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Onboard Plotters / GPS. True or Magnetic?


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Wondering who sets their plotters and Gps up as true, who sets them up as magnetic?

 

It's a preference thing obviously, but there must be a good reason you personally choose one over the other.

 

Someone told me they set theirs all up to give magnetic readings and headings so that they don't have to do the maths when they then steer via their compass,,,,however, I was thinking that if your compass is not swung correctly (as I suspect most are not on your average joes boat) then this could put you a wee bit crook. Yes / No?

 

Actually, speaking of compasses, one guy told me that he sailed his Cat from Sth Africa to NZ and didn't even have a traditional compass on board! 

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when gps is working and used correctly the compass isn't essential

 

but there are so so many accidents when gps is either not working correctly or used poorly

 

remember 1  story of a cruising cat that set it's autopilot to cross the baja gulf?, a simple straight line, and then partied on

 

they failed to account drift into account and wrecked on a shoal they thought they'd clear easily

 

you can be lucky lots

 

but that won't keep you as safe as skill + knowledge

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Everything in true, but I haven't really used a paper chart in years other than planning. I still use  a hand bearing compass occasionally but for clearance bearings and such not for a running fix or triangulation.

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And the reason for that was simplicity, keep everything written down as true then at the end you produce a course to steer which would be noted in the log. Now you get information from all sorts of pieces of equipment with different yaw rates and nothing reads the same at the same moment. GPS, back up GPS, the plotter, the laptop the phone the autopilot all telling you stuff. When I go somewhere I've never been that has a tide or current effect after I've done the planning I just use the plotter on my phone and the helm compass to keep me sane.

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Hmm, either works fine. I normally use magnetic, and the compass when last swung had no error >5 deg. So I don't worry about it - hard to keep a course accuracy within 5 degrees when. Manually steering anyway. This way, as mentioned, if I were to lose the electronics, the course in the log can be immediately used at the compass.

I've said this before - never trust a single source of navigation info. Set a course, then check with a different system that you are maintaining it, and it's safe.

The example above of the cat steering a compass course is stupid! That's incompetence. And it's also what Nav mode is for on the Ap. Still, it needs monitoring.

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 If you're using magnetic then you still have to make an adjustment for your compass but not for a hand bearing one. I found it easier to stay and think in true, for the most part you only have to make a variation and deviation adjustment once, if you use only magnetic every action or plot requires adjustment. And for long courses using true the variation changes and is more likely to be accounted for by habit. Anyhow horses for courses as long as the fridge is cold.

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Do be aware that a compass may still read in error every now and then, even if it has recently been swung and that includes electronic compasses on AP's. Simply due to magnetic anomalies in the ground under us. The Sounds are really bad for it for instance. There are a couple of places where the AP will make a significant alteration in path. But it is not for long. Where as a GPS set to Mag is a calculated heading rather than following of a magnetic field. None of that is usually a cause for navigational concern of course, just more of interest.

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If everything is in magnetic there is nothing to convert. The plotter reports Mag, the compass (all of them) report Mag, the weather I use is reported in Mag, just as easy to plot on a chart in Mag as it is true. What is there to convert, nothing.

 

I can't understand why you have one compass that needs correcting and one that doesn't. That's either a compass with a deviation issue which can be corrected or the compass is rooted.

No, compasses give uncorrected (deviation) magnetic, IT said his compass is less than 5 degrees deviation, for me 5 is quite a high figure to ignore. I take it all and then give a simplified course heading. I generally don't bother reducing the annual variation unless it's really bad.

A hand bearing compass doesn't have a deviation card.

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a hand bearing compass will suffer deviation though - its just that it varies depending on too many variables to bother with a card.

 

Raises another point - where do most people swing their compass' in Auckland

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a hand bearing compass will suffer deviation though - its just that it varies depending on too many variables to bother with a card.

 

Like a big chunky 1980's casio digital watch with calculator

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Pummba, you might think 5 deg is large, but if you're hand steering, without a plotter or gps, I'd be very surprised if you (or anyone) can steer within 5 degrees over a long period. Yes, easy to do in reasonable conditions for a short period, but not across an ocean. Fixes will be required, and course corrections made.

Gps and plotters are great, with an autopilot interface even better!

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No IT i'm talking about ignoring the 5 degrees of deviation not about holding a 5 degree course tolerance, I add up all the adjustments to the nearest degree (everything in true), heading,tidal offset, drift, var,dev and then I round the total adjusted figure up or down, normally to the nearest 5 degrees but at night I might make it to the nearest 10 degree line on the compass so it's easier to see for a compass course especially if you're having to hand steer.

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I always run everything true.

Why would you take a technology that is capable of constant and introduce "an error" ?

I don't know any long distance cruisers who steer to a mag compass. Its only there as a last ditch back up. Adjustment for leeway with an auto pilot set using mag or true is easy and the same.

Set a waypoint track ...then check the xte (cross track error) adjust the bearing a few degrees until the xte is as stable as possible. Check as part of normal watch keeping duties.

I only hand steer if something is broken.

By the way..steering to a star if possible is way more acurate than trying to follow a compass needle.

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Now you do. Me.

Steering to a star is fine,but you better check its bearing every 15 mins or better. Stars move.

In the event of a failure of electronics, however unlikely, the magnetic course in the log can simply be used directly on the magnetic compass. No conversion or mathematics required, in what may well be a stressful situation. I'll not be wanting the Admiral to be trying to figure it out, and remembering which way to apply variation etc if something has happened to me.

An alternative is to use true bearings, but record both in the log.

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Ok ..firstly I don't have the energy or accuracy to compete with my most competent helm..so am glad to let Albert steer. Albert dosn't care if its mag or true.

All my charts are in true. My chart plotter charts are true. In fact the only thing that is not is my least used least accurate device...the compass.

On a small steel vessel the compass table of offsets needs to be done regularly (along with swinging the compass) local magnetic annomolys along with changing variation from your charts rose all add up to traditional paper navigation...which I don't do. Its not accurate enough for me.

So..if all of my three main plus two electronic systems fail then I will be a little surprised and revert to tradnav.

And except a very likely 10 degrees of inaccuracy.

I guess its just when its cold wet and dark I don't enjoy standing at the helm..yep lazy sailor.

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I'd just use true on everything. Haven't had to use a compass for a while, but practice occasionally.

 

I don't think it's that hard converting to true and magnetic. You just have to remember which way they go.

 

Timid Virgins Make Dull Companions, add Whiskey :-)

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