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A Compass is More Than a Navigation Tool - Here Are Some Other Tricks It Can Do

By: Captain Bill Jennings

compass on boat

How to get multiple uses out of your compass for little money


In this day of smartphones, GPS plotters, and the myriad of available electronic navigational tools, we can easily forget about the good 'ole compass. This is a big mistake! This article will tell you things you didn't know about a compass, including some slick ways to put them to good use. If you don't feel 'compass smart' after reading this blurb, you get your money back. Wait -- this is a free article.


Your direction relative to north has always been a basic part of every boat navigation system. A compass operation is very simple. It reads in frames of reference in degrees to north, south, east and west, relative to the surface of the Earth. There are different types of compasses, but they all deliver the same reference information to keep you off the rocks.


The common liquid magnetic compass contains a magnet that is sensitized so that it will interact with the Earth's magnetic fields to point towards Earth's magnetic north. Fluid is used to dampen and stabilize the magnetized needle or reader card to prevent excessive needle swing caused by your boat's movements. The reference point is 'magnetic north.' The fluid used is usually alcohol, oil, or kerosene. You can't drink it, even in an emergency, but you can refill it if some evaporates. A digital compass works the same way, but uses liquid numbers in the readout, like a digital watch.


One important thing to remember when using a magnetic compass is that the needle points towards what is called magnetic north, not true north. The strength of magnetic fields on the surface of the Earth vary from place to place and this difference must be accounted for. You can find this variation for a given area measured in degrees on your marine chart. You must adjust the magnetic reading shown on the compass to adjust for the variation in your area in order to find true north. The difference between the direction of the field’s horizontal component, which is what the magnetic compass reads, and the direction for true north, is called "declination" or "variation."


A magnetic compass has another concern to consider when looking for true north from where you sit. A metal object or electrical field, such as your boat gauges being too close to your compass, will also move the needle, creating a magnetic north error. The difference that such a disturbance creates is called 'deviation' and must also be adjusted for. If you are not planning to navigate safe passage across the Pacific, don't worry too much about the formulas to adjust your compass at this time. With a casual glance your compass will still tell you which way is up.


Another type of compass is called a fluxgate compass. It also detects the earth’s magnetic field, but it measures it electronically rather than using magnets. Two or more coils of wire are wound around a core of highly magnetic material to sense the direction of the horizontal component of the magnetic field of the Earth. It operates using the electric power in your vessel. Because it has no moving parts, it is more accurate and its readings can be easily transmitted to an autopilot or digital read out. As you would expect, they cost a little more than a pure magnetic compass.


A third type of compass is the gyrocompass. It contains one or more electrically powered, rapidly spinning discs, or 'gyros'. The rotation interacts dynamically with the rotation of the Earth. As rotation slows due to friction, the axis of the gyro rotation becomes parallel with the Earth's. I get that spinning gyros are more accurate than magnets in a fluid, but to be honest, I am still confused as to how they "align to Earth rotation." I guess we just need to remember "Spin the gyro compass --- read true north".


Did you know that most people carry a compass everywhere they go? True. There is one in every smartphone. Ask your phone to load a free compass app and there it is. A sensor called a magnetometer measures strength and direction of magnetic fields and determines which direction you are pointing. Again, this is how they work, but I couldn't build one. Reports say that phone compasses can have reading errors up to 4%, but having one in your pocket is much better than no compass at all.


Here are some lesser know uses of a compass that you can use for recreational boating: On small lakes, we travel similar course directions on a regular basis. The next time you are running straight towards one of your popular destinations, jot down the compass heading for that direction. Note beside the compass heading, the from/to that it represents, then place this information in your boat's glovebox. You can then refer to this information on a dark night or in poor visibility to confirm that the direction you are heading is correct.


Here's another compass tip for boaters. If you come across a single lateral day marker in open water and are not sure on which side you are to pass, your compass heading can give you the answer. Remember that whether it is a lake or a river, there is always a directional flow of water. A chart will provide this information with arrows. The rules of markers tell us that if travelling upstream, all red markers must be kept on your right (.e., starboard side). Red markers are also triangular in shape and have odd numbers.

Lets look at an example. We come across a single marker. We know from experience or looking at our chart that the water flow is north to south. Next, we glance at our compass and see that we are travelling 'northish'. (i.e., clockwise between 270 and 90 degrees). We now know from marker rules that we should keep this red marker to our right side. Conversely, if your compass told you that you are traveling southish in that north to south current, all red markers must be kept on your left side.


When telling friends how to find your cottage, give them a magnetic compass radial heading from a nearby landmark, or buoy, to your place. As an example, "From green marker number 36, head 97 degrees magnetic, directly to our place." They will then boat straight to your dock. This beats telling visitors that you live in 'the brown cottage on the bay.' It's also good practice for learning how to give simple and effective directions when boating.


Once you understand the importance of a compass, here are some buying tips: In general, a larger compass provides better accuracy, stability, and readability. Look for a recognized brand name and don't be tempted to buy one at a cheap price. Test the compass before buying by moving a magnet or heavy metal object back and forth beside the compass. When the magnet or metal is removed, the card should return smoothly without oscillating excessively.


Choose a compass that is designed to work on your type of boat. A sailboat compass may require a gimballed mounting, while a powerboat compass needs to be mounted far enough from electrical instruments to avoid interference that will cause deviation.


Top-reading compasses with a flat card have more detailed scales and are preferred over front-reading models. A quality compass will also have 'repeatable readings.' This means that if you turn the compass in different directions, then back to the original heading, the number should be the same each time. And be sure there are no bubbles above the fluid in the bowl.


In short, the compass should 'feel' rugged and reliable. When installing, be sure your compass is mounted in your line of sight for easier and more accurate reading.


A quality boat compass will cost you just over $100, but if you want one for cruising you could easily spend north of $400. Whatever compass you choose, remember -- the magnetic compass is an essential navigation tool that has stood the test of time. I hope that this brief overview gives you a better appreciation of their value. #tips #quicktips

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