Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 02-02-2013, 05:10   #61
Registered User
 
GrowleyMonster's Avatar

Join Date: Dec 2012
Location: New Orleans
Boat: 1976 Cal 2-27
Posts: 1,298
Re: True or Magnetic for Electronic Nav

For all magnet-heads... your homework assignment is to read Bowditch and then report back what it says. Note, please, that Bowditch is the main reference cited for the USCG Navigation module in the Deck exam question pool. It is also a required pub aboard ship. It is the Navigator's Bible, if there ever was one. Can we at least agree that American Practical Navigator is a rock solid authority on Navigation?

And that's my final argument.
__________________

__________________
GrowleyMonster
1976 Cal 2-27, MR WIGGLES
Now with clean, dependable electric propulsion!
GrowleyMonster is online now   Reply With Quote
Old 02-02-2013, 05:14   #62
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: True or Magnetic for Electronic Nav

Quote:
If true north was defined in any other way, latitude would remain the same as it now is.
Thus speed is not dependant on the north reference whether that be true north, chart north, magnetic north or any other north.
huh....!

True north is a fixed reference point, magnetic north is a wandering minstrel. we use Magnetic because for many years we couldnt sense or measure anything else, thats all.

Speed is a scalar quantity , Speed with Direction is a vector and must have a frame of reference. for that we common use True. ( as in when we plot on a chart). Thats becuase almost everthing we get is resolved to True, ( Tides, directions to objects, running fixes, three point fixes etc).

Dave
__________________

__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 02-02-2013, 06:18   #63
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by daddle View Post
Not if I cannot clearly explain it

It is obvious that when using True headings, bearings and such that one is using the well known and simple coordinate system of latitudes, and longitudes. All the numbers we use work together nicely. Direction, location, speed, and time (and velocity) all relate in simple ways. The coordinate system is all nicely regular, has nice (spherical) right angles, and so forth. Computations in your head, on charts, and in the chart plotter software are simple and well known.

What it less obvious is that the Magnetic headings, bearings and such that we use are also in a coordinate system. It's a crazy one. The latitudes and longitudes are not regular, straight, or consistent in any way. They even change with time! Computations in a Magnetic coordinate system would lead to insanity. Smart guy Bowditch (the book anyway) will back me up on this.

So when one sets their chart plotter to Magnetic all they are really doing is changing a few of the outputs based on the local magnetic variation. Don't fool yourself that you are working in Magnetic. Personally I think trying to mix the two is a bit nutty. I convert from one to the other at the companionway stairs when climbing up with a course to steer or climbing down with a bearing to an object. That is simple - especially so in SE Asia where the variation is nearly zero everywhere.
OK, this makes sense and what makes even more sense is that you and I have been talking about two different things.

I have been referring to magnetic bearings / headings in the same sense of the OP i.e. the electronic nav system just converting the true reading to magnetic and displaying same rather than you doing the conversion and I have not been considering a complete magnetic co-ordinate system.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 02-02-2013, 06:55   #64
Registered User

Join Date: Dec 2009
Posts: 3,016
Images: 4
Re: True or Magnetic for Electronic Nav

Yes, but the OP wrote "giving you all data in true or magnetic" and others seemed to be expressing similar thoughts that most of the navigation work could be done in Magnetic coordinates. I then jumped in with my nerdy geeky arguments ...
__________________
daddle is offline   Reply With Quote
Old 02-02-2013, 23:00   #65
Sponsoring Vendor

Join Date: Nov 2012
Location: Australia
Boat: Catalina 387
Posts: 21
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by goboatingnow View Post
in my opinion all plotting should be in True and only when steering orders are been issues are conversions done into magnetic. The only reason we have magnetic on board is we still rely on a traditional compass ships and others using gyroscopes have long since moved to True. Magnetic directions should remain in the realm of steering and nothing more.
Anyone looking for logic in this would agree that doing any type of conversion is additional effort and a point of potential error, in simple terms,if you have waypoints in the plotter and the plotter is showing bearing to waypoint of 250deg it's straight forward to steer a course of 250 without any conversion or trying to remember the rules such as error east compass least, error west compass best.
Your point of gyros makes sense if you have a gyro compass, I have never sailing on a recreational vessel with gyro compass so anyone reading this post should consider the type of compass which aids in steering there boat.
__________________
iLog Sailing Companion (new iPad app)
www.ilogsailing.com

[SIGPIC][/SIGPIC]
iLog Sailing is offline   Reply With Quote
Old 02-02-2013, 23:19   #66
Registered User

Join Date: Feb 2013
Posts: 2
Re: True or Magnetic for Electronic Nav

Excuse me. Can someone help me? Where is more difficult and dangerous sailing, in Mediterranean or along Atlantic coast (from Portugal to Shetland islands) , because of weather, windstorms, sea currents, fog, tides etc.

If there was a discussion about this on the site, please give a link???
__________________
Stan IV is offline   Reply With Quote
Old 02-02-2013, 23:43   #67
Senior Cruiser

Cruisers Forum Supporter

Join Date: Mar 2007
Location: Santa Cruz
Boat: Boatless Again
Posts: 4,310
Re: True or Magnetic for Electronic Nav

I can swing either way, having used CM93 and OpenCPN, but I have one word of advice--don't try to calibrate your autopilot compass to True North. If you have to ask why, you need more school.
__________________
donradcliffe is offline   Reply With Quote
Old 02-02-2013, 23:54   #68
Registered User

Join Date: Feb 2013
Posts: 2
Re: True or Magnetic for Electronic Nav

You got me wrong , That was off-top question from me. One man is trying to prove me that sailing in Mediterranean area was more difficult and dangerous in the past to compare with North Atlantic . I can't agree with him. i just need facts prove otherwise. That's it
__________________
Stan IV is offline   Reply With Quote
Old 03-02-2013, 00:02   #69
Registered User
 
Iskoteo's Avatar

Join Date: Jun 2011
Location: Campbell River BC
Boat: Roberts 434 pilothouse
Posts: 18
Re: True or Magnetic for Electronic Nav

We have the furono SC 30 sat compass networked to both chartplotters and the auto pilot, all set to true north. We use the magnetic compass as a backup. Cheers
__________________
Iskoteo is offline   Reply With Quote
Old 03-02-2013, 02:59   #70
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by Stan IV View Post
Excuse me. Can someone help me? Where is more difficult and dangerous sailing, in Mediterranean or along Atlantic coast (from Portugal to Shetland islands) , because of weather, windstorms, sea currents, fog, tides etc.

If there was a discussion about this on the site, please give a link???
Quote:
Originally Posted by Stan IV View Post
You got me wrong , That was off-top question from me. One man is trying to prove me that sailing in Mediterranean area was more difficult and dangerous in the past to compare with North Atlantic . I can't agree with him. i just need facts prove otherwise. That's it
Hi Stan IV, welcome aboard. I think you will get a better response to your question if you start a new thread.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 03-02-2013, 03:45   #71
CF Adviser
 
Pelagic's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 6,880
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by donradcliffe View Post
I can swing either way, having used CM93 and OpenCPN, but I have one word of advice--don't try to calibrate your autopilot compass to True North. If you have to ask why, you need more school.
Don…..These kind of blanket warnings annoy me.

We basically calibrate ‘correct’ every type of compass to True in order to verify our compass error.

That is something a prudent navigator does everytime he checks the log entries or takes bearings on a range


From one Flux-Gate Compass Manufacturer

Q: Can a fluxgate compass show True North?
A:
Since fluxgate compasses are magnetic compasses, they can only reference the earth's magnetic field. They can not reference the earth's inertia and therefore cannot be made to know True North.

If you wish to navigate in true heading, you have a couple of options. KVH's Azimuth 1000, Azimuth 314AC, GyroTrac, and Sailcomp 103AC allow you to enter an offset angle.

This means that you can enter your local variation value (either positive or negative) and the compass display will show true rather than magnetic heading. When your variation changes because you have traveled a significant distance, however, that offset angle will not update automatically. You must update it manually.

Sailcomp

The GyroTrac can also interface with a vessel's GPS, allowing the compass to learn the variation from the GPS. With this option, your GyroTrac will give true heading and will automatically update to the correct variation.


This more modular Furuno, C-500 will do exactly what you say it cannot.

http://www.furuno.com/en/business_pr...arine/c500.pdf
__________________
Pelagic is offline   Reply With Quote
Old 03-02-2013, 06:26   #72
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by daddle View Post
Yes, but the OP wrote "giving you all data in true or magnetic" and others seemed to be expressing similar thoughts that most of the navigation work could be done in Magnetic coordinates. I then jumped in with my nerdy geeky arguments ...
Hey, I'm happy to partake in nerdy geeky arguments
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 03-02-2013, 10:49   #73
Senior Cruiser

Cruisers Forum Supporter

Join Date: Mar 2007
Location: Santa Cruz
Boat: Boatless Again
Posts: 4,310
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by Pelagic View Post
Don…..These kind of blanket warnings annoy me.

We basically calibrate ‘correct’ every type of compass to True in order to verify our compass error.

That is something a prudent navigator does everytime he checks the log entries or takes bearings on a range


From one Flux-Gate Compass Manufacturer

Q: Can a fluxgate compass show True North?
A:
Since fluxgate compasses are magnetic compasses, they can only reference the earth's magnetic field. They can not reference the earth's inertia and therefore cannot be made to know True North.

If you wish to navigate in true heading, you have a couple of options. KVH's Azimuth 1000, Azimuth 314AC, GyroTrac, and Sailcomp 103AC allow you to enter an offset angle.

This means that you can enter your local variation value (either positive or negative) and the compass display will show true rather than magnetic heading. When your variation changes because you have traveled a significant distance, however, that offset angle will not update automatically. You must update it manually.

Sailcomp

The GyroTrac can also interface with a vessel's GPS, allowing the compass to learn the variation from the GPS. With this option, your GyroTrac will give true heading and will automatically update to the correct variation.


This more modular Furuno, C-500 will do exactly what you say it cannot.

http://www.furuno.com/en/business_pr...arine/c500.pdf

Thanks for illustrating my point so eloquently, but my warning was not directed at folks like you.
__________________
donradcliffe is offline   Reply With Quote
Old 05-02-2013, 04:04   #74
cruiser

Join Date: May 2008
Posts: 164
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by iLog Sailing View Post
A further observation, assuming you sail with paper charts......
Actually not intending to. I can buy charts for Inavx via navionics for a fraction of the cost of paper charts.

Maybe a couple ocean charts, but that's it.

Now someone will say "what if your electrics fail".

Well, I guess that is the same sort argument celestial aficionado's had with
gps only users in the early days of gps. You can drop a Sextant overboard, charts can get wet.

Two Ipads with all the charts downloaded on each, and 2 or 3 cheap gps handhelds. They are not all going to fail, surely.
__________________
LBW399 is offline   Reply With Quote
Old 05-02-2013, 04:32   #75
cruiser

Join Date: Dec 2012
Location: Seattle
Posts: 1,130
Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by Kiwi399 View Post
They are not all going to fail, surely.
Uh, oh. Break out the popcorn.

__________________

__________________
Jammer Six is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Charts II: NGA - 2700 Charts cagney OpenCPN 1292 23-12-2013 09:46
For Sale: BRISTOL 45.5 DESTINY avail. for viewing in South FL sail_destiny Classifieds Archive 7 17-01-2013 10:04
Differences Between Ground, Apparent, and True Wind Direction twistedtree Navigation 92 26-12-2011 14:48
True North or Magnetic North ? Mary Farmer OpenCPN 24 31-10-2011 13:39



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 05:37.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.