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While electronics are great, analog backups are critical for when things go south. Keep your bearings with these magnetic compass care and maintenance tips
Photo: Frank Lanier
While often overshadowed by modern electronics, a magnetic compass remains the only piece of navigational gear that can show direction and the way home during a loss of onboard power. Although its construction is relatively robust and straightforward, a compass still requires maintenance, attention, and an occasional bit of guidance. Here’s how to ensure your compass will provide years of reliable service.
It’s the little things that count
- Show your compass some love at the end of each trip: Give it a gentle freshwater rinse to remove corrosion-inducing salt residue. Never use waxes or abrasive cleaners, which can scratch the compass dome (the same reason to avoid wiping saltwater spray off with a rag or your hand), and let your compass air dry before installing its protective cover.
- Inspect your compass regularly for both physical damage and installation issues that could affect operation. One of the more noticeable problems a compass can develop is the dreaded air bubble. Compasses are routinely exposed to temperature extremes. With each cycle, internal pressure can force fluid past hardened O-rings, cracked domes, or damaged expansion bellows (which accommodate normal fluid expansion) leading to fluid loss and the formation of air bubbles. Moisture in these bubbles eventually causes pitting of the compass card and damage to other internal components. So contact the manufacturer for the recommended dampening fluid to use (and instructions for adding it), or have your compass serviced by a professional at the first sign of bubble formation.
- Keep your compass covered with a light-colored cover when not in use. Not only will it reduce heat (and fluid loss), but it also protects plastic domes from crazing due to prolonged UV exposure. Store your compass ashore during the winter months or when the vessel faces long-term storage.
Leaving tools or other metal objects near your compass can throw it off
Internal pressure can force fluid past hardened O-rings, cracked domes, or damaged expansion bellows (which accommodate normal fluid expansion) leading to fluid loss and the formation of air bubbles, as seen here. Photo: Frank Lanier
Covering your compass will reduce crazing and oxidation from ultraviolet rays. Frank Lanier
Installation issues
- Ensure the compass is properly oriented by sighting along the “lubber line” (the fixed line on a compass showing the direction straight ahead) and the center pin of the compass to verify the line is parallel to the vessel’s keel. If not, you’ll have to slew (rotate) the compass to bring it into alignment.
- This is also a good time to verify that the compass light is operating properly, a must for nighttime use.
Deviation
Deviation is the unintentional effect of metal, electrical, or magnetic items near your compass, which can throw your compass off enough to give you erroneous headings. Although there are ways to reduce the effects of deviation, you won’t be able to totally rid your boat of it. That’s one reason every compass requires a deviation card or table indicating how many degrees to add or subtract from your heading to obtain the correct magnetic reading.
- Inspect the immediate area around your compass (out to a distance of 6 to 8 feet in all directions) for items that can cause deviation. These include speakers (often the worst offender due to their large magnets), canned goods, propane tanks, handheld VHF radios, wire runs, tools, and so on.
Keep an open mind during your inspection, no matter how seemingly unlikely the source. Unusual examples include bimini or dodger frames having different effects on a compass when raised or lowered and steering wheels that can cause compass deviation while turning because of metal spokes. Also keep in mind that how items affect compass readings may vary wildly on different headings.
When checking the effects of electronics or electrical equipment on compass deviation (e.g., windshield wipers, radars), test the equipment in both the on and off positions. The same thing goes for the engine, which should be checked while off, idling, and at various rpms as well. You can only adjust a compass for one set of variables (e.g., engine on, radar on, windshield wipers off). Any variations (such as having the engine, radar, and windshield wipers on at the same time) will require a second or even third card to account for each variable.
Here’s a great example of why you need a deviation card for your magnetic compass. Just look at all those electrical items nearby that are likely to throw off the readings! Photo: Frank Lanier
The compass card in this unit is not moving properly. The compass will need to be repaired or replaced. Photo: Frank Lanier
Considering a new compass?
Most boats come with a compass already installed by the builder, however, the question is, was it selected based on functionality or cost?
Most recreational vessels have compass cards or dials between 2- and 7 inches in diameter. Smaller compasses are less expensive and popular on vessels with limited mounting space, but compasses with larger cards are more stable, making them more accurate and easier to read.
A general rule of thumb is to select the largest compass that your helm and wallet can accommodate. A smaller compass may work OK for shorter runs. But imagine staring at the same unit (with its microscopic numbers) for hours on end should the need arise due to navigation equipment failure. — F.L.
Sensitivity
- Next up, check compass sensitivity. This is a simple test that can be performed onboard (using whatever heading it happens to read) but is easier to do ashore.
1. Place the compass on a wooden table in a place free from all magnetic influences (except the earth’s magnetic field) with the forward lubber line lined up with the north indicator.
2. Touch the side of the compass with a small magnet or any ferrous metal object (such as a screwdriver) causing the heading to deflect +/-5 degrees or so, then remove it; the compass should return to its original heading.
3. Repeat the above for each of the remaining three cardinal points as well (south, east, and west). If the compass doesn’t return to its original heading in each case, it likely has a problem with the pivot, jewel, or magnetic strength of the compass needle and needs to be serviced by a professional.
Compensation
- Compensation is the act of adjusting the compass to address the effects of deviation and provide a correct bearing. Most magnetic compasses have two built-in correcting magnets to aid in this. The magnets themselves are mounted on two adjustable rods, the ends of which are slotted, allowing you to adjust the magnets with a small nonmagnetic screwdriver (one normally comes with the compass when purchased).
Many methods have been developed over the years to verify compass accuracy using equipment such as sextants, gyros, azimuth circles, or a pelorus, however two of the simplest methods I’ve seen can be found on the Ritchie Navigation website (ritchienavigation.com) under the Resource Center.
If you have any doubts about your ability to perform the above correctly, then having a compass adjuster perform the process and develop the deviation card is the best option.
Prudent mariners never let the sirenian call of modern electronics lure them from the safe dependability of the humble magnetic compass. Invest the time to care for your compass, and it will always have your back.