How Can I Adjust My Own Compass?

[sketchrbob]

HOW CAN I ADJUST MY OWN COMPASS?

OK, this is the topic many of you have been asking for. First of all we need to understand why a compass needs adjusting. Some of you know all about this but some of you do not, so those who know, please bear with me.

Without anything in a boat, so the boat is a mere shell, unless the boat is made of iron or steel, every compass will point to magnetic north. This will differ from true north by different amounts in different places. This is called Variation. It is automatically indicated on charts, or figured mathematically if you know the amount and direction (east or west). It is also computed for your location on your GPS automatically when you have it set on “Magnetic”. This information can be found in nautical almanacs. You cannot correct for Variation by adjusting the compass.

When you start adding equipment to our shell of a boat some of that equipment may set up a magnetic field that will draw a compass off its heading of magnetic north. Some of this equipment may be heavy metals like anchors or engines, but some may be set up by competing magnets in music speakers, and microphones, and some fields may be set up in electrical circuits like we learned in Physics Class in High School.

Some equipment such as a Radar Screen may not bother a compass while other such as a mechanical tachometer will set up a field requiring the moving of the compass. I wrote about this in the Post --Installing your own compass -- the point is to keep known offensive equipment like fire extinguishers away from the compass (or keep the compass away from the equipment.) In that post we also discussed how to test the compass to make sure the Deviation Error (error caused by movable equipment) is not more than 20° so we can adjust the compass. (This is the maximum correction that can be obtained by the compass’ built in compensators.)

So the object of adjusting a compass is to neutralize the effect of other magnetic fields. We can do this by any one of several methods. In my adjusting work I use an electronic gyro, but since these cost about $3,000, we will use an alternative method. We can do it by running ranges or using a hand bearing compass. However, since most of you have a GPS, it will probably be the easiest way to do it. If you want to try one of the other ways, let me know.

Step 1. Set your GPS to read “Magnetic” (instead of True -- your Manual will tell you how to do this). Also set your GPS to give “Bearing” instead of “Course”. Course will give a COG or course over ground which is subject to the currents and wind while the Bearing will give a constant direction of the target. If there are no wind and current effects, and the helmsman is steering a straight course, Course and Bearing will be identical.

Step 2. Find a Target Waypoint and enter it on your Waypoint List. This should be buoy or landmark that you can run toward on a course of N or S or E or W. You will probably need at least two or more. If you can find a buoy with plenty of clear water around it you may be able to do the whole job with that. Position your boat so you can run East toward a Target. You must be able to run at least 3/4 mile on a day with as little wind and current as possible. You do not have to be able to run exactly on East but it should be within 10° (from 80° to 100°).

Step 3. Set the GPS in the “GO TO” mode with the the Waypoint due East plugged in. Have one person steer directly for the “target” while the other person uses the non-magnetic screwdriver to turn the screw on the bottom edge of the compass facing you until the compass just matches the GPS. In order to minimize course variation, speed should be 10 knots more or less. At 10 knts, you will have 3 minutes to make your adjustment in 1/2 a mile. If the error is not reduced by turning the adjustment screw one way, turn it the opposite way.

Step 4. Using a Waypoint for North , put that in the GPS Go-To mode. Again have one person steer for that waypoint while the other person turns the adjusting screw. ONLY THIS TIME USE THE ADJUSTING SCREW ON THE SIDE of THE COMPASS. The Waypoint should be between 350° and 010°. The compass should match the GPS. If the GPS shows 004° That is what the compass should show. Now with some of these smaller compasses that only show every 5° this is very difficult. So if you can find a Waypoint that is exactly North or South it will be easier.

Step 5. Alignment: Now go to a Waypoint you can run to heading South. Check the compass, if it does not agree with the GPS unfasten the tape holding the compass and physically move the compass to remove one half the error. e.g. The GPS shows 180° but the compass shows 190°. Turn the compass so it shows 185°.

Step 6 Repeat Steps 4 and 5 until the alignment is correct and then repeat Step 3.

Step 7 . Check the GPS with the compass on the following courses, 045°, 135°, 225° and 315°. Write down any differences. If there are significant differences you can make a steering table as follows:

GPS Compass Difference Steer

045 042 +3° 048°

135 133 +2° 137

225 228 -3° 222

315 315 0 315

Note that on the headings where there was a difference the GPS was greater or less than the compass. For example the 045 ° heading was greater so the compass was under reading and the 3° must be added to make up the difference.

To EVERYONE:

Have you adjusted your own compasses? If so, what method did you use? Have you been satisfied with the result? Any difficulties? Any other comments?

Compasses with 45° compensating system such as the Ritchie Explorer and Angler.

These are the compasses I wrote about on my post - “Another Suggestion for a C Dory Compass”.
On these compasses the adjusting screws are not on the rear facing you and on the side, but set 45° to the left and right of where the rear screw would normally be. The adjusting is on what we call the Intercardinal points of compass (Cardinal points being NSEW). Therefore our target waypoints will be NE and NW . The adjusting screw controlling the NW direction will be the screw on the left side while the screw affecting the NE direction will be on the right side. When you think about it, a NW - SE direction will have a right angle to the screw on the left side. These screws control a shaft that goes all the way under the compass . Holes are drilled through each end of this shaft and tiny magnets are set in these holes. When you turn the adjusting screws you are rotating these tiny magnets which in turn are changing the magnetic field of the compass.

So to adjust the Explorer or Angler you follow the same steps as for the other compasses except instead of using the E target you will use a NE target and the right screw. Instead of using the N target, you will use a NW target and the left screw. After making the initial adjustments you will select a course of SE and rotate the compass to take out 1/2 of any error. Then repeat the steps as above. Instead of checking the Intercardinal points as with a regular compass, you will check the cardinal points (NSEW) of the Explorer - Angler compasses.

To Everyone:

Do any of you have either of these compasses? If so, how do you like it? If not, what compass do you have now? Are you happy with it? If so, what brand and size do you have? Do you use your compass? If so, how often and in what way? Where do you have it mounted?

I am not sure that this explanation on adjusting is much clearer than mud. However, post me your questions and I will try to clear it up. I appreciate any feedback you supply. Some of your questions won’t come until you actually go out on the boat and attempt to do it. I will be available for questions then too. sketchrbob@roadrunner.com

Bob W

[RogerJuntunen]

Thank you for the great information and keep it up. My education in this subject goes back to my army days and I am overdue for a review! Roger

[Sea Angel]

Bob;
What a nice simple menthod.

One area I have to be careful of in our local area is the small areas with variation abnormalities from the charts. These are nice area on the surface for convience, but the 'field makeup' will drive you nutts with its irregularities. So, we keep away from large vessels anchored and known ferrous heavy locations.

Swinging the compass on my CD25 is a lot harder than swinging a compass on a compass rose at an airport. But, thanks to the GPS and even an autopilot as a 2nd hand, I have been able to follow your method with a minimum of effort.

You can never have enough backup to navigation, including practice...

Art

[smittypaddler]

I hope nobody else makes this mistake. When I first installed a compass on my C-Dory, and I wanted to build a deviation table for it, I drove around my brother-in-law's circular driveway with the chartplotter on, stopping every few degrees to record the magnetic compass heading versus what the GPS was reporting. My table had some pretty wierd deviations. It wasn't until much later I realized my mistake. I'd neglected to go in a straight line for a little bit on each heading before stopping. The GPS was recording my heading on a curved path.

[thataway]

I have used a sun compass for many years. The beauty of this is that you can swing the compass before any voyage in a matter of a few minutes and check the deviation of your compass-and be sure that it has not be changed--either by intent or accidentally. (The loss of the Schooner Goodwill was most likely the result of a disgruntled person changing the compass compensation) There is a nice pelorus/Sun compass (the card is marked counter clockwise, since the shadow is recprocical of the actual bearing. These cost $60 now--mine cost considerably less 45 years ago. You can make one yourself, using a CD and an upright (best all metal be brass, to avoid any magnetic influence.

I submit that you can have a more accurate compensation with a sun compass than with using the GPS, and more rapidly.