Visit our Popular Forums

**irwinsailor** · 24-03-2006, 07:15

I need to have my compass calibrated. Where can I find out who does it?

markpj23 · 24-03-2006, 10:16

I know this is not the answer to your specific question ... but why not do it yourself?

Swing Ship procedures are in Bowditch APN Chapter 6 - starting at para 608 is the discussion of how to do this. APN refers to using the ship's gyrocompass as a reference, but if you have a GPS that shows course made good I'd expect that to be a sufficiently accurate reference.

If you have the manufacturer's guide on how to adjust the compass, maybe using the GPS as a reference you can do this yourself and save some $$$.

**irwinsailor** · 24-03-2006, 11:50

Mark I tried it using the GPS and it helped but it is not as good as it should be. I bought a plastic screwdriver and did circles in lake Michigan.

AnchorageGuy · 24-03-2006, 11:56

Here is some good information

http://www.woodfreeman.com/pdf/compasses-nautical.pdf

rsn48 · 24-03-2006, 12:26

On a calm day, look at the difference between your GPS reading for North and your compass. Lets say the difference is 3 degrees east on your boat compass, then just subtract the 3 degrees from what you want to get your true North. So if your boat compass reads 3 degrees east, subtract three and you will pointing North. Just rememer the 3 deviation. You might want to check this out a couple of times a season, but I have read of this technique in a British Yachting magazine and it is what I use.

**Pblais** · 24-03-2006, 13:29

Better to do the same process for each major point of the compass. I would start with the basic 4 points then add the next 4 mid points at least. When there is a general deviation then you can usually adjust the compass to correct it. But at some point it won't do it all unless you get fancy and install apliances here and there.. Make a little card for the helm and one for the Nav station with the readings and date it. It's not a hard thing to do so you can do it yourself because you'll need to redo it regualrly.

Also what you get on Lake Michigan won't be the same down at 15 degrees north lattitude. You need to repeat the process in each geographical region of a global voyage.

One of the really great books on the subject is called "Compass" by Alan Gurney. It's a scholarly work that details the history of the compass.

It's the best way to understand why the magnetic compass is actually a pretty poor piece of equipment that needs a lot of TLC to remain accurate. It's a less than a 300 page paperback book you can get at most bookstores or order from Amazon. It's not written like a text book but more like a factual story. It's got funny parts too. The modern compass is really not all that old! It's also amazing how most compasses on most boats are not adjusted or even close.

"Longitude" by Dava Sobel is the other one to read. This one is pretty interesting as well. You learn a lot as you go through each book as it starts out with all the silly ideas they had back in the beginning and how they figured it all out is an interesting process. You'll come away understanding why things are the way they are.

markpj23 · 24-03-2006, 14:21

So how good is "good enough?" 5 degrees - 10 degrees - what??

I'd think it would be way too expensive to have it calibrated, especially since as others have pointed out the settings must change based upon your operating region. That could be a significant recurring cost for an item [probably] used as a backup reference to GPS & autopilot fluxgate compasses anyway.

You're not wearing one of those Texas-sized belt buckles at the helm are ya?

**GordMay** · 24-03-2006, 14:29

Compass Calibration is performed at the factory. Compass Adjustment (compensation) is performed on an installed compass, aboard ship. Search for a Compass Adjuster, or ask the Manufacturer of your compass for a list of factory authorised repairers/adjusters.
Expect to pay between $100 to $200 plus travel time.

Compass Compensation
http://www.ritchienavigation.com/

To assure accuracy on all headings, check for deviation every thirty degrees and record any deviation on a deviation card. Ritchie Navigation (compass manufacturer) recommends checking at the start of each boating season for changes in deviation. If you feel that the deviation on your boat is of an unusual nature, the services of a professional compass adjuster will be a wise investment.

A built-in correcting magnet system, standard on all compass models, consists of two sets of magnets fixed to two adjusting rods with slotted ends. The slots should be horizontal before starting the adjusting procedure. A small non-magnetic screwdriver is provided for this purpose. On bracket and deck models, the black plastic inserts must be removed to gain access to the slotted compensator rod ends. Before starting compensation, check the area around the compass to make sure all material of a magnetic nature is secure and in its sea-going position. Through the years, many methods and devices have been used to compensate compasses, such as gyros, azimuth circles, sextants, or a pelorus. However, we are going to show you one simple, yet effective method, using equipment that you should have on your boat, and another method using equipment you might have on your boat.

Method 1

Step 1 With the compass in its intended position, but not finally secured, select a course on your chart using two identifiable marks, buoys or landmarks that are within ten degrees (10̊) of the North/South line. Try to select this course so that you can maneuver your boat "down range" of the marks selected (See example).

Step 2 From a position down range of the North/South marks, and keeping the marks lined up, run the boat visually along the Northerly course selected. Turn the port/starboard compensator until the compass reads correctly.

Step 3 Reversing direction, run the boat Southerly, again keeping the marks lined up. If the compass is not correct at this time, there is an alignment error. To correct, rotate the compass itself to remove one half of this error. Repeat Steps 1 and 2 and then recheck this Step 3.

Step 4 Simply repeat the procedures of Steps 1, 2 and 3, except this time, using and East/West course and the fore/aft compensator, although at this time any alignment error should have been eliminated.

Step 5 Upon completing the procedure, secure the compass in its final position.

Method 2 (Requires the use of GPS or Loran)

Step 1 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 2 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 3 Check the Southerly course by steering away from the mark, to a bearing 180̊ 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 4 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 5 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.

Bowditch - Chapter 6 - Compasses:
http://www.nga.mil/MSISiteContent/St...N/Chapt-06.pdf

**Boracay** · 24-03-2006, 18:04

My memory is hazy but didnt the old yachties make up a compass correction card?
My memory is also that it may be unwise to rely on a compass to be any better than 30 degrees, and that if better accuracy is needed then it must be cross checked with other methods.
A single act of commission or omission could put a magnetic compass way out.

**GordMay** · 25-03-2006, 03:01

A Cruising Boat (Magnetic) Compass should be capable of accuracies to within about 1 degree of error (I cannot steer that accurately). For a variety of reasons, including the resolution of my compass card, I’ve always assumed a compass accuracy of 2 degrees. This permits a cross-track error of 2 miles over a 60 mile run, which I’ve never experienced.

The Magnetic Compass, used on a large commercial ship, should have an Accuracy of 0.5 degrees, whereas an electronic compass to should have 0.2 degree RMS Accuracy, and 0.1 deg. Resolution .

However, there are a variety of reasons that a compass may give an incorrect bearing. The most commonly encountered problems are summarized below.

SHIPBOARD INSTALLATION:
1. The Compass must be aligned with the keel, or it will suffer from a constant Alignment Error (on all headings).
2. The initial deviation must be less than 20 degrees on all Cardinal Headings. The integral compensation rod cannot correct more than 20 deg error.
3. It’;s impossible to adjust a compass that’s subjected to variable magnetic fields, such as caused by motors (autopilot), etc.

DECLINATION:
It is generally agreed that the IGRF (International Geomagnetic Reference Field) achieves an overall accuracy of better than 1degree in declination; the accuracy is better than this in densely surveyed areas such as Europe and North America, and worse in oceanic areas such as the south Pacific. The accuracy of all models decreases in the Arctic near the North Magnetic Pole. Global model calculations are produced every five years.

MAGNETIC DIP:
The horizontal force of the magnetic field, responsible for the direction in which a compass needle is oriented, decreases in strength as it approaches the North Magnetic Pole, where it is zero. Close to the pole, an area is reached where the frictional forces in the pivot are comparable to the horizontal forces of the magnetic field.

Ie: In Ketchikan the free compass needle points 17 degrees from the vertical: in the Nome and Anchorage areas, about 15 degrees; at Whitehorse, 14 degrees; at Fairbanks, 13 degrees; and at Barrow, only 11 degrees from vertical. Hence in Alaska and northern Canada, the magnetic field exerts little horizontal force on a compass needle. The force that is exerted causes the compass to point horizontally well east of north, by amounts ranging from less than ten degrees in the Aleutians to as much as 40 degrees near Barter Island.

LOCAL ANOMALIES:
In various parts of the world, magnetic ores on or just below the seabed may give rise to local magnetic anomalies resulting in the temporary deflection of the magnetic compass needle when a ship passes over them. The areas of disturbance are usually small unless there are many anomalies close together. The amount of the deflection will depend on the depth of water and the strength of the magnetic force generated by the magnetic ores. However, the magnetic force will seldom be strong enough to deflect the compass needle in depths greater than about 5,000 Feet. Similarly, a ship would have to be within a mile of a nearby land mass containing magnetic ores for a deflection of the needle to occur. Local magnetic anomalies are depicted by a special symbol on most charts, and are mentioned in Sailing Directions. The amount and direction of the deflection of the compass needle is also given, if known.

Deflections may also be due to wrecks lying on the bottom in moderate depths, but investigations have proved that, while deflections of unpredictable amount may be expected when very close to such wrecks, it is unlikely that deflections in excess of 7 degrees will be experienced, nor should the disturbance be felt beyond a distance of 300 Yards.

Greater deflections may be experienced when in close quarters with a ship carrying a large cargo such as iron ore, which readily reacts to induced magnetism.

Power cables carrying direct current can cause deflection of the compass needle. The amount of the deflection depends on the magnitude of the electric current and the angle the cable makes with the magnetic meridian. Small vessels with an auto-pilot dependent upon a magnetic sensor may experience steering difficulties if crossing such a cable.

SOLAR STORMS:
The effects of magnetic and ionospheric storms, which may persist with varying intensity for several days, are usually greatest in higher latitudes, causing radio 'black-outs' and simultaneous deviations of the magnetic compass needle by several degrees (in and around auroral zones).

Accuracy and durability are the most important points to consider when buying a compass. Some key specifications to check are:
1. Compass accuracy
2. Bezel resolution (5 degree, 2 degree, 1 degree, 1/2 degree - lower is better)
3. Dampening time (how long it takes for the needle to settle)

GPS HEADING ACCURACY:
GPS collects data about our very recent track or course over the ground, providing a “Historical” track. That track includes all of the other forces working on the boat in addition to where we’re pointing the vessel. These other forces include tides, currents, leeway, and any other anomalies. GPS generates a HAS BEEN FUNCTION (ie the course & speed average HAS BEEN 162 degrees @ 6.2 knots). It does not generate real time information.

HTH,
Gord May