OK, just a brief warning up front…this is going to get a bit complicated. But it’s an important topic and one that many do not fully understand even through it’s a requirement for all OPFACs.
The deviation card is a necessary item for all facilities and is simply the difference between magnetic and compass headings. There is also the other component of overall compass error which is variation (difference between True and Magnetic North) but that’s a topic for another day. Deviation is a characteristic of the boat, and consists of both permanent and temporary magnetization.
Why do we care about this stuff? Besides the fact that it’s a facility requirement, a large deviation can make the compass grossly inaccurate and result in huge navigational errors. With today’s universal reliance on chartplotters, this all seems antiquated and unnecessary but all mariners must maintain the ability to navigate without electronics should the need arise. I’ve been on facilities where I’ve noticed that the deviation is 10 degrees or more, and with sloppy installations of electronics next to the compass it can be 30-40 degrees off! This renders the compass useless. A 10 degree error on heading would result in being one mile off track every six miles.
What does a deviation card look like? It is a table (which should be posted at the helm) that shows the correction to be applied to a magnetic course to derive the proper compass course. Here’s an example:
Let’s say your GPS or chartwork indicates the magnetic bearing to a desired waypoint is 170 degrees Magnetic. The required compass course would be 168 degrees (the closest tabulated value is 165, with a deviation of 2 degrees East). Westerly errors are added and easterly are subtracted.
The tricky part is the development of the deviation card for your vessel. And changes to the vessel such as installing new equipment or steel railing will change the vessel’s magnetic signature and require a new card. The process is called “swinging ship” and observing the bearing to known objects, measuring them on the chart, and comparing to the compass. A compass adjuster can be hired to tweak the compass to minimize deviation but it will never be totally eliminated.
There are a few different methods to determine deviation. A very accurate way is to find a natural range between two landmarks that are shown on the chart. This will be used in combination with sightings to figure out the values.
In this example, your boat is on the east side of Treasure Island and you can see both the tank on Alcatraz and the end of the breakwater on TI. On the chart (or on a chartplotter) you can determine that the magnetic bearing is 249 degrees. Now it is necessary to sight the relative bearing of this range as the boat swings through the entire compass. To do this you can make a pelorus that consists of a compass card and a rotating element with two points.
The zero degree mark is aligned with the centerline of the boat, and then all sightings are relative degrees.
Now it’s time to swing ship while holding position on that line, and measure the relative bearing which is then added to the heading for every 15 degrees of the compass. In this way the deviation can be determined. Wow, really? That sounds complicated and hard to do. Yes, it is a challenge to do all that on a small boat – definitely requires a calm day and good crew coordination.
Another method is to pick landmarks all around the compass at 15 degree intervals and compare the compass reading with the bearing as given from the chartplotter. This is also difficult as it’s hard to find charted objects you can sight on all the way around.
There is another slick way to determine deviation – using the Sun! The Nautical Almanac (published by the US Naval Observatory) tabulates values for the Sun and other heavenly bodies which can be used to determine true azimuth (bearing) of the Sun for every second of the day. Unless you want to geek out on celestial navigation it is not trivial to figure out all of the calculations. Fortunately there is a MUCH easier way.
On my phone I have a free app called Nautical Almanac. It does a lot of stuff, but for this purpose a simple touch of a button gives the instantaneous current true azimuth of the Sun. But we also need to be able to sight where the Sun is based on the compass and compare the two values to come up with the deviation.
This simple homemade device uses a blank plotting sheet and a metal rod which is installed perpendicular to the board.
It is critical to align the board precisely parallel to the boat to give accurate relative bearings. A secondary short metal rod at 180 can be used for aligning with the boat. It’s also necessary to make sure it is level by using whatever shims are required.
That’s the hard part. After that, it is simply a matter of swinging around and reading values around the compass card, and at the same time noting the true azimuth of the Sun. To use it you obviously need a flat calm day. But it can be done anywhere, even in the harbor right next to your slip! You just need to be able to see the Sun.
In the app you select Sun from the pulldown of celestial bodies. Then, every time you press the Update button, the current azimuth of the sun is shown.
Variation needs to be factored in to figure out true bearings which are then compared with the app values.
With some styles of compass, the shadow of the needle on the compass card can also be used as a sighting device. One nice thing about this method is that the compass card gimbals and is always level. Note that with either method, the reciprocal of the shadow is the bearing to the Sun.
While the celestial method is not complicated, it does require some math that might seem a bit spooky at first. Another method that is sometimes suggested is to compare compass readings to the COG function on a GPS. While this method is tempting, it is fraught with error from current, leeway, and the slow update frequency of the GPS’s internal calculations. This method is NOT reliable – don’t use it.
Whichever method you use, make sure to recheck your deviation card if any changes to equipment are made. Even relocating the anchor could change your magnetic signature. And make sure you and your crew practice steering the proper compass course when navigating to a destination.