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.

Cruising World - - Front Page - BY DAVID SCH­MIDT

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.

Hand-bear­ing com­passes

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.

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