R-25 Magnetic Compass errors

[henrym]

The back access door to the instrument panel has magnetic latch that assists in holding the door closed. It is redundant, what really holds the door closed is the teak swing down latch. The magnetic latch is about 8" from the magnetic compass. After removing the magnetic latch, my compass swung -5°. Whenever I placed the magnetic latch within 12" of the compass the card started to swing. I realize very few boaters use the magnetic compass today, but come the day you need it, why compensate the compass for an unnecessary error. You just don't put magnets near the compass unless you have a steel hulled boat that sometimes require large compensating magnets.
Henry

[Nan-C]

I have a similar issue. I am agonizing over where to mount low-magnetic-field Poly Planar box speakers. When I started to mount them where I really want them, I notice the compass deflected -5 degrees. I had not expected this. Do I need to be concerned about the effect on the electronics mounted in the same area as the compass? The speakers have no apparent effect when positioned in the alternative location that is my distant second choice.

Regards to all,

Greg

[acustis]

The magnet does serve a purpose on our tugs. With "Autopilot installed there are too many wires for that magnet to hold the door up." Thats why you have a teak latch. We have yet to here that this was interfering with the compass until now. We will look further into this.

[Lobo]

I have a modest priced compass mounted on the dash in front of the helm. I also have GPS and I have my track bearing as one of the pieces of information displayed. I have a list of all bearings, 0 to 360 on a sheet of paper, and as I cruise I write down what my compass shows at each 5 degree mark. I think this is a modern way to "swing your compass". In due course I will have the sheet completed and then print out a "Compass Deviation Sheet" so for the day when all my electronics die, the fog rolls in, and I have to go back to using my Power Squadron navigation skills, I will at least know where the boat is pointed.

[thataway]

The GPS Course over ground is not aways equal to the compass course. Drift and set will influence GPS course over ground. Probably best to do a proper swinging of the compass, rather than relying on GPS courses. Also there is the possiable errors of magnetic influences of items placed near a compass--as noted above.

As I have noted in the past, one of the easiest ways to swing the compass is to use a "sun compass"--that way you can check the compass easily at regular intervals. Basically this is a compass card with a perpendicular pin, which casts a shadow on the card and allows to both take an azmuth of the sun and to quickly swing the compass.

A brief description of a technique similar to what I use is at:
http://www.cs.brown.edu/~jfh/boats/compass-calibration.html

I'll try and post a photo of my card/pin suncompass in the next couple of days.

[Les Lampman]

I've maintained for years, and still do, that there is no good way to mount a magnetic compass (especially at the price level usually chosen) on a small boat and have it compensated properly unless there are no ferrous metals aboard. Since that isn't going to happen compasses do not work properly on small modern boats, especially those mounted near the helm. We've demanded from manufacturers, and received, better wipers motor with large permanent magnet motors, more electronics, stereo systems, ad infinitum. Each and everyone of these adds to the problem of getting a proper heading from a magnetic compass. If you read the guidelines for installation on the compass (and for all the other items stuffed into a small boat for that matter) you'll find that you can not meet all the criteria.

A magnetic compass on the dash of a small fully equipped boat is, for the most part, eye candy. If it makes you feel good put one there but then you can't really expect it to work well. Either install an electronic compass (like the KVH Azimuth 1000 which also feeds fast heading update to the nav system) or just use the nav system itself. If and when a magnetic compass is ever needed (that means for me that my main nav system has failed, my KVH 1000 electronic compass has failed, my handheld battery-powered GPS/chartplotter has failed, and my second battery-powered GPS unit has failed) I'd grab the quality hand-held bearing compass I carry in a safe spot (used for checking on a dragging anchor) and then I can at least get it far enough away from most of the dash area interference to get a good bearing. I should say, other than for checking on a dragging anchor, it's never been used as I've never had four systems fail and required its use.

[Westfield 11]

[thataway]

I would love to give "Lobo" the credit for using GPS to check compass heading, but it has been described for quite some time. Ritchie thus suggests how GPS can be used to check the compass headings:

"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."


This still gives a relitatively rough idea of the compass bearings at each 5 degree incriment. (The reason I recommend the Sun Compass technique is that it will compensate for each degree, if necessary)

I first used a GPS I took to a friend in Australia in 1990/91 when SA was off during Desert Storm, and used it to check compass headings and found some errors with this technique--and I am sure that I am not the first to do that. Loran was utalized prior to that time for similar compass checks.

I agree with much which Les says. However, there are specifics when a compass is useful. Specifically when the boat is at rest or anchor. We always take a heading out of our anchorage--since GPS will not accurately show a course until underway, and the course will vary depending on how averaging is set. On several occasions we have had to leave anchorages quickly because of a lee shore in limited visability as wind velocity made the anchorage unsafe--and that pre taken compass bearing was the best way to get a safe course out of the bearing. Thus, I do think it is worthwhile compensating and making a Napier Diagram for each fixed compass--and the fluxgate (electronic) units.

We have a remote fluxgate compass which is installed in the Tom Cat. But even with a fluxgate or electric compass you need to do a Napier diagram (which shows the deviation of the compass with each piece of electrical equiptment running--for example with the windshield wipers on and off, nav lights on and off, wipers and lights on etc. Also the small compass cards are hard to read closer than 5 degrees. I spent many hours when young steering with a true 4" compass (now in my living room) where it was possiable to steer a coarse to one degree.

We also have a "Hockey puck", as well as the binocular hand bearing compass as back ups (and to take ranges for navigation if necessary--and specifically when setting anchors to check for dragging.)

As a rough check for the compass bearing--yes the GPS can be useful, but other techniques are better for accurace compensation.