Swinging the Compass

Tech Tips: Swinging the Compass
By Bill Whitney

Year after year we repeat the springtime rituals of deviation exceeds 5° on any course, it’s time to hire a profes-
launching and outfitting our boats. Being creatures of habit we __ sional compass adjuster to correct the compass.
also have a tendency to stow items aboard where they have been The 2018 Eldridge Tide and Pilot Book gives a good
for years. But things change. Something new arrives aboard; a description of using your GPS; on page 213 for creating a devia-
new set of tools, a new GPS, or we replace that beloved Teflon- tion card, and on page 209 for adjusting the compass and deter-
coated frying pan with a new cast iron one. mining any misalignment problems that
Little things that may seem inconsequential N may exist between the boat’s centerline
can add up over time. Big changes like an and steering compass, not as rare a
engine refit are more obvious, but both the problem as you may think!
big and the cumulative small changes can RN A oe S, Pen A third method, although limited to
have a significant impact on the magnetic I~ EKGS one bearing line because of the timing of
environment that the ship’s compass lives f IN a by fa is es the ome aus
within. of the sun at sunrise or sunset. Once

Checking out the compass, or W- = re Oo E, again, your 2018 Eldridge gives a good
“swinging the compass” is a procedure that ” WA NY: i] description of the process on page 232.
should be conducted annually or whenever One potential drawback with this method,

rate! <2 ib. pe garages the location of anything made of steel or S& BA aside from being limited to one bearing, iron changes. It establishes the amount of wy NdeI is the cognitive condition of the person deviation that the boat’s magnetic field taking the bearing. A sunrise bearing imposes on the compass. When the boat S assumes one is awake and can recognize was new, a compass adjuster added é a sun Srey comes ". 1 The senses compensation magnets, or adjusted those earing has its own set of issues. e already installed, to counteract the magnetic field of the boat so seductive comforts of a quiet cockpit while waiting for sunset that the compass pointed to the correct magnetic heading regard- may induce additional cognitive problems. The clear liquid less of the boat’s heading. When the compass adjuster finished refreshments, crackers and cheese, dips, chips and good installing or adjusting these magnets, a compass deviation card company can easily cause one to forget entirely about sunset, not was provided that recorded any remaining deviation error that recognize that the sun is going down, or be unable to locate the could not be removed. (You do have it, right? Oh, so it was compass. This method may not be terribly productive, but installed 42 years ago? And you have checked it since, right?). certainly sounds like the most fun! The accuracy of your steering compass on all angles of the ship’s heading is really important. This is the one instrument that won’t fail when the power goes out. A check of its perform- — ance should be part of your commissioning checklist. It is not only easy but can be accomplished several different ways, using several different “tools” that are already aboard the boat or in your pocket. Of course you could always hire a compass adjuster to come aboard and go through the exacting procedure of comparing the compass headings with the modern version of a gyro-compass, and pay whatever the current cost is, but why not check things out yourself to see if there is a problem before going that route and possibly spending money unnecessarily? There are several ways to do it, but the quickest and easiest is to have someone steer due north (000°C) on the steer- ing compass while another person uses a hand bearing compass to sight along the centerline of the boat while standing at the stern, well clear of any magnetic interference. You can also use the compass app on a smart phone (set for magnetic, not true) as Jong as you can sight along it or keep it aligned with the vessel’s centerline. Have a note pad handy to record the readings of both the steering and handheld compasses and/or smart phone heading. Continue recording bearings every 30° until you have completed a full 360°. Compare the differences between the steering compass and handheld/smart phone headings and record =~ - = lei. the amount and direction of deviation for each course steered. If