ELECTRONICS KNOW-HOW Making Sense of HEADING SENSOR DATA
Understanding how the various heading sensors on a sailboat work — and what to do when they start reporting “alternative facts” — will help you make sense of where you think you’re going.
Growing up, I always looked forward to our annual Down East cruises along the Maine coast, but I was never a fan of loading provisions and other assorted gear aboard. At the wise age of 10, it seemed to me that my dad, an engineer, physicist and expert navigator, took what I considered to be dawdling steps when it came to stowing canned goods, tools and spares aboard Windancer, the family’s C&C 37. After all, unloading groceries and suitcases at home was a quick and easy job, so why should it be any different aboard a boat?
Flash forward 30-plus years and many thousands of miles on the wheel, and I’ve gained appreciation for my dad’s meticulous preparations, especially considering some of the self-inflicted headaches that I’ve witnessed on boats when skippers have been less concerned with where ferrous metals (read: iron and iron alloys) were stowed, triggering cascading compass and heading-sensor problems.
In theory, driving a sailboat in a straight line is simple enough. Just hold the helm steady, right? But as anyone who has sailed on waters larger than a millpond knows, numerous factors can influence one’s heading — say, set, drift and wind — and even skilled helmsmen struggle to maintain less than 5 degrees of heading inaccuracy over helm sessions longer than 15 minutes.
Moreover, once offshore and away from fixed visual references, driving becomes a matter of trusting one’s steering inputs. Most modern boats typically carry a range of equipment capable of providing accurate heading information. However, troubles arise when onboard compasses and heading sensors start quarreling or acting catawampus. Here’s a look at different types of heading inputs, how to troubleshoot conflicting metrics and how to determine the best source of information in a sea of alternative facts.
Get Your Bearings
Trusty magnetic compasses were developed in China during the Han dynasty (206 B.C. to A.D. 220), followed some 2,000 years later by liquid-filled, or “wet,” compasses that use oil or liquid to dampen the motion of moving parts. Like their forebears, wet compasses point toward Earth’s magnetic north pole (not the true north pole, which is some 1,000 miles away). Modern binnacle, or ball-style, compasses are fixed-mounted and house a gimbaled compass card that sits atop a fixed pivot and rotates to always point north. Users determine their heading by lining up the compass’s lubber line, which corresponds to the boat’s centerline, with the closest mark on the card, which reads from zero to 359 degrees.
While magnetic compasses are reliable, they need to be adjusted for two factors. Magnetic declination, known as variation, refers to the angle, on a flat plane, between the planet’s magnetic north pole and its true north pole. This angle varies depending on one’s latitude and longitude and oscillates over time, with areas closer to the poles experiencing more oscillation than equatorial zones. Since charts are presented in true-north-up orientation, navigators must correct for this difference when determining their best heading. The second factor, magnetic deviation, refers to locally introduced errors. These range from onboard ferrous metals, such as an engine block, to external magnetic anomalies, such as large iron deposits. To correct for small amounts of magnetic deviation (say, onboard tools), careful navigators swing their compass during slack tides and prepare deviation cards that allow them to make real-time corrections to the vessel’s heading. To correct for bigger problems, or for initial setup calibration, magnets are placed inside the compass housing by a professional to create equal but opposing magnetic fields that nullify deviation caused by
in situ ferrous objects. This is known as compass adjustment or calibration.
“Compasses are underappreciated in the electronic age, but they work when all else fails, unless you break the glass globe,” says Jim Mcgowan, Flir/raymarine’s Americas marketing manager. Others agree. “The bottom line is that the ship’s compass will get you home,” says Bill Haines, owner of Island Marine Instrument in Everett, Washington, and an expert compass adjuster.
Says Jon Josephson, Garmin’s regional sales manager, high-end ball compasses are accurate to roughly 2.5 degrees, while mere-mortal compasses are good to 5 or 10 degrees.
Ball compasses require little maintenance, aside from occasional calibration testing. However, Haines notes that over time and with weather changes, air bubbles or fluid leaks can corrode the compass’s pivot and cause the motion of the bearing to stick.
work like binnacle compasses except they’re mobile. Depending on the model, navigators can determine their heading by using the compass’s magnifying glass or sight lines. “A hand-bearing compass is a great reference because there’s not much constant ferrous interference,” says Alan Davis, B&G’S product-line director. Still, says Mcgowan, it’s important to remember that shaky hands reduce the accuracy of these compasses.
While magnetic compasses require a watchful eye, modern heading sensors, such as fluxgate and solid-state compasses, supply a vessel’s heading information electronically to the boat’s autopilot via NMEA 0183 or NMEA 2000 connections, or over a proprietary data backbone. The information can also be fed to a chart plotter and other networked instrumentation.
Fluxgate compasses have historically been used to provide heading information electronically. The devices consist of a magnetic core that’s wrapped with two coils of wire. These wires drive an alternating cycle of magnetic saturation within the core, canceling each other out and allowing the fluxgate compass to detect Earth’s background field. While more sophisticated than an analog compass, fluxgate compasses are subject to the same declination and deviation issues that affect all magnetic-based instruments.
Because of this, newer autopilots increasingly employ solid-state compasses, some of which automatically selfcalibrate and self-correct for declination and deviation. These compasses use either solid-state or
B&G’S Precision 9 compass (left) and Raymarine’s EV-1 sensor (right) are nine-axis solid-state compasses. Furuno’s SC-50 (center) is an even more accurate satellite compass.
A binnacle-mounted ball compass, such as this one by Ritchie, is a trusty standby when the power goes out.