We’ve written about GPS many times. It’s truly one of the simplest yet most powerful aids to navigation ever invented, and it just keeps getting better. So, who needs a compass? You do. This column is about that.


A Candle Held Where?

What if I told you that the signal from the GPS satellites reaches your boat with the intensity of a candle – held in Los Angeles while you are in New York? Yes, that’s how it’s designed. What happens if the weather really becomes foul? You can lose your GPS signal, that’s what! It takes a lot because of the built-in redundancies, but it can happen. I know it, firsthand. And if you have to leave your boat in an emergency, are you going to rip your GPS out of your dashboard and take it with you into the raft? No. As a matter of fact and of safety, right next to my compass, which sits above my in-dashboard GPS system, is an old-fashioned handheld compass. If I leave that boat, the handheld compass comes with me.


Where is the Magnetic North Pole?

Most of us have seen pictures of magnetic waves, like those from household magnets, coming out of the North and South Poles, encircling the Earth. The Earth’s iron core spins at high speed, creating this magnetic field. Of interest, the magnetic forces don’t emanate from the top of the world, i.e. the true North Pole. Right now, the “Magnetic North Pole” is just north of Hudson Bay. When Washington was President, Magnetic North was near Norway. If you look on any paper chart for the “compass rose,” it shows in the very center what is called “variation,” i.e. from the area that the chart covers, what is the angular difference – variation – from True North to Magnetic North. Here, it is 14-degrees west, so your compass points 14 degrees too far west at Magnetic North versus where True North lies. This means that when your compass is pointing to Magnetic North, you would turn the boat 14 degrees to the east (014 degrees) to be pointing to True North! This is interesting, but largely meaningless since all compasses sold above the Equator point to Magnetic North. But it’s important to be aware of variation.


What is Deviation?

Frankly, more important than variation to the average boater is deviation. Deviation is the sum of all the forces within your boat that keeps your compass from pointing to Magnetic North. What? Case in point: several years ago, I was doing USCGAux vessel exams at a marina when a skipper asked me if I could look at his compass because it wasn’t working properly.

Now, fixing an errant compass is a relatively complicated process that requires specialized hardware. But away I went to see if I could at least isolate the problem. We stepped on his boat and, just before taking his seat at the helm, he removed his wallet from his hip pocket (which held his police badge within) and placed it next to his compass. While he was fumbling with the boat keys, I literally watched his compass clock around and point at his wallet! I asked, “Skipper, why do you put your wallet there?” He said, “It kills my sacroiliac if I sit on my wallet!” I said, “Keep your eye on your compass while I move your wallet.” As I lifted it away, the compass clocked back and pointed to Magnetic North. “You fixed my compass!” No, I simply removed a source of deviation.

Metallic objects (or magnetic objects like radio speakers) near your compass will “fool” it into thinking that object is Magnetic North. How can you determine the Deviation of your compass? Well, if you have a GPS, it’s easy. All you need is mile or so of calm water and you can run down the rhumblines of the four cardinal points and record the differences between what the physical compass is reading from the GPS course you are running.

Of interest, deviation “deviates” differently at a given compass course, so you need to check at least the four cardinal courses (when we develop our deviation tables for new boats, we measure at least sixteen compass headings). You need to know what your boat’s compass deviation is, so that if you do have to use your compass in lieu of your GPS, you can compensate appropriately. Over enough distance, even a degree or two can add up to significant differences. If you don’t have a GPS, it’s a bit more complicated but it can be done. Get your paper charts out, mark a rhumbline between two points that lie at a given magnetic course between each other. Run down that line and record what your compass is reading versus what your paper chart told you the compass should be registering. The difference is deviation.


My GPS Has Failed and I Don’t Have a Compass!

Well, happily for this sorry skipper, there is a way to create a crude compass with an analog wristwatch. Simply point the hour hand at the sun. Halfway between the hour hand (the sun) and 12 on your watch lies south. If you know where south is, you know where north, east and west are.

Don’t have an old-fashioned watch? Draw one and line it up as it were on your wrist. It works!

If you are interested in being part of USCG Forces, email me at JoinUSCGAux@aol.com or go directly to the D1SR Human Resources Department, which is in charge of new members matters, at DSO-HR and we will help you “get in this thing . . . ■