The Marine Compass: Its Care and Adjustment


By Captain John J. Jordan

The following article was written by the late Capt. Jordan, and appeared in the August 1948 issue of The Fisherman, published monthly by Marine Publishing Co., Grand Haven, Michigan


In discussing the subject of compasses it is important, at the outset, to emphasize the fact that a compass is a finely constructed instrument with a number of delicate parts, and should be handled with extreme care at all times. Many fishermen consider only the strong metal bowl, and the heavy glass, and the sturdy box, and forget entirely the precision ground pivot and delicate jeweled bearing. A compass, properly handled handled and maintained, should give satisfactory service for years.

There are two types of compasses, the dry and liquid compass. Both of these are of similar construction, consisting of a bowl, pivot, card, and magnets, and a jeweled bearing; in additionto the outer frame , glass, gimbals, and a box or binnacle. The liquid compass is far superior to the dry compass for use aboard ship, as it is much steadier in rough weather, and has greater directional force by virtue of its larger magnets.

The liquid compass is available in two distinct types, namely the spirit of wine (alcohol-water) type and the oil model. Of these two types, the spirit of wine model is preferred by most fishermen. In the alcohol-water compass the magnets are encased in small tubes, while in the oil type they are exposed. It is therefore rather easy to distinguish between these two types of compasses, as the exposed magnets in the oil compass are bright and shining.

The spirit of wine compass, in addition to having a faster action, is also able to withstand extreme cold. The oil compass has a tendency to become sluggish in cold weather, and where the drop in temperatures are extreme the compass may become inactive due to the thickening of the oil.

Many liquid compasses have been ruined by fishermen who have refilled them with the wrong type fluid. Water, when used in an oil compass, will rust the magnets and discolor the card. It will affect the spirit of wine compass in the same manner, and in addition to ruining the card, may cause the card to float up off the pivot. There is also danger of adding too much water to the mixture in a spitit of wine compass, causing it to freeze in cold weather. The proper mixture for most compasses is 45% pure grain alcohol and 55% pure distilled water. Pure benzine is recommended for most oil compasses.

To Remove An Air Bubble

It is often necessary to remove an air bubble in a liquid compass. Before starting the operation, mark the position of the false bottom in relation to the bowl, so that it can be returned to its proper position when the job is completed. This is important because the false bottom is usually weighted, to make the compass hang level in the box or binnacle. If the bottom is not returned to its proper position the compass may be off balance. The expansion chamber of the compass will be exposes when the false bottom is removed. Place the compass on edge with the fill hole up. Maneuver the compass back and forth until the bubble comes to rest immediately under the filling hole, then begin to apply pressure to the expansion chamber and at the same time loosen the screw in the filler hole. The pressure on the expansion chamber should cause the bubble to escape around the threads of the screw as it is loosened. Complete the operation by tightening the screw again and releasing the pressure on the expansion chamber, as undue pressure will damage the thin metal covering. If the bubble has not disappeared completely it may be necessary to add a small amount of liquid to the compass. This can be done very easily by using an eye dropper or an ear syringe.

Handle With Care

A compass, as mentioned before, should be handled with extreme care. Many expensive compasses have been damaged by a sudden jar caused by the boat striking the dock or some other obstruction. If the pivot or jewel or both, are damaged, the compass will not operate accurately.

A fisherman should check his compass at least once a year to determine whether or not the jewel or pivot have been damaged, or the magnets are getting weak. This check can be made in the following manner. Place the compass in a location that is free from all local magnetic attraction, and in such a position that the North mark on the compass card points directly north. Then with a magnet, deflect the card eleven degrees to the east, holding it there for several minutes. In withdrawing the magnet, it should be done in such a manner that the card is neither pushed or pulled. Then watch the action of the card very carefully, and when the North point passes line for the first time, check the time on the second hand of a watch. Take a second reading on your watch when the North point (going in the opposite direction) passes the lubber-line for the second time.

The proper time for this vibration, for a six inch compass, is 14 seconds, at the room temperature of 70 degrees Fahrenheit. To assure maximum accuracy in this test the compass should be left in the room for several hours, so the liquid will be the proper temperature. Should the compass fail to complete the second passage of the lubber-line, or be much slower than the prescribed time, the compass should be sent in for repairs. The test should be made at least once a year, or oftener if you suspect the compass is sluggish.

Storing The Compass

Do not expose a compass to the direct rays of the sun. There have been a number of cases where the liquid in a compass has been expanded by the heat of the sun to a point where either the glass was broken or a gasket ruptured. During closed seasons, or when the boat is laid up for any length of time it is advisable to remove the compass from the ship and store it with the face down, to stop all wear on the pivot and jewel. Care should be taken to store the compass away from large masses of metal, or electric motors that would affect the magnets. If a compass is left aboard an idle boat for a long time it will gradually wear the pivot jewel in one position, as the working of the boat at the dock, through the action of wind and currents will cause the card to work back and forth almost continually. If this happens it will be necessary from time to time when on the run, to tap the compass with a finger to bring it past the worn spot

Locating The Compass

Designers and builders of fish tugs have given very little consideration to compass location. It is pitiful to come aboard a fish tug to adjust the compass, and find it sitting on a steel shelf, or in a specially constructed steel angle frame, with a steel steering drum and chain directly below it that pulls the compass every time the wheel is moved. Under such a setup it is almost impossible to adjust a compass so a residual deviation will hold. Before building a tug it would be wise to submit the plans to some compass manufacturer for their comments. During the war the U.S. Navy insisted on a seven foot nonmagnetic circle around the location of a compass.

I have found from experience that regardless of how badly you pound a steel tug in ice, the compass deviation can be held if the pilothouse of the boat is constructed of wood, with at least five feet of the deck or roof immediately ahead of the pilot house is also constructed of a non-magnetic material, and the same arrangement aft of the pilothouse is small. I have adjusted the compasses on several tugs that have wooden pilothouses, with wood forward and aft and to the sides of the compass, and in all cases the adjustments held for more than a year with little or no change.

Compasses should be located directly over the keel of the ship, as the center of the pilothouse is a particularily neutral position as far as the sides of the ship are concerned. Ammeters, tachometers, and other electrical instruments should be mounted at least five feet from the compass. Radio sets should be located as far away from the compass as possible because of the powerful magnets in the speaker. A steel ship is a magnet in itself, with a definite North and South electrical pole, similar to a permanent magnet. For this reason a one wire electrical system, where the is used as a neutral wire, should never be used. Any current passing through the hull will have a tendency to change the magnetism of the ship, and affect the compass. An electric horn, or even the electric heater plugs on some diesel engines will affect the compass unless they are properly shielded. Moveable iron, such as steel net boxes or steel doors will, in many cases, affect a compass.

Last year, while making the initial compass adjustments on several new tugs, I found that steel doors pulled the compass at least half a point on certain headings, causing a westerly deviation on some headings, and an easterly deviation on others. Such a condition should not be permitted, as a crew member could easily open or close a door without the captain's knowledge, and throw the ship off the true course.

I do not agree with fishermen who claim that steel net boxes do not affect their compasses. I have adjusted compasses on several boats only to find that they change three to four degrees on some headings when steel boxes are brought on board. Fishermen should make it a practice to carry the same number of boxes at all times, and in the same location if at all possible. It is a good idea, in all cases, to keep them as far away from the compass as possible. Care should also be taken to see that tools, lunch buckets, drinking cans, and other metal objects be kept away from the compass.

The list of a vessel will also affect the compass in steel boats, and to a lesser degree on wooden crafts that have heavy ice iron. This effect is called heeling error and cannot be compensated. It is therefore important to keep the vessel on an even keel at all times. If this is impossible, it is well to keep the following rule in mind and apply it whenever necessary. When going North, with a port list you will invariably fetch up to the starboard, or to the high side of the ship. When going South, with a port list you will fetch up to the port or low side. The amount in each case will depend on how much your vessel is listing, and how close to North or South you are heading.

Although you should have the utmost confidence in your compass, it is a good policy to check it whenever possible. Do not place too much confidence in compass headings taken inside a harbor, as local attractions caused by metal piers or catwalks, steel sheet pilings, bridges, iron ore deposits, and even other ships, may be drawing on your compass and giving you a false reading. It is much better to check your compass on established ranges, several miles from shore. To be absolutely accurate you should have some sight parallel to the keel, that can be used in your sights

Many compasses have little peculiarities of their own; but in most cases these are not serious. A good navigator can soon learn to know his compass and its slight differences from other compasses he might have occasion to use. Likewise, many vessels have peculiarities in holding a course, or the way they respond to the action of a sea from a certain quarter, that must be considered in steering a good course.

Local Variations

The magnetic field is slowly changing over the whole earth, making a swing in one direction and then returning in the opposite direction and extending over a period of a great many years. The direction of the swing, and the amount, varies considerably over the different parts of the earth. This change is called variation, and must be taken into consideration when adjusting the compass, or plotting courses.

There are also periodic oscillations of the compass needle which are minor and of short period and small in amount. Besides the periodic oscillations of the compass needle there are at times irregular changes of greater amounts than the minor oscillations, remaining but for a short time in any definite direction and continually moving about irregularly. Magnetic storms are said to prevail at these times. On rare occasions the compass needle has been deflected about three eights of a point during these storms, but as a rule it will not amount to over one degree on one side or the other.

There are other irregularities of the compass needle, with the largest to be found along the north shore of Lake Superior, the north end of Lake Huron, near the Magnetic Reefs, and Lake Ontario, at the entrance to Kingston Harbor. These large irregularities are along shore and do not extend far over the water, probably becoming normal three or four miles out.

In some instances it has been found that a compass that behaves well in calm weather will be very unsteady and wild in rough weather, which makes steering very difficult or impossible. This may be caused by some of the fore-going reasons. Relocating the compass or substituting a larger size compass will often correct this. Some compass manufacturers are beginning to take some interest in the fishermens' compass problems. Tests are being conducted at the present time with a newly designed compass, which if found successful under all weather conditions, should prove to be a big help to fishermen.



Posted to the Web on Sept. 30, 2008, for educational purposes only.