By Biff Matthews

A few months ago I said I’d explain how to create a deviation chart in one of my earlier articles.

Many of you are wondering, what is a deviation chart and better still, why do I need one? The quick answer is … ”Bad Stuff Happens.”

A deviation chart is necessary to navigate accurately by compass. When navigating by compass to Ocracoke, you don’t want to land at Cape Hatteras. Use your deviation chart!

For background, the earth’s magnetic fields are constantly changing. The term for this change of plus or minus is variation, earth’s magnetic signature as noted on a nautical chart’s compass rose.

Like the earth, each boat has a unique magnetic signature, deviation. Boats have magnetic fields from a combination of iron on the boat (natural magnetism) and electronics (electronic or electro-magnetism). Deviation is the plus or minus from the boat’s true heading.

A smart boater is a safe boater and uses variation and deviation in calculating an accurate course. With all our electronics, apps and gizmos, we still need a deviation chart because electronics and apps fail at times … period.

To create a deviation chart for your boat, you need an electronic compass linked to a heading display. Headings display on a GPS, fish finder, even an app or GPS on your cell phone.

1. Create a ledger of 0 degrees (north) through 350 degrees in 10-degree increments

2. On a very calm day take you boat a safe area without boat traffic

3. Turn off every electronic device on the boat except what you are using for the heading display. Do not place anything with iron or that generate an electro-magnetic field (smart phone) close to the compass.

4. Turn you boat to north, 0 degrees, by its compass. Note the heading on your electronic read out. That difference is the degree of deviation from north for that boat. Write that difference on the ledger beside 0 degrees. The deviation is plus if the electronic compass displays anything greater than 1 with a minus deviation when the electronic compass displays anything less than 360.

5. For simplicity turn the boat to east, 90 degrees by its compass and record that degree of deviation.

6. Repeat this process for south, 180 degrees and west, 270 degrees, recording the degree of deviation for each.

7. Repeat this process for every 10 degrees on the compass.

You have just created a deviation chart for your boat enabling you to navigate accurately using the compass.

I recommend redoing your initial chart to make it easier to read. Plus date your deviation chart since you’ll need that date as a future point of reference.

Laminating your redone deviation chart will protect it from the elements. Don’t have a laminator, clear box tape properly applied works well. Or put your deviation chart in a plastic zip lock bag. Keep your deviation chart on the boat where you can easily find it.

Safely store an original deviation chart with the date you created it should something happen to the one on your boat.

How to use your deviation chart? For instance, a home is normally 120 degrees by your chart plotter. Your deviation chart for 120 degrees has a value of –9 degrees. Then steer a course of 111 degrees by your compass to accurately steer 120 degrees and home.

A word of caution – If you add something to your boat with iron or change electronics, test your boat’s deviation chart. If the degree of deviation changed, create and date a new deviation chart, destroying the old, now inaccurate, deviation charts.

For local information on boating safety and education we invite you to go to our website www.pamlicosailandpoersquadron.org and review the calendar of class and seminar opportunities or email our Education Officer, Tom Graham at tomgraham@suddenlink.net.

 

Biff Matthews represents Pamlico Sail & Power Squadron.