Boating Safety; Reading a Mariner’s Compass: part 8 of 16
In this day of inexpensive GPS many boaters have little or no need of a compass. However electronics sometimes fail and the smarter captain will have some knowledge on how to navigate with a mariner’s compass. To do this the captain should have access to NOAA navigation charts and have determined the DEVIATION of their compass in relation to the other electronics on the vessel.
The compass works by using specific metal bars that can detect the earth’s magnetic field. What many do not know is that the magnetic field of the earth is not in a fixed location and is rarely aligned with the geographic north of a map/chart. In other words your compass is not pointing to the “true” north (geographic north), it is pointing towards the magnetic north.
A mariner’s compass is composed of a housing in which there small magnetic bars aligned on a compass card indicating north. This magnet/card is fixed within the housing on gimbals that allow it to move with the earth’s magnet field and the housing is filled with oil or alcohol to allow the magnet/gimbal system to rotate smoothly despite the rocky motion of the vessel. The compass should be mounted on the he vessel so that the lubber line (the fixed line on the compass that indicates your direction) is parallel with the keel of the vessel. You should mount the compass as far from other electronic devices as possible and in a position that it can be easily read by the helmsman. Correction for nearby electronic devices will be explained in a moment.
As mentioned above, the earth’s magnetic field does not align with the geographic north/south of a chart; it either varies east or west of it. How far it varies depends on your location on earth and what year it is (the earth’s magnetic field moves over time). NOAA navigation charts include a printed compass on the chart called the compass rose. Using a chart for the location where you are you should find this compass rose. In the center of the compass rose you see what the variation for this location is and when it was measured (var 15oE – 1998). This is known as the VARIATION (how far the magnetic north varies from the geographic north at that point on earth).
As mentioned above, electronic devices such as electric watches, marine radio, etc. can cause enough interference to move the compass card’s direction slightly. This is known as DEVIATION. You will have to determine the deviation of your vessel. I determined mine by aligning the range markers at the entrance of Pensacola Pass, determining what the compass heading SHOULD be based on the chart, checking my true compass heading, and noting this deviation in my log. I would check with electronic devices on and off and log both.
CORRECTING FOR VARIATION AND DEVIATION
This is typically done for logging purposes, which most recreational boaters do not do, but good to understand the terminology. Let’s say you have a compass heading of 61o (logged as 061). The compass rose on your chart indicates a variation of 11oE (var 11oE 1998). This indicates that the magnetic north your compass is indicating is actually 11oE of the geographic north on your chart. Your compass is telling you that you are heading 61o from north but this is the MAGNETIC NORTH. Your chart is telling you that magnetic north is 11oE of the geographic north. So by adding 11o to your compass heading (add east, subtract west) you would have your TRUE heading on a geographic chart. It would be logged as 061 compass, 072 true. Again, this is for logging purposes. The captain is navigating with a magnetic compass and should maintain the 061 heading.
Another example: your compass heading is 68o (068). Chart states var 14oW. Correction means subtract west (068-014) = 054 true. Keep the 068 heading but on a chart you are truly heading 054. If you have determined the deviation of your compass and logged you should include this in your correction.
This skill will be needed when we address how to use navigation charts to plot course from the open Gulf back to your home port. That will be in another article to come. If you are interested in learning more about navigating with a compass I recommend one of the boating courses offered by your local U.S. Coast Guard Auxiliary or your local U.S. Power Squadron. If you have questions yay contact one of the above agencies or your local Sea Grant Extension Agent.