Boating USA — January 2018

FEBRUARY 2016 / BOATINGMAG.COM / P.NO

Boat

Doctor

Q&A

By

Michael “Mick”

Hannock

B OAT

DOCTOR

BOATINGMAG.COM

JANUARY 2018

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Michael “Mick”

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CHANGE THOSE O-RINGS Fuel and water fi lls, deck plates, and hatches all come

fi tted with O-rings to seal out water from rain, spray and washing. These degrade over time and

should be replaced every fi ve years. Often the maker of the fi tting is stamped on the underside

of its cap or lid. Contact the maker to purchase replacement O-rings.

PHOTOS: COURTESY GARMIN (GPS), COURTESY SILVA (COMPASS)

A:

Tim, fi rst, check the settings of your GPS to

make sure it is displaying “magnetic” rather than

“true” headings. For most boaters, magnetic will

more closely match the readout of your compass.

But it will never be exact. So-called “compass

error” is normal. Compass error comes in two

forms. What is known as variation is a result of

the diff erence between magnetic north and true

north from your particular location on Earth.

You can fi nd this diff erence on the nautical chart

for your area; it is noted inside the compass rose.

Your GPS, set to magnetic, likely takes this into

account and displays an accurate heading. But

you’d have to add or subtract the degrees of vari-

ation for your area to correct your compass.

The second kind of compass error is called

deviation. This occurs, as you have surmised, as

a result of metals and wiring aboard your boat

aff ecting the magnetic fi eld of your compass.

(continued on page 22)

COMPASS

DIRECTION

Q:

Hi, Doc. The compass mounted atop the helm of my

Bayliner does not agree with headings displayed on

the screen of my GPS. Moreover, it appears that it

is off by a different amount of degrees at different

times. I make sure not to place my keys, fishing pli-

ers, or iPhone next to the compass. What could be

causing this?

Tim Schlieff

Providence, Rhode Island

True North

The direction

read on a chart,

representing the

geographic pole.

Magnetic

North

The direction

an error-free

compass needle

points.