Compass problem checklist

COMPASS CHECKOFF LIST

Dock Side

1. Check alignment of the compass with the fore-aft line of the ship: (A-Coefficient)
Pedestal mount sail/ Centerline mounted: Sight the compass center pivot pin, forward lubbers line, and boat mast / bow. All should line up. If not, slew the compass to bring them into alignment with the mast/bow.

2. Check steering system: With chain drive system, turn wheel slowly lock to lock. On any type system check for change in compass heading caused by the spokes of the steering wheel. IF THE ANSWER IS YES consult a Professional.

3. Check for position of ampmeter gauges, tachometers: If they are near the compass, do they affect the compass when the engine R.P.M.'s are changed. IF YES consult a Professional.

4. Check for Electronics, Windshield wiper error: IF YES a second deviation card is required. A compass can only be adjusted for one set of variables; i.e., Engine ON or Engine OFF, Wipers ON or Wipers OFF, Radar ON Radar OFF. A second or third card can be generated for other variable, but you can only adjust for ONE SET OF VARIABLES

5. Check dodger bows for effect on compass. IF YES see # 4

6. Check within 8 (eight) feet of the compass in all directions, for miscellaneous ferrous metal objects, i.e., anchors, stereo speakers, propane tanks, air horns and hand held VHF radios SECURE ALL ITEMS.

7. Many of the above problems are correctable, either by relocating the compass, the equipment causing the problem, or by de-magnetizing the equipment (when possible). Two of the worst offenders are radio speakers (they have a magnet in them) and some types of autopilots. If you cannot correct these problems yourself, call Merrimack Compass Service to resolve the problem, compensate the compass, or determine the deviation at each heading.

7. When not using your boat, keep the compass COVERED AT ALL TIMES. Use a light colored cover to reflect the UV rays which cause crazing on your plastic dome.

Under Way

1. Check the compass for error at least once a year after commissioning. Whenever you change anything of a ferrous metal nature or a power supply source (within 6 feet of the compass), the compass should be rechecked.

North/ South (C-Coefficient): Check compass heading off a fixed land range within +/- 10 degrees of N/S

East/ West (B-Coefficient): Check compass heading off a fixed land range within +/- 10 degrees of E/W

NOTE- I recommend using Fixed Land Ranges, because they do not move. The problem with floating Navigational Aids is that you must assume that they are on station and that is not always true. Use caution when checking your compass against Loran or GPS. Both generate a HAS BEEN FUNCTION (the course & speed average HAS BEEN 162° , & 6.2 knots). It does not generate real time information. Even if you maintain a steady straight course for a number of minutes you will not be close to the accuracy of fixed land ranges.

2. Upon completion of your voyage, avoid wiping salt water spray off the dome with your hand (it will dull the plastic dome surface) COMPASSES LOVE FRESH WATER BATHS. Never use abrasive cleaners or waxes on the DOME.

Winter Storage and Care

I always recommend that my customers remove their compasses in the winter and store them in a constant temperature environment.
In the days gone by when boats where stored at boat yards and professionally maintained, compasses where removed along with electronics and stored in heated locker space. Today more and more boat owners are taking their vessels home or to self service facilities. Keep in mind that a vessel stored outside under cover can reach temperatures in the 60 degree + range during the day, and plummet to freezing temperatures at dusk. Each of these cycles of heating and cooling cause the compass fluid to expand and contract. If there is a crazed or cracked dome, a hardened O-Ring, or worn expansion membrane, you WILL GET A BUBBLE in your compass (It is the path of least resistance).

1. Air bubble indicates need of repair.

2. Check for compass sensitivity: Place the compass on a wooden table, rotating the compass so that North is lined up with the forward lubbers line. Using either a magnet, or a ferrous metal object, i.e., wrench, hammer, or screw driver, cause the compass heading to change +/- 4-5 degrees. Rotate the compass and repeat the procedure for the other three cardinal points (East, South, West). The compass MUST return to the original heading. If it does not, then the compass pivot & jewel, or dial assembly must be replaced.

Bubbles

Whenever you get a bubble in your compass, it is an indication of a failure in your sealing system. A bubble in almost all cases requires a trip to the repair shop. Left untended, the bubble (which is air and contains particles of water) will cause contamination of the compass fluid. In the worst case's water droplets can cause pitting of the compass dial and interior paint, which means more $$$$ to repair. Do yourself a cost saving favor, when you get a bubble do not wait to get it repaired.

Electronic Instrumentation: Course vs. Heading

One of the most common questions I receive these days is why does the compass not match my Global Positioning System (GPS) readings. There are two reasons for this descrepancy: the earth's magnetic deviation and differences in what each instrument measures. While the first problem is well known (and printed on every navigation chart) the second is not obvious to most recreational boaters. A magnetic compass measures the direction the boat is pointed, i.e. its heading. A GPS,on the other hand, takes sequential measurements of the boats position to determine the direction of travel, i.e. its course. A vessel's course is affected by other factors besides the boats heading and speed, including the effect of wind and current. A GPS cannot measure a vessel's heading, unless it also has a compass or gyroscope within the instrument.

Poor GPS installation is also a common problem that I find when a compass and GPS disagree. Improper alignment of the GPS components with the vessel's keel is often the cause and is easily correctable. If you need help fixing these problems, please contact Merrimack Compass Service.