Wrong Way Corrigan

[kas919]

I rarely get out of site of land, but I am noticing that my Garmin MFD and my Ritchie Compass are not reading the same heading. I would say it may be off as much as +/- 20 deg. I was thinking the other day that I should probably figure out who to believe. Of course we all assume that GPS is never wrong right? Should I have the compass professionally calibrated? Any thoughts or wisdom would be greatly appreciated. Thanks

 

[JJF]

Depending on the model of your compass, there could be adjustment screws on it.

 

[kas919]

Depending on the model of your compass, there could be adjustment screws on it.

 

[JJF]

Compensator Module (SR-CM2) - Ritchie Navigation

SR-CM2 Easily installed (see instructions) Provides up to 15 degrees of compensation Exclusive 5 Year Ritchie Warranty Made specifically for the: SS-PR2 (standard) and SR-2 (optional) Approx. Shipping Weight: .5 lb. Warning: This product may contain Cables, Printed Circuit Boards, Brass...

www.ritchienavigation.com

 

[JJF]

There appear to be three sub-models of the SR-2. Each has its own manual.

Venture
SR-2	SH-0221, BK-0023, BK-0042
SS-PR2	SH-0221, BK-0029, BK-0042

 

[DennisG01]

First check to see if you have your Garmin set to read "magnetic" as opposed to "true north".

Second... do you have any speakers/etc (sources of magnetic disturbance) close to the compass?

With the boat in your slip, does turning the wheel affect the compass? Does using/turning on any nearby electronics affect the compass?

Then, yes, follow the compensation procedure (plenty of info out there about this). Look for two brass (usually brass) screws on the back side. This is a normal thing to be done based on where someone boats.

 

[seasick]

Based on the OP location, the declination would be about 10 degrees so that doesn't explain the 20 degree difference.

One simple test for magnetic interference is to turn off all battery switches and note the compass heading. Not turn on the battery switches, note the heading again. Turn on things one at a time and look for changes, Don't forget the stereo if you have one, especially if there are speakers mounted close to the compass.
The above test will test for electromagnetic interference, the speakers can cause both magnetic and electro magnetic interference.
Phone chargers, magcase phones, magnetic mounts, heavy current cables and the like are all possible sources
It could be adjustment as noted but test for magnetic and electromagnetic interference first

 

[SkunkBoat]

When you are traveling in a channel and your chart shows you in the channel, the GPS is correct.
When you are offshore and you see Venice to the east on your chart and your track takes you back to the inlet, the Garmin is correct.

Compasses are notoriously inaccurate due to things that Dennis mentioned and the declination in venice Fl is about 8 degrees west

I never use a compass anymore. Your Chart allows you to see where you are going and where you've been vs pointing in a direction and trying to watch a compass and drive.
I wouldn't spend time or money trying to make a compass "correct" but yes you should figure out if there is something nearby effecting it.

You probably have a compass on your smartphone you can compare to.

 

[Halfhitch]

Kas, you can get your Garmin to do a self-check on its compass readings by finding an area with calm water and drive your boat in slow 360 degree circles at least twice, taking one minute or more per circle. I live in Venice, Florida also and when I do this when syncing the compass with the autopilot, I go to Roberts Bay, about a half mile south of Snake island. It's protected water and there is a wide area outside of the ICW channel to mess around. Also, what SkunkBoat said is a good idea, turning on the compass on your smartphone and comparing it to your compass and to the reading on your chart plotter to see who is lying.

 

[wspitler]

To be accurate, if using a magnetic compass,(wet compass) you need to develop a compass deviation card. It will tell you the difference between your compass reading and actual magnetic heading at the various quadrants. The GPS does not provide heading. It provides course over ground. Not that significant in a boat, but real significant in an airplane with a crosswind so think of it that way. Your heading comes from a compass, your course over ground comes from your GPS. Your auto pilot, if you have one, has some sort of heading sensor associated with it and that may be where you see the heading that is transmitted to your multifunctional display. Most earlier auto pilots use a magnetic compass that is known as a Flux gate compass. It is a fancy magnetic compass that provides an electric signal to your auto pilot. There are also GPS corrected heading sensors such as the Garmin 24Xd that use special sensors to provide magnetic heading.