Amid all the tech­nol­ogy found on cruis­ing boats, com­pass ac­cu­racy can eas­ily err. Ad­juster Jo Robin­son tells us why it’s still es­sen­tial to check your com­pass ac­cu­racy reg­u­larly

Yachting Monthly - - CONTENTS -

Swing your com­pass. Why mod­ern elec­tron­ics make ad­just­ing your com­pass more im­por­tant than ever

Jo Robin­son is a spe­cial­ist in an age of gen­er­al­ists. Fo­cus­ing on one in­stru­ment, the fixed com­pass, she plies her trade across al­most ev­ery size of craft, from day boats to oil tankers; even the Royal Navy is a loyal cus­tomer. Her role is to tame the most fickle of nav­i­ga­tional in­stru­ments, the fixed com­pass. But why is com­pass ad­just­ment still so im­por­tant on a rel­a­tively small mod­ern yacht?

‘A prop­erly ad­justed com­pass and de­vi­a­tion card re­mains the pri­mary source of di­rec­tional in­for­ma­tion for non-elec­tronic nav­i­ga­tion. If you are sail­ing any dis­tance, er­ror mat­ters. Sail 60 miles and a de­gree’s er­ror will put you a mile off course. If you’re sail­ing any fur­ther then it re­ally starts to make a dif­fer­ence to where you end up at the end of the pas­sage!’ ex­plains Jo.

Hav­ing served an ex­tended ap­pren­tice­ship with her fa­ther Rob Robin­son, who ad­justed com­passes in an age be­fore GPS even ex­isted on big ships, Jo has been jump­ing from boat to boat in the Solent for the last 20 years. It’s given her both a wealth of tech­ni­cal ex­pe­ri­ence and a feel for what’s most likely to be affecting de­vi­a­tion, an in­vis­i­ble force present on ev­ery boat. ‘How hard it is to re­duce er­ror varies enor­mously from boat to boat. It’s a process. First we de­ter­mine what er­ror is present, then get rid of any ma­jor anom­alies, then fine tune. As a rule, any­thing less than 10° is con­sid­ered quite ac­cu­rate and it usu­ally takes no more than three cir­cles of the boat to cal­i­brate each com­pass.’


Com­pass er­ror is the dif­fer­ence be­tween Mag­netic North and the di­rec­tion in which the com­pass is point­ing. Both vari­a­tion and de­vi­a­tion are mea­sured in de­grees east or west. The aim of the com­pass ad­juster is to nul­lify the ef­fect of the un­wanted mag­netic fields by plac­ing cor­rec­tors (mag­nets and soft iron) ad­ja­cent to the com­pass. These

cre­ate equal but op­pos­ing mag­netic fields, thus elim­i­nat­ing the de­vi­at­ing fields around the com­pass, en­abling it to align cor­rectly.

Why does a com­pass need ad­just­ing?

It’s not just some­thing for big ships. ‘Most of my clients own yachts un­der 40ft in length. Some new boat own­ers make the as­sump­tion that their com­pass is right when it’s in­stalled at the yard,’ ex­plains Jo. ‘The re­al­ity is that ev­ery newly com­mis­sioned yacht needs to be prop­erly swung once ev­ery­thing has been in­stalled as part of the com­mis­sion­ing process. The amount of kit in prox­im­ity to the com­pass, par­tic­u­larly VHF ra­dios, stereos and elec­tronic equip­ment, can have a sig­nif­i­cant in­flu­ence. If you in­stall new equip­ment on an older boat, the same will ap­ply and it’s a good idea to have the com­pass re-swung when you change the elec­tron­ics. As a rule, ev­ery two years it’s

also worth check­ing as a mat­ter of rou­tine. Peo­ple are al­ways sur­prised what can throw a com­pass off – even a pair of glasses can have an af­fect, and a mo­bile phone can eas­ily cause a 10° er­ror. If your com­pass is sud­denly not be­hav­ing, think, what could be affecting it, what’s changed?’


By swing­ing the yacht through 360°, ev­ery an­gle she will sail at and com­par­ing true head­ings with the com­pass read­ing.

Mo­tor­ing out into a busy Southampton Wa­ter, we find some space with nearby buoy­age to start the process. This is al­ways done in an area where there are no known mag­netic anom­alies marked on the chart. Tra­di­tion­ally, ships will fly flags Os­car and Que­bec above each other to in­di­cate they are swing­ing the com­pass as it ne­ces­si­tates a lot of steer­ing in cir­cles, some­thing that can eas­ily con­fuse other ma­rine traf­fic.

On a yacht, it’s worth hav­ing some­one keep­ing a good look­out whilst some­one else con­cen­trates on swing­ing the com­pass.

‘A com­pass ad­juster is only as good as the helms­man. Be­ing able to hold head­ing on a car­di­nal and us­ing tran­sits is how we make the as­sess­ment of how much er­ror we are deal­ing with. A hand-bear­ing com­pass is also used in dif­fer­ent parts of the boat (to mit­i­gate the in­flu­ence of on-board mag­netic anom­alies) to cross-check,’ says Jo. ‘With the yacht held steady on each of the eight pri­mary com­pass points, the ex­ist­ing com­pass bear­ing and head­ings are then com­pared with what we know the head­ings should be; this then gives us a de­vi­a­tion in de­grees. As a rule of thumb, any­thing be­low 10° is usu­ally con­sid­ered ac­cept­able. We will knock out er­ror above 8°.’

To check the dif­fer­ence of true an­gles, a pelorus is used. Ef­fec­tively a dumb sight­ing com­pass, it ac­cu­rately mea­sures the dif­fer­ence in de­grees but pro­vides no mag­netic in­for­ma­tion.

To take the er­ror out of a mod­ern yacht com­pass, the outer cas­ing must be re­moved re­veal­ing the mag­net hous­ing. Ini­tially, a box of loose mag­nets is used to cal­i­brate, then small mag­nets are in­stalled in the cas­ing at var­i­ous points around the com­pass so er­ror can be com­pen­sated for. On the new cata­ma­ran on which the corrections were be­ing made, the big­gest er­ror found was 30°.

Tak­ing the er­ror out of an elec­tronic com­pass is the same process. How­ever, elec­tronic plot­ter de­sign­ers have made it easy to do your­self, largely be­cause an elec­tronic com­pass in cal­i­bra­tion mode will remember the er­ror that it en­coun­ters as you swing the yacht round in cir­cles. Most will ask you to swing the boat around at least two or three times be­fore man­u­ally ad­just­ing the head­ing. So, it’s well worth ad­just­ing the man­ual com­pass be­fore you do this. Flux­gate com­passes are just as sus­cep­ti­ble to mag­netic in­ter­fer­ence as their tra­di­tional coun­ter­parts.

Jo Robin­son has been cor­rect­ing com­passes for over 20 years af­ter be­ing ap­pren­ticed in the trade by her fa­ther, Rob Robin­son

The tools of the trade are lit­tle more than a pelorus and a box of mag­nets

Af­ter corrections have been made, any re­main­ing er­ror is recorded on a de­vi­a­tion card

the prin­ci­ples of ad­just­ing a com­pass are sim­ple, but do­ing so ac­cu­rately is an art

A hand-bear­ing com­pass is used for com­par­i­son, away from mag­netic in­ter­fer­ence A pelorus is used to ac­cu­rately mea­sure an­gles from the ships head, which can then be com­pared to the com­pass

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