If your boat driver’s license has expired “beyond grace”. That means that in order to renew it, you must go back to square one and re- submit all the paperwork, and re-take the dreaded exam. Some folks say it has gotten harder since I took it last century. Anyway, I have been installing a compass in the boat, which prompted some of the schooling from my earlier years to bubble back up. I guess it would be a good thing to write some of it out for practice, since I will be re-taking the test. One thing I learned at sea school and did not forget:
“True Virgins Make Dull Company add Whiskey”
As Father Guido Sarducci, of Saturday Night Live fame would say, “and that’s all-a you need-a to know about-a that”.
OK, but if you are curious or masochistic, read on.
The nautical chart shows True north and Magnetic North on a thing called the “compass rose” (see above photo). It is a circular pattern with the degrees of a circle and a few other things marked on it. Compass roses are put there to help you figure out which way to steer the boat. They are scattered about on the chart in convenient places for quick reference. First, you have to realize there is a difference between true headings and magnetic headings. You might know one, but need to know the other. The Compass Rose shows both.
The North Pole, is located at the axis of the Earth’s rotation, on top of the world. Its direction is known as True North
The Magnetic North Pole, is a different thing entirely, and is located several hundred miles away from the North Pole. At present it lies somewhere inside the arctic circle. A magnetic compass only points to the magnetic north Pole.
The Variation is the the difference in degrees between True North’s bearing from an observer, say a sea traveler, and what a magnetic compass would point to- the Magnetic North Pole. My brain hurts already.
A Magnetic compass will always point to whichever direction the magnetic pole is in. However, it will only point towards True North, if and only when the North Pole happens to line up with the Magnetic North Pole. It could even point to a southerly direction. For instance, If a traveling person was way up in the arctic, and happened to be exactly between the North Pole and the Magnetic Pole, his magnetic compass would point to the Magnetic Pole- due South! You can see this with an orange or a tennis ball or something, and a couple of pins. The Variation for the travelers location in that rare case would be 180 degrees. To find true north, he would add 180 degrees, (the variation) to his magnetic compass heading.
The Variation of a given area on the chart, is printed on the chart inside the compass rose. It also “varies”, depending on where you happen to be on the Earths surface, in relation to these two poles. To make matters even more complex, the variation changes from year to year, in small increments, because the magnetic north pole and the magnetic field of the earth wanders around. The Magnetic North Pole is at the axis of the earths magnetic field, where it exits the Earth in the Northern Hemisphere, and it is not fixed in one place like the True North Pole (the axis of the earth’s rotation) is. The magnetic pole is affected by the Iron composition of the Earth’s interior, which moves around because the Earth’s core is liquid below and the Earths crust floats on top of it. In geologic history, the magnetic poles have even flipped. That would be a real calamity if that happened again, which it probably will ( and if it does, Virgins will no longer be dull company).
So, for example, the compass rose on your chart might have something printed in it like “Variation 3 deg West, annual increase 5’, 1999’ “ . The annual increase is the amount of change, measured in degrees, based on past history, of the actual physical location on the Earth of the Magnetic North Pole, each year, since 1999. Now, this is where the “add whiskey” comes in.
True Variation Magnetic Deviation Compass add Westerly
So, moving left to right, TVMDC, just like it reads, you always add (+) westerly. If the Variation is 3 deg W, you add that to the true heading to find the magnetic heading, OR, now get this, if you are reading it right to left, i.e. Magnetic to True, you would SUBTRACT the westerly variation, so if your magnetic reading is, say, 000 deg., and you want to know where true North is, then subtracting 3 deg, would give you 357 deg- the bearing of True North.
Stated differently, the “Variation”, is a number that is applied to the True heading to find the magnetic heading, or applied to the magnetic heading to find the true heading. If you know two, you can find the third with this formula. Add westerly variation going left to right, subtract Easterly variation, going left to right. OR subtract westerly variation going right to left, ADD easterly variation going right to left. The annual increase of the Variation mentioned above should also be tallied up and applied to the number. If the annual increase is 5’ when the chart was printed in 1999 and it is the year 2001, then add 10’ (two years). Year 2005, add 30’, etc. Needless to say it gets trickier and more important as you go north because the relative positions to the observer of the two poles (the Variation) can become more and more pronounced as one gets closer to them.
This stuff is often inconsequential in the real world, but it depends on where you are, and how much the variation is, and how accurate you need to be. A half degree (30’), is negligible in most circumstances on a small boat. You couldn’t physically steer with nearly that much accuracy. A steering error of one degree will put you one mile off course after about 57 miles.
Ready for some whiskey?
Not so fast. What about Deviation?
So, Variation then, is a corrective number applied between True North and Magnetic North, that is used to find the bearing of the other. Using the formula above, all you need are two values and you can calculate the other. Just remember to ADD Westerly, or Subtract Easterly, when moving left to right.,or vice-versa moving right to left in the equation.
The Deviation, is the same kind of number, between the Magnetic and the actual Compass bearing, (TVMDC), that is applied to one, to find the value of the other.
Magnetic compasses are affected by all things ferrous, whether it is the earths iron ore deposits or a sack of nuts and bolts left carelessly on the bridge deck, and also by all things electrical, since electrical fields are really the same as magnetic fields. Depending on the amount of Iron on the boat, and electronics, and their proximity to the compass, any of these can have a disastrous effect on the accuracy of the instrument. In a perfect world, without any local interference, a Magnetic Compass would give a bearing from an observer to the Magnetic North Pole. End of story. But every Boat is different. The distribution of iron implements, fasteners, rigging, chain, radios, etc will uniquely affect the compass. So, just as Variation must be applied to compensate for difference between True North and Magnetic North, Deviation must be applied to compensate for the difference between Magnetic North, i.e., where the compass should be pointing- Magnetic-and where it actually is pointing- Compass due to local interference, or stuff on the boat.
A Deviation table is printed up on a Compass Card, by a compass adjuster, who “swings” the instrument, after it is installed, by tweaking the adjustment screws found on the compass. The screws are supposed to allow the adjuster to calibrate for Deviation, which he usually can do, but sometimes he cannot, and so he prints up a card that shows how much to add or subtract from any given heading. Usually it is divided up into the different points of the compass. It might say for instance, N,+2, NNE-1, NE-0, ENE+1, etc. The closest relevant Deviation number, taken from the Compass card, supplied by the compass adjuster, then is also plugged into the equation TVMDC, and the same rules apply- ADD westerly going left to right SUBTRACT westerly going right to left.SUBTRACT easterly going left to right, ADD easterly going right to left.
Cheers.