all that good. Some have some errors which is a worry. I am using Navionics too, on my next trip in a week.

cheers

Phil