Chartplotter compass v fixed boat's compass

[PerformanceSailor]

Garmin = Highly Magnetized

Hi,

Went to the boat today to inspect my lovely new Garmin GPS Map 750 chartplotter. It's a great unit. Unfortunately, the new binnacle pod means that the boat's compass is not visible when standing at the helm. On the whole, it shouldnt be a problem as I tend to be sitting when I helm, but on the occasions when I am standing, I thought i would use the Chartplotter's compass. It reads differently to the boat's compass (am assuming as a consequence of no deviation?). Can anyone advise me on whether navigating off a chartplotter's compass would be any different than than a ship's compass?

Thanks all.

Dave

When you scroll down to page 8 of the Appendix for the install instructions you will find out, indirectly, that the little door over the chip slot is secured by a powerful permanent magnet.
It tells you that the compass safe distance = 32". Some other Garmin models specify that this distance be a minimum of 39".
Only my opinion, but these were probably never initially engineered for any boating application, especially a sailboat.

So, visibility of the ship's compass may not be your first concern...

Talk about gross Deviation! A clear triumph of marketing over engineering.


L

 

[nimbusgb]

There are 'GPS Compasses' as mentioned higher up the thread they use multiple antennae and actually measure the different times that signals arrive at each antenna to calculate orientation. Perhaps a 'satellite compass' would be a better term or even a 'satellite attitude indicator'?

JRC makes one if you have a couple of grand to spare http://www.marconi-marine.com/products/jrc/pdf/JLR-20-30.pdf

 

[mcframe]

A tall ship and a star to steer her by

Provided you are aware that the stars (except, effectively, Polaris) are moving & how the one you are using is moving.

I've only ever done that once - by luck - Polaris was due[0] to appear *just* above where the signal halyard met the crosstrees on a trip back across the channel.

It did, and it was nice - as long as I sat in the same place at the helm.

[0] estimate by "digital sextant", given assumed latitude; arm outstretched, pinkie & thumb as far apart as possible ~= 15 deg, interpolate as required ;->

 

[Stu Jackson]

You may or may not be confused yourself, but you certainly risk confusing other people. You use the term "GPS heading" to mean COG. Most people would restrict the use of "heading" to "the way the bow is pointing", which you refer to as "compass heading". This caveat apart, I would agree with what you say as regards a standard GPS set or chartplotter...

alan, you're right. Boat bow is "heading" (traditionally in piloting) but when used on a GPS output, it's called "heading" (the way you're moving, which is course over ground COG as far as the simple GPS can handle it) and not the traditional and correct boat "heading" the way the bow is pointing - which we both agree has been explained earlier. On the GPS, where you want to go (next waypoint) is "bearing." The difference between the traditional non-GPS heading and the straight line course from point A to point B was the "course," since the boat was always yawing around. Interesting discussion.

 

[CharlesSwallow]

A small ship I do some work on, only had a magnetic compass when it was bought (40yo and 2nd hand).

A GPS compass has been fitted to allow autopilot use, and has effectively pushed the magnetic compass into standby. All nav is now by the GPS compass.

(The GPS compass aerial array is made up of 3 aerials in triangular setting about 15cm apart. Very stable.)

So I would say that your GPS compass from your plotter or GPS would be quite adequate. You always have your magnetic compass if you have an electrical/plotter/GPS failure.

I only use the plotter compass, but have the magnetic if all else fails. (I also have a separate GPS).

The autopilot doesn't have a remotely sited fluxgate compass as it's primary heading input then? What happens when Barrack decides to degrade or switch off GPS?

Chas

 

[Silent Lady]

CCU

The Garmin is the same as all the others . Thats why they list a CCU ( Course Compass Unit ) to use with their Autopilots . The system HAS to know the compass heading.

Here

Bit on the expensive side though but it does give turn rate / yaw etc.

The Raymarine System does the same with its separate Flux Gate Compass and other bits of kit.

The Garmin Unit certainly looks the part but it could be costly to fit an Autohelm.

Compass can manage without the Chart Plotter but the Chart Plotter is useless without a Compass Heading. Trying to calculate one will not be close enough as you have already seen in the difference between Compass and Plotter. It will be near but never spot on .

.

 

[fireball]

Umm - was it something like this?

Along those lines - but not the same - this one had 4 mushrooms in a diamond - seems like the same principle.

 

[fireball]

I beg to differ. The ONLY information the GPS has is the boat's heading, which is COG.

No NO NO NO NO No NO NO NO

a GPS has one bit of 'true' information - it's position ... the rest of the information is derived from this. COG and SOG is a calculation based on where the unit IS now against where it WAS before.
Hold a GPS still and COG is 0 - actually it often gives strange headings not related to your actual heading - this is because it relies on MOVEMENT to decide your COG.

GPS Compass relies on 2 or more GPS mushrooms and will use their relative positions to CALCULATE a HEADING.

 

[fireball]

There are 'GPS Compasses' as mentioned higher up the thread they use multiple antennae and actually measure the different times that signals arrive at each antenna to calculate orientation. Perhaps a 'satellite compass' would be a better term or even a 'satellite attitude indicator'?

JRC makes one if you have a couple of grand to spare http://www.marconi-marine.com/products/jrc/pdf/JLR-20-30.pdf

Ah - someone else who KNOWs! ...

Well - I didn't know how it calculates the heading - it was based on guesswork - and not knowing what information the mushrooms actually get ... 3 points would make a triangulation though ... so sounds right.

 

[Stu Jackson]

No NO NO NO NO No NO NO NO

a GPS has one bit of 'true' information - it's position ... the rest of the information is derived from this. COG and SOG is a calculation based on where the unit IS now against where it WAS before.
Hold a GPS still and COG is 0 - actually it often gives strange headings not related to your actual heading - this is because it relies on MOVEMENT to decide your COG.

GPS Compass relies on 2 or more GPS mushrooms and will use their relative positions to CALCULATE a HEADING.

Not sure about the no, no, no...but I think we do agree that the GPS definition of heading is exactly what you describe: compared to what it was "before." And I agree with the need for movement. My reply #24 above discussed the difference between traditional piloting use of the word "heading" and the use of that same word with new GPS unit displays, at least the simple ones with single receivers.

With more sophisticated multiple receiver GPS units, of course, the more position "fixes" will be available (pun intended).

 

[Silent Lady]

Hmmm

So the Marconi Unit relies on the 3 way head unit which works out its position and then sends it to the head unit. Bit like a compass really that sends out its actual orientation to the poles. Wonder if they know that Raymarine do one thats a fraction of the cost. Think they call it a Flux Gate Compass

 

[whipper_snapper]

I've only ever done that once - by luck - Polaris was due[0] to appear *just* above where the signal halyard met the crosstrees on a trip back across the channel.

It did, and it was nice - as long as I sat in the same place at the helm.

[0] estimate by "digital sextant", given assumed latitude; arm outstretched, pinkie & thumb as far apart as possible ~= 15 deg, interpolate as required ;->

I am surprised. I use a star to steer by all the time - and never with Polaris. It is SO much easier and nicer at night to just plonk a star in the right place against the boat. Hold that for 20 minutes or so, then check the compass and find a new star/bit of the boat.

 

[Signed Out]

A fair few true GPS "gyros" out there.

Have a look here if you're interested-

http://www.hydro-international.com/productsurvey/id16-GPS_Compasses.html

Phase difference, ie phase of signal wave/code (or something like that, I forget and can't bothered to read up right now).

As long as calibrated properly, like all sensors, is very accurate, has minimal run-up time, and far, far more sturdy than a traditional spinning gyro. Of course there's fibre-optic gyros (FOGs) too...

But not what the OP wanted to know.

If using a plotter compass to steer when sailing, I'd want a rose display, not a chart. But a fluxgate is probably as good as most small magnetics.

 

[pugwash60]

To answer the original question, it's fine to steer by but don't expect to find it easy, just pick a course relative to something; star, wind, sun, church, bushy topped tree, etc. and see what course the gps gives you then turn left or right a bit if you need to. Don't worry about cross tides, you'll only notice the difference if you are sailing against somebody else. Probably more than half of people that sail or motor do so on a track not a heading anyway so you'll be in the majority even if technically not sailing the shortest route.

What most of the posters are missing is the advantage of the gps compass, as in the unit designed as a compass with multiple receivers, which is that it not only gives headings which are 'true' so you don't need to muck about with deviation and variation, thus saving some dull virgins, but it also is very stable, so doesn't swing about as you bounce around. They are obviously a lot of money for a small boat but not really designed for that, this is because they are a cheap alternative to a proper gyro compass which senses true north from the earths rotation and are what is normally fitted to ships.

Flux compasses are a cheaper alternative but less accurate, stable and only give magnetic heading which is two corrections removed from any true heading and in the majority of cases on a yacht only one of the corrections (variation) is ascertained.

 

[mcframe]

I am surprised. I use a star to steer by all the time - and never with Polaris. It is SO much easier and nicer at night to just plonk a star in the right place against the boat. Hold that for 20 minutes or so, then check the compass and find a new star/bit of the boat.

David Lewis, _We, The Navigators_ explains how the Polynesians worked their star paths.