Let's begin with a piece of advice: Don't cast your compass to Neptune-it's not dead yet! Of all the navigational tools and instruments employed on boats, the compass is still the single most valuable. Its sole moving part is governed by a fundamental physical force of Earth itself. Electronic instruments communicating with each other and with artificial satellites may provide more information at any time, but the simple compass provides the crucial piece of information that no other device can: the direction the vessel is headed. With that knowledge, anyone aboard the vessel can orient the surroundings-be they sea, sky, land, or all three-relative to the boat and therefore relative to a chart.
Why, in this world of GPS and other networked marvels, is that so important? Isn't it enough to know where the boat's going, and isn't that the same thing?
Well, no. Where the boat's pointed and where it's going aren't always the same. In fact, they rarely are. A current will cause a boat to move over the ground in a direction different from that in which the boat is pointed. Even where no current exists, a boat under sail (and, to a lesser degree, a powerboat) carries leeway, which has a similar effect. Together, leeway and current can cause a boat's track to be very different from its heading, which can bring unwelcome surprises to a mariner unaware of what's happening.
|Steering by COG without a Compass
In this example, the boat is being steered to a course over the ground (COG) calculated by the GPS. The COG coincides very closely to the GPS' "Bearing to Waypoint," making the skipper assume that the boat is bow on to the bouy. On approaching the waypoint, the skipper searches ahead for the red mark, but it's hidden behind the sails. Without a compass to provide the actual heading, the skipper has been correcting for the current that's been setting him across his course.
Let's take our compass away for a moment and steer the boat toward a GPS waypoint by matching course over the ground (COG) to the bearing to the waypoint. We're doing well, and the track on the plotter shows our course as a straight line with the waypoint dead ahead. As we approach our waypoint, which happens to be a navigation aid, we begin looking for it to confirm how totally reliable our instruments are. Where should we look? Dead ahead, like the plotter says? Maybe not. We're on port tack, on a close reach, and making 10 degrees leeway, and a current is also setting us left to right (which we weren't aware of). The mark isn't dead ahead; it's 30 degrees to starboard, behind the genoa. Clang!
If we'd had our compass, we'd have known, from the bearing information provided by the GPS, where exactly to look for the mark, and we wouldn't now be facing a charge of damaging government property.
No matter what vehicle we're steering, a fixed reference of some kind is an essential aid to pointing it in the desired direction. A star can serve that purpose as well in life as it can in poetry, but in the absence of stars or distinguishable points on the horizon, the compass is our only option. In fact, it's a better option than a land-based or anchored reference.
While the compass can't tell us where we're going, it can tell us what direction we're steering, which makes it unique. And far from being made redundant by electronic devices, the compass adds to their value. For example, by comparing COG from our GPS to the course steered by the compass, we can detect the presence of a crosscurrent. A little math involving boat speed through the water (from the log), compass course, and COG (magnetic) and speed over the ground (SOG) from the GPS can provide the direction and velocity of that current. Knowing that, we can calculate the course to steer to offset the current, something our GPS can't do (although it can tell us if we're off track). The GPS can return the favor by providing a means to check the accuracy of our compass.
Adjusting the Compass
A magnetic compass, as long as it's in good physical order, will always point in accordance to the magnetic field surrounding it. If that field was the Earth's alone, it would be utterly reliable, but the compass isn't monogamous, and it's easily led astray. Onboard magnetic influences-such as those created in some electrical devices and by iron-cause a compass to point askew from magnetic north, an error called deviation. Deviation can vary depending on what direction the boat lies in relation to the Earth's magnetic field. An important step when commissioning a new or refitted boat, once its equipment has been installed or stowed in its permanent locations, is to determine what the compass's deviation is on all headings so that you can apply it to courses you lift off your chart.