GPS>> Distance from Compass

[trueblue]

If you can't be arsed adjusting your compass, make a note of how the GPS affects it from true (ie Number of degrees variation from true). Then if you ever need to use your compass due to GPS failure, just add or subtract the variation (effect of your gps) to your back bearing to base.

kev

Unfortunately not quite as simple as that. All compasses are factory set, however there is no guarantee that they are dead true when you take it out of the box.

Then there is the small error that often happens when the compass is installed as it is difficult to get the compass body perfectly aligned with the centreline of the hull.

Then anything steel nearby such as the mechanics of your steering assembly affect it as well, or framing for biminis etc. Even "Stainless Steels" can affect compasses because they are not 100% non ferrous and different grades cause different magnetic interferences.

Then the last thing is electro magnetic interference from electric cables and instruments that are installed nearby.

Add these errors together and it is called cumulative error.

The guts of it is that for a compass to be considered to be reading true, it must be adjusted, which is a little known but critical fact that is usually overlooked.

It is vital that the compass be reading completely accurately at all points, so that you can plot a course from your estimated position back to home, by chart. Along with that, it is important that you always know exactly where you are so that you can actually use the chart to plot a course back home. It is a good idea to have a laminated list of your GPS coordinates in the boat, so that if you are fishing at spot X # 19 in the GPS, you can look up that Lat / Long for spot X # 19 on the laminated sheet and then plot that position onto the chart so that you know exactly where you are.

That said, most compasses that are installed out of the box and screwed into the boat generally in line with the hull are reading somewhere near true. But the risk is that the odd few will be grossly affected by something in the boat and can be up to 20 degrees or more off proper Magnetic North.

Just a few degrees of error can land you a long way off where you expected to land on the beach, especially if you started out a long way offshore.

Also remember that you can have a compass that reads perfectly accurate to Magnetic North, but the east / west reading can be out... This is why there are 2 adjustment bars under the compass.

All compasses need to be at least checked at the compass adjustment buoy to see how close they are. Even if you don't adjust it, go to the Compass Adjustment Buoy and line the boat up will all of the landmark bearings and check them against the chart to see how close your compass is.

You don't want to be one of the few who has a grossly inaccurate compass who didn't know it was reading wrong and didn't get it adjusted.... and then get lost...

Cheers

Mick

[kingtin]

Mick, I agree fully with what you say, particularly your use of the words "cumulative error"

My comments were based on the fact that the compass was initially reading true prior to the installation of the GPs and that nothing metallic other than the GPS had been added after the compass was set to true. It would therefore follow that if you knew the degree of interference from the GPS, then you could factor that into your calculations.

Ideally, you should have your compass re-set but this isn't a perfect world and I was offering a simple quick solution for a situation whereby someone adding something metallic may want to instantly duck out onto the water prior to having to re-set the compass.

With regards to "cumulative error". Not only can this be applied to the accumulation of items likely to affect the compass but also the number of "legs" of a journey influenced by the number of "operator errors" ie if one forgets to add or subtract mag variation on one short leg, then the error may be "manageable" but if one forgets this for all legs of the journey, then one is in deep poo.

I admit that I am sloppy with my calculations, particularly with regard to my trips to the caper via Combyuro. I simply point the boat in the direction I am heading (a straight line via GPS), make a note of the compass bearing, and then get the deckie to take a note of the back bearing for the return (in case the GPS carks it). I don't bother with variation (I am a naughty boy, says the deckie) as the back bearing will get me roughly to my point of return and the last part of the trip relies on visibility which has always been good. Bad practice really as it only takes a few seconds to do a bit of maths

Back in the UK, I was meticulous and the deckie checked everything. Fog and haze were ever present, even in summer. You had to know how to use a compass flawlessly as there was no GPS then. Just developed bad habits i guess

Off topic but just musing and reminding myself what a lazy bastar* I am

kev

[trueblue]

My comments were based on the fact that the compass was initially reading true prior to the installation of the GPs and that nothing metallic other than the GPS had been added after the compass was set to true. It would therefore follow that if you knew the degree of interference from the GPS, then you could factor that into your calculations.

Very true

Cheers

Mick

[Spaniard_King]

Thanks guys

you have basically answered my questions... I will certainly check the compass before and after installing the GPS...altho not used very often it is handy to know if your heading west and not EAST

[Chine]

SK,

Another good method to calibrate (swing) your installed magnetic compass is to compare your compass heading with GPS COG. To achieve this you need to be ideally motoring at over 5 knots in calm water (free of current and wind) so that COG is close to a true heading.

This will give you an idea of the total magnetic compass error, inclusive of both external (variation) and internal (deviation) magnetic forces on your installed compass for different headings.

By steering 000, 045, 090, 135, 180, 225, 270 & 315 degrees true (COG) and noting the compass heading you will have a record of compass heading to steer to achieve a certain true heading.

This is a little academic as, generally, the recreational magnetic compass is both poorly damped and compensated although will give you a rough idea of which way to head.

Rgds

Chine

[netfish]

Don't forget the Epirb.

I noticed the other day the compass reacted to the epirb when I was looking for a place to mount it

[Edencraft]

Never use a small gimbal mount compass on small boats as i find it is very dificult to take a bearing with one so I use a good quality hand bearing compass, a bit more expensive but it has been on about 5 boats with me now and I find it a far more usefull navigational tool on a small boat.

[wessel]

That was probably put there for legal reasons i.e. to prevent some idiot taking the manufacturer to court and sink the company because the end user did not realise that metal parts can affect the readings of a piece of equipment that works on magnetics......

If it has an effect on your compass, then it will probably be 10 degrees at the most..... That amount of error over relatively short distances is not a problem. Not sure what type of fishing you do, but if you are a person who heads offshore in one direction, and in order to get back home all you have to do is to just turn around and drive until you get back to land again. If the GPS failed, you drive on a bearing that will take you back to land again. Once you get close, check where you are and then turn either left or right for the last stretch.

A completely different ballgame when you have to do this thing called "navigation" using a chart and a compass due to unmarked offshore reefs you have to make your way through if the GPS is on the blink....
If ever I should loose my GPS while I am offshore, then I will definately be in serious trouble trying to get back through some of the reefs around here. It doesnt hurt to have a spare GPS though, just hope the yanks never switch those satelites off overhead of us...

Wessel

[STUIE63]

Is this right? I was always taught to check compass heading when going out even if I wasn't skipper so that if something happened that I could always reverse the course. But is what you are saying that it can be 10 degrees out even in this scenario.
Stuie

[wessel]

Stuie

What I meant to say was that if you install a piece f electronic equipment close to a compass on the dash of a boat, you most probably kind of definately but it wont hurt to actually check yourself will have an effect on your compass.
My thumbsuck gues from experience with my own setup and having observed the course indication on the GPS and what the compass says on other boats, that there is almost always a margin of error present.

That margin of error for recreational use is minimal and not really something to loose sleep over. Just keep in mind also that the magnetic pole is always shifting position and that this will effect the way the compass is pointing from a day to day basis.......


Wessel

(That last bit was said with serious amount of tongue in cheek, so please, no flames)