Plotters & Compasses, how does that work then?

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kcrane

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Just struggling to get my head around it, but a question for the experienced forumites.

I just calibrated the fluxgate compass on the boat. This meant sailing slowly in a circle twice. We thoughtlessly did this in Portsmouth Harbour, which drew the attention of Seaman Plod as apparently we were a threat to the warships.

It reported a 1 degree deviation, which as the limit is 15 degrees, we took to be quite good. We then sailed in a straight line at 3knts + and set the fluxgate to equal the boat's compass as instructed.

The plotter will now take this as its primary input re heading, which is a magnetic bearing, but presumably the charts (RL70, so C-MAP) are aligned to a true north, so when I plonk a waypoint on the chart and say "Sail to that, you smarta**e boat" where is the conversion between magnetic and true being done?
 
Reading your post I think you are looking at the raymarine unit difference not the chart, the difference from true to magnetic is in the settings menu, if I remember there is an "auto" setting or a "manual" input, either should be around 3W as I said before.
 
Re: Plotters & Compasses, how does that work then?

Ah, that may be the answer... the plotter has a setting for the variation, tho why it can't know variation for its charts, oh, maybe that's the auto setting
 
Re: Plotters & Compasses, how does that work then?

Why are you using Magnetic? One of the great things about gps is that you can use true.
If you have Raymarine stuff set it to true. Then sail in straight line take the true heading from the gps and set the fluxgate reading to this heading.
 
Re: Plotters & Compasses, how does that work then?

Why would you do that?

Why not convert everything on the boat to magnetic, GPS included, as this way everything agrees. Otherwise all your fancy instruments agree, but don't agree with the main compass, or the hand held compass if you're trying to identify something. Having everything the same means you can just forget about the difference.

Plus, when motoring in a straight line, don't forget that any tide, or even wind, can push you sideways slightly and your COG will not be the same as your heading. I'd be very dubious of any calibration done on that basis.
 
Re: Plotters & Compasses, how does that work then?

I dont have a compass.... it's 2006 you know. Have a standby compass in case of elec failure. Why use magnetic if true is available?
 
Re: Plotters & Compasses, how does that work then?

Because the simplest instrument on the boat uses it. The rest are clever enough not to care if they're using magnetic or true, so why not make everything the same?
 
Re: Plotters & Compasses, how does that work then?

Yes make everything true. As I said before I dont have a compass.
 
Re: Plotters & Compasses, how does that work then?

I'm intrigued, how do you sail a compass course without a compass?
 
Re: Plotters & Compasses, how does that work then?

I have ST60 instruments. Autopilot has fluxgate compass, this is set to true. The autopilot display reads heading when in standby and desired heading in auto.
I have a graphic display unit. I have set this to read heading and course over the ground.
So in manual I can read the heading from either the Autopilot or the Graphic. In auto I read the heading from the graphic.
Absolutely no need for magnetic headings with this set up.
Should the electrics fail I have a hand bearing compass which slots into a bracket in the cockpit, then it's back to the nineteenth century.
I flew airliners 8 million miles only using fluxgate/INS/GPS----never once had to use the "real" standby compass.
 
Re: Plotters & Compasses, how does that work then?

Question asked, question answered!

Guess you must be pretty used to just a 3 digit compass reading with a flying background. Don't think I'd be tempted to ditch the compass simply because the card makes sense checking the bearing before turning onto a new course easier.
 
Re: Plotters & Compasses, how does that work then?

Great as long as you have amps.....I'll stick to magnetism thanks...electrickery is a useful comfirmation of observations. I've even got a walker log....
 
Re: Plotters & Compasses, how does that work then?

I find it a lot easier to steer a course using a digital readout-----the autopilot finds it even easier!
 
First, as we know, the world is set up on 'True'.
Next, and consequently, so are most of the charts we're likely to come across.

Magnetic-sensing compasses 'point' in the general direction of the Magnetic Pole - or more correctly, 'align with the direction of the local magnetic meridian' - 'cos that's all that was available for many centuries, before the Coming of GPS. ( we know how reactionary some of the 'old sea captains' on here can get ).

The 'local' bit matters, for in some local areas, that alignment means the compass points a little east of True North, and in others somewhat west of Tn. It has all been surveyed, and the 'compass roses' on Hydrographic Office charts give the value of Local Magnetic Variation when the chart was printed. So heading/course information taken from your main steering compass needs to be modified - by you - by this value, before it can be used for charting purposes.

The fluxgate device is magnetic-sensing, too. Its electronic-signal output can be tweaked electronically to 'input' this Local Variation as an error-correction signal. Whether the value used is fixed manually by you, or whether it uses a variable value from an internal algorithm ( as many can ) depends on the make and features.

Most pro navigators - including airline pilots of a certain vintage, used to wicker armchairs and astrodomes - can readily use Mag and True, converting effortlessly as appropriate. That's familiarity with the processes, but occasionally one still manages to get it all backside-foremost. Perhaps that's why there's a SuperSeacat ferry in Poole ( another thread ) instead of Weymouth..........

Should you happen to know, reliably, that your magnetic steering compass is calibrated accurate to 1º on all headings, and that there is no residual deviation on all headings more than 1º, then it would make sense to 'align' your expensive fluxgate device onto that. Otherwise, I'd be tempted to get a readable booklet on the subject ( such as 'Practical Guide To Compass Correction' by G.H Reid ) and align it myself on a sound, accurate transit. Otherwise, as said before, most yot clubs have some ould soak propping up the bar who knows this stuff backwards. For the cost of a couple of G&Ts, you could get it all done for you.

As you will need all the available accuracy when your radar and ( soon ) AIS-overlaid plotter tries to work out Closest Point of Approach of an approaching High Speed Ship, I don't see the point of spending several hundred pounds on a fine electronic compass system, then degrading its potential accuracy down to that which it was designed to supercede.

But I'm sure someone will point out a reason.....

/forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
 
EDIT>>

post deleted by DogWatch, I can't be arsed getting into a thread suggesting a compass is not required on a boat and an owner of a 38ft super large cat who does not understand basic navigation.

Yes this is a miserable and totally unhelpful response, but I am totally flabbergasted.

Sorry!
 
[ QUOTE ]
<span style="color:blue"> Dogs are better than cats! </span>

[/ QUOTE ]

I've heard it said that the HT26 is rather of 'dog' of a 'cat', but I reckon you're just plain jealous of Cap'y.

As am I....

But don't get your knickers in a twist.

Who cares that he doesn't know his amphidromic point from his Prime Meridian? Maybe he'll ask me to come and do all that boring stuff seamlessly for him....

/forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
 
Of course I am jealous!

But also shocked that big boats like that are thundering up the harbour with inexperienced skips at the helm. Yes PBO is the place to ask, but I would have expected the purchaser of an Ocean going vessel would have rudimentary knowledge of navigation.

I am just offending people now, so I will shut up and leave you all to it.
 
Re: Plotters &amp; Compasses, how does that work then?

[ QUOTE ]
never once had to use the "real" standby compass.

[/ QUOTE ]

True story: the standby P2 compasses fitted to the RAF's fleet of SA-330 'Puma' helichoppers were ALL found to be attached to the airframes using magnetic set-screws. French-built, of course, and the French airframes reportedly never had such a problem!! /forums/images/graemlins/blush.gif

This was discovered following a routine - but accurate - compass swing which would not resolve, and after about 12 years of regular compass swings by a generation of Wing Navigators.

Seems that they had all - to a man - frigged the P2 figures over the years to make the compass logbooks 'come out right' - and signed them! /forums/images/graemlins/blush.gif

Guess who was hugely unpopular for tripping unawares over this 'professional shortcut', and consequently making the whole Puma fleet 'Unserviceable' for other than airtests and circuit flying in VMC conditions, until new set-screws could be sourced, procured, certified and fitted? Guess who's boss took to snarling like a rabid dog when someone's name was mentioned? Guess who's Senior Reporting Officer ran a short, intense competition to find the worst 'bad boy' posting - worldwide - in the 3 Services and Commonwealth Forces for someone?

Guess who hasn't done a compass swing since....?


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[ QUOTE ]
<span style="color:blue"> I am just offending people now </span>

[/ QUOTE ]

Not at all, Doggy. I'm probably just as narked as you at the implications, but I've come across this eccentricity before. I'm still only halfway through my Rehabilitation 'Anger Management Course', and I'm working on the techniques. /forums/images/graemlins/grin.gif

Anyway, there's probably only thee 'n me on here, getting grumpy.

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