During my delivery of a new up-market sloop from the Chesapeake Bay to Long Island Sound, the binnacle compass deviated approximately 40-60 degrees E of magnetic North, depending on the boat's heading.

Aboard a 40' racer being re-positioned from Long Island Sound to the Hudson River, I noticed that the binnacle compass was permanently installed approximately 5 degrees off the boat's fore-and-aft axis.

While delivering a pre-owned motor yacht from New York Harbor to Vineyard Sound, the binnacle compass consistently deviated 5 degrees E of the electronic compass in the boat's chartplotter, and 10 degrees E of the fluxgate compass driving the autopilot.

In each case, accurate steering courses were determined with my personal hand-held GPS receiver, and all three owners were counseled on how to take corrective action. However, one wonders about the many other incorrect, improperly installed, or non-compensated compasses on our boats. They should all be evaluated and verified as accurate, then corrected if necessary. I also recommend checking your compass for deviation at the start of each ocean voyage, or each boating season, and every time you install new equipment near the compass. Clearly, failure to compensate a deviant compass may result in significant steering and piloting errors.

Three ways to compensate your magnetic steering compass

Every quality marine compass has a built-in system with which to correct deviation errors. This system consists of two sets of small magnets fixed to two adjusting rods with slotted ends. The entire arrangement is contained within the compass binnacle mount. On bracket and deck compass models, the black plastic inserts must be removed to gain access to the slotted compensator rod ends.

The slotted ends of the adjusting rods should be horizontal - - or nearly so - - before starting the adjusting procedure. Also, check the area around the compass to make sure any installed equipment that may be magnetic (binnacle table or cup-holder, VHF radio, GPS receiver/chartplotter, radar display) is secure and in its sea-going position. Remove anything portable and possibly magnetic, such as your toolbox, cell phone, beverage can, binoculars, and keys.

Then follow one of these techniques:

Easy technique any boater can apply

1. Select two navigational aids that are in reasonably close proximity (+/- 1/4 mile) and within 10 degrees of the North/South line. Two buoys will serve, if they are nearly North/South, but fixed aids - - e.g., a stake and a flagpole - - are better. For example, in my home waters, Stamford CT, there's an ideal place to do this: Stamford Harbor has a fixed navigation range indicating the middle of the primary channel. This range axis is 010/190M. In your local boating waters, any similar range, even a buoy and a flagpole, will serve if the range is nearly North/South.

2. Pick a calm day with little or no wind and current. From a position on the range and mindful of traffic, run your boat slowly along the Northerly course, steering to keep the range markers lined up one behind the other. As you do this, turn the port/starboard compass compensator magnet until the compass reads correctly (in Stamford, which would be 010M). Be sure to use a stainless steel (or other non-magnetic) screwdriver.

3. Reversing direction, run the boat Southerly, again keeping the range marks lined up on the boat's axis. You should be steering 180M (190M in Stamford) and the range should bear 000M (010M in Stamford). If necessary, make a further small correction in the post/starboard compass compensator magnet. Your actual steering course North and then South while on "your" range should match the axis of the range as plotted on your local chart.

4. If the compass is not correct at this time, there may be an alignment error. To correct, rotate the compass assembly within the binnacle mount to remove one-half of this error. Repeat Steps 1-3 and then recheck this Step 4.

5. Repeat the procedures of Steps 1-4, except this time, run the boat East and West across the range, and if necessary make a small adjustment in the fore/aft compensator magnet. When you cross the range steering East (090M), the range should be abeam and should bear 000M (010M on the Stamford range). When you cross the range steering West (270M), the range should again bear 000M (010M on the Stamford range) when abeam. Most times, any alignment error will have already been eliminated.

Easy technique (requires a GPS receiver or chartplotter)

Pictured is the E-Series Chartplotter by Raymarine.

1. While at sea, on a calm day and mindful of traffic, position yourself a mile or two directly South of a lighthouse or other prominent fixed landmark. As a temporary waypoint, enter the landmark's exact La and Lo coordinates (three decimal places equal +/- 6 feet) into your GPS receiver.

2. Steer your boat North as precisely as possible toward the waypoint. Turn the port/starboard compensator until the compass heading exactly matches the GPS bearing (ideally, 000M) to the fixed landmark.

3. Check for a Southerly compass error by steering directly away from the waypoint, that is, South. Although your compass heading should be 180M, on the GPS display, the waypoint will still bear 000M. If any error is present, there may be an alignment error within the binnacle, so you should rotate the compass assembly to correct for one-half of this error. Repeat Steps 1 and 2 and then recheck this Step 3.

4. Repeat the procedures of Steps 1, 2 and 3 for the East/West course, using the fore/aft compensator, although, at this time, any alignment error should have been eliminated and no alignment correction should be required. Of course, the temporary waypoint/fixed landmark should bear 000M when it is abeam.

5. Always use the bearing "To" or "From" mode on the GPS display. Do not use the heading information or track information because in real time, the heading or track may vary due to the boat's motion; it is therefore unreliable for this purpose.

Easy and fast (requires a hand-held compass)

Perhaps you have a second magnetic compass aboard; it may be a little hand-held "hockey puck," or an integral part of your binoculars. Or perhaps you have one of those trendy electronic digital compasses or a flux-gate compass. Whatever - - dig it out, level it, and then stand with it near your primary steering compass. Compare the hand-held readings on at least the four cardinal points with those on your primary steering compass. They should be much the same.

Then take the secondary compass forward to the bow. With an assist from your crew, again compare readings of the two compasses. If the readings are not the same, you should consider evaluating and adjusting the primary compass as described in Methods 1 and 2 above.

Also, to assure accuracy on all headings, expert mariners advise checking for deviation every 30 or 45 degrees and to record deviation (if any) on a so-called deviation card. This card should be consulted when laying courses and piloting. This is especially useful as back-up for long voyaging, but in practical terms, for recreational buoy-hopping it may not be necessary - - if you have adjusted your compass at the four cardinal points to correct the major magnetic deviation errors on your boat, and if you have verified that the compass is accurate to within about 5 degrees. In my experience, even the best helmsperson does well to maintain a margin of error of plus or minus 5 degrees. The important point to remember is that your course made should at least average your intended rhumb line course.

Conclusion

Your steering compass must provide accurate and reliable headings by which to navigate, independent of all on-board electronic navigation and piloting systems. This is frequently overlooked amidst all the buzz about GPS and other electronic technology. But it is vital.

At the first opportunity, take time to verify - - and correct if necessary - - the accuracy of your compass. As you now know, this is not a difficult procedure and it may even be a mildly rewarding experience; there almost certainly will come a time when you will compliment yourself for being so foresighted.

Captain Bernie Weiss is a delivery skipper based in Stamford, CT. As Atlantic Yacht Delivery (www.AtlanticYachtDelivery.com), Cap'n Bernie re-positions sailing and motor yachts along the Atlantic coast, Maine to Florida. When not at sea, he trains, lectures, and organizes workshops on electronic navigation, seamanship, boat-handling, and related subjects.