In Creating a Route on Charts, Do You Use True or Magnetic ?

[markpierce]

Originally Posted by beetle

Scanning the discussion was fascinating!

Everything on my boat is set to to True, including the mechanical compass at the helm station.

I'd like to know how you eliminate all variation and deviation from a "mechanical" compass.

[David M]

If when you lay out the track line you label it correctly then you don't need to remember if it was plotted as true or magnetic. Either works, but be consistent so the chances of making a mistake are less.

[lordgeoff]

Originally Posted by Jim Cate

G'Day all,

Pedantic arguments about true vs. other plotting methods aside, the real world beckons to us.

Once away from daysailing in ones own backyard, my observation is that most folks actually sail by watching cross track error. Doesn't matter how they plotted their course, doesn't matter if they are hand steering, auto-piloting, or windvaning, the bottom line is where is the boat relative to where you intend to go. And to worry about fractional degree changes in variation, or minor changes in deviation wrought by moving magnetic masses about the boat is usually over the top. Few helmsmen, autopilots or windvanes can maintain a course with less than a few degrees of wandering about over the length of a watch, so the theoretical errors we've been ranting about really don't apply.

Yes, I believe that most cruisers in small yachts check their position every now and then, see if they are on track, and if not, change the desired course to compensate. Some do it with electronic plotters, some with XTE on their GPS, some by plotting positions on paper charts... doesn't matter! In mid ocean one might only do this every few hours, coastally or anywhere there are more impediments to progress more frequent checks make sense.

Another set of factors that affect where you actually go are leeway and drift from current. Very few of us actually know how much leeway our boats make in different sailing conditions. I suspect that it is usually more than we think! Then there are currents. For years we sorta believed the pilot charts, where there are these great broad arrows across the sea, labeled "x miles/day". The advent of GPS on board has shown that there are LOTS of eddies, whirlpools and other deviations from the general flow, that can and do make significant changes in your COG as you traverse them. These deviations are far larger than the minor errors that come from using dated variation data, etc. Incidentally, it is really educational to check one of the websites that show real-time pictures of surface currents and temperatures... they are incredibly complicated!

I expect that professional navigators, trained to operate large vessels with gyro compasses, large crews and (usually) very precise autopilots will disagree with this approach. But in the real world of shorthanded cruising in small yachts, something like what I have described is, I believe, what really goes on. At any rate, this sort of navigation has served Ann and I well for some 150K miles now, and I'm still happy with it.

Cheers to all,

Jim and Ann s/v Insatiable II lying Manly Qld Oz, southbound

Give Jim and Ann a standing ovation. Absolutely spot on. Only done 14K myself but that's they way I see it.

[beetle]

Originally Posted by markpierce

I'd like to know how you eliminate all variation and deviation from a "mechanical" compass.

You don't, just as you don't when you swing the compass for magnetic north - you compensate for variation and note the deviation at different compass points, this is how I normally swing a compass.

The point is that the compass is swung for true north, not magnetic north.

[Jim Cate]

Beetle, just what is a"mechanical" compass? I've never heard the term.

JIm

[beetle]

Originally Posted by Jim Cate

Beetle, just what is a"mechanical" compass? I've never heard the term.

JIm

A compass that is your normal, typical helmsman compass mounted in the cockpit - no batteries, no electricity.

That term was put into the race rules for the SSS TransPac when race committee was trying to differentiate a 'compass' from the fluxgate compass as part of the minimum equipment requirements. The issue arose when someone showed up for the race with a boat that had loads of GPS devices, fluxgate compasses for slaving the autopilot to, and he'd even mounted a dedicated fluxgate compass display in the cockpit... but he didn't have a single ordinary simple compass (e.g., a mechanical compass) anywhere on the boat.

- rob/beetle

[Jim Cate]

Originally Posted by beetle

A compass that is your normal, typical helmsman compass mounted in the cockpit - no batteries, no electricity.

That term was put into the race rules for the SSS TransPac when race committee was trying to differentiate a 'compass' from the fluxgate compass as part of the minimum equipment requirements. The issue arose when someone showed up for the race with a boat that had loads of GPS devices, fluxgate compasses for slaving the autopilot to, and he'd even mounted a dedicated fluxgate compass display in the cockpit... but he didn't have a single ordinary simple compass (e.g., a mechanical compass) anywhere on the boat.

- rob/beetle

G'Day Rob,

Ahhh.... what I have always referred to as a Magnetic Compass!

So, I don't understand how you swing a magnetic compass for true north.
If you mean that you somehow tweak the compensation magnets so that the compass points toward true north whilst you are doing the swinging (which seems unlikely to me), what happens as you voyage to an area with a different variation? I believe that it now would be in error by whatever the difference in variation was.

Most electronic compasses can indeed be set to read out in True if you input the local variation, and many GPS units can actually calculate the local variation and do continuous correction to true readings. Gyro compasses are usually set to read true as well, and because of their operation, continue to point true as one voyages. But I don't see how that could be done with the old magnetic compasses. Perhaps you could explain your method.

Cheers,

Jim

[beetle]

Originally Posted by Jim Cate

Ahhh.... what I have always referred to as a Magnetic Compass!

All compasses are magnetic, in one way or another. That was the problem...

Originally Posted by Jim Cate

So, I don't understand how you swing a magnetic compass for true north.
If you mean that you somehow tweak the compensation magnets so that the compass points toward true north whilst you are doing the swinging (which seems unlikely to me)

That's exactly how you do it. You can adjust compensation magnets to force the compass to point to magnetic north (backing out local variation), or you can force the compass to point to true north (again, backing out local variation).

Originally Posted by Jim Cate

what happens as you voyage to an area with a different variation? I believe that it now would be in error by whatever the difference in variation was.

Regardless of how you swung the compass initially, if you travel to somewhere new then you need to swing the compass again to adjust for changes in local variation.

One thing I would love to have is a spun gyro compass - but they are enormously expensive and consume a fair amount of power to keep up and running.

[Jim Cate]

Originally Posted by beetle

All compasses are magnetic, in one way or another. That was the problem...


That's exactly how you do it. You can adjust compensation magnets to force the compass to point to magnetic north (backing out local variation), or you can force the compass to point to true north (again, backing out local variation).


Regardless of how you swung the compass initially, if you travel to somewhere new then you need to swing the compass again to adjust for changes in local variation.

One thing I would love to have is a spun gyro compass - but they are enormously expensive and consume a fair amount of power to keep up and running.

But Rob, When one does the normal "point to magnetic north" adjustment, then if one wishes to use true headings, one does the simple calculation using local variation, no matter what it is. So, as you voyage and the variation changes, you just use a different number in the calculation... you don't re-swing the compass. Swinging the compass is done to eliminate local deviation peculiar to your vessel, not to compensate for variation.

Cheers,

Jim

[wolfenzee]

mechanical compass?.....is this something other than a magnetic, flux gate (electronic) or GPS bearing ?........ I have always steered by a magnetic compass, GPS bearings require an update which at slow speeds takes awhile. As I sail in northern waters the compass deviation is significant (I believe something like 7.8), though my backpacking compass lets me set that, it is not something you want to do to a compass traveling on long passages. If you don't use a magnetic compass and just electronic gizmos plotting a course would be fine....until your power goes down.

[lordgeoff]

Originally Posted by wolfenzee

mechanical compass?.....is this something other than a magnetic, flux gate (electronic) or GPS bearing ?........ I have always steered by a magnetic compass, GPS bearings require an update which at slow speeds takes awhile. As I sail in northern waters the compass deviation is significant (I believe something like 7.8), though my backpacking compass lets me set that, it is not something you want to do to a compass traveling on long passages. If you don't use a magnetic compass and just electronic gizmos plotting a course would be fine....until your power goes down.

I mentioned a while ago. A box of GPSs and an even bigger box of AA Batteries is the go. If all goes pear shaped on an ocean crossing, turn a GPS on once every couple of hours to get an accurate position and you can navigate just fine for weeks on end. GPSs and batteries are cheap.

[wolfenzee]

With a magnetic compass you move the tiller, the compass card moves, with GPS you move the tiller....and after you have been on that course long enough for the satellites to notice a difference....that bearing changes + or - the GPS error. I am just not comfortable with the GPS lag.

[lordgeoff]

Originally Posted by wolfenzee

With a magnetic compass you move the tiller, the compass card moves, with GPS you move the tiller....and after you have been on that course long enough for the satellites to notice a difference....that bearing changes + or - the GPS error. I am just not comfortable with the GPS lag.

GPS combined with Autohelm means the course held is much more accurate than any hand steering using the compass can be. Because the GPS and Autohelm are making perhaps 100 adjustments a minute it is better than the human hand reacting to a floating movement of the compass. I use GPS/Autohelm combination over hand steer when crossing a rough dangerous bar as you never get oversteer.

[beetle]

Originally Posted by Jim Cate

But Rob, When one does the normal "point to magnetic north" adjustment, then if one wishes to use true headings, one does the simple calculation using local variation, no matter what it is. So, as you voyage and the variation changes, you just use a different number in the calculation... you don't re-swing the compass. Swinging the compass is done to eliminate local deviation peculiar to your vessel, not to compensate for variation.

Yes, that would be normal and keep things straightforward. I singlehand and when I get really tired I find that I can make stupid mistakes such as adding when I should be subtracting. To that end, I'll happily put in the effort up front to keep things as simple as possible (e.g., work hard to make things look easy).

One of the things I've found that makes life simple for me is to swing the compass to true before heading off, and, upon arrival, swing the compass again if I need to (which takes about an hour - I'm getting better at it).

I'd rather spend the hour adjusting the compass in the current locale such that it agrees with everything else on the boat including the charts, rather than depend upon a tired me to make the correct calculation in my head as to how lines on the chart correlate to what the cockpit compass is telling me.

- rob/beetle

[GordMay]

Originally Posted by beetle

... One of the things I've found that makes life simple for me is to swing the compass to true before heading off, and, upon arrival, swing the compass again if I need to (which takes about an hour - I'm getting better at it)...

Unfortunately, that wouldn't work when crossing isogonic lines at high latitudes, or near magnetic anomalies, where the variation/declination can change at a rate of over 1.5 degrees per mile.

I didn't believe that the TransPac folks used such an imprecise (& technicaly inaccurate) term as "mechanical compass"; but apparently they have:
➥ 2004 Transpac Inspection