Compass vs VHF Radio - Moderated Discussion Areas

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: The Whaler GAM or General Area
  Compass vs VHF Radio

search | FAQ | profile | register | author help

Author Topic:   Compass vs VHF Radio

Jimm posted 04-02-2003 10:42 AM ET (US)
I noticed in another topic that a member was asking where to mount a VHF radio in his new boat. Naturally the question of compass deviation came up. Is this deviation caused by the speaker magnet in the radio or the transmit power from the antenna? Would shielding the compass from the raqdio with sheet lead help?...Jim
triblet posted 04-02-2003 11:07 AM ET (US)
It's the speaker magnet. Opposite sides of
the console is probably sufficient separation.
Lead is nonmagnetic and won't help. Steel
(not stainless, also nonmagnetic) might help.
Also some radios have better speakers (stronger
magnets), some have the speaker on one side,
some on the other.

Chuck
Jerry Townsend posted 04-02-2003 01:15 PM ET (US)
"Blocking" the VHF from the compass also requires "rebalancing, realigning, recalibrating - (what-ever the correct words are)" the compass. ---- Jerry/Idaho
jameso posted 04-02-2003 02:05 PM ET (US)
Jerry,
It's called "swing the compass" you do it on a "compass rose". Not to be confused with "swinging the cat'
Jim
jimh posted 04-02-2003 05:21 PM ET (US)
It is possible to have an accurate magnetic compass in the presence of many metalic objects, as for example on a large ship built of steel.
In order to do this you must house the compass in a binnacle. You have many flinders bars to adjust, other compensating magnets (which look like big bowling balls on either side of the compass itself).
In my older two-volume edition of Bowditch's Practical Navigator, there is a long section, perhaps 10-pages or more, devoted to describing the process of compensation of the compass. It is far more difficult that anything you could imagine.
To assess compass accuracy one "swings ship"
not the compass. Hopefully the compass never swings, that is, it remains pointed at magnetic north.
The term compass "variation" refers to local geographic magnetic variation, which can be significant. In Nova Scotia their variation is something like 35-degrees west. Here in the Great Lakes we are blessed with much smaller variation, and in Lake Michigan it is often zero.
Deviation is the error in the compass resulting from onboard effects, like improper mounting, presence of metallic objects, etc. This is often deduced by observation and noted on a chart kept near the compass.
jimp posted 04-02-2003 05:31 PM ET (US)
As another compass option, look at a "flux-gate" or electric compass. I installed one several years ago - Ritchie Mag 1. Its worked great. Compensation is very easy... make two slow 360 degree turns the first time you use it and all deviation errors are removed from the compass. For the last several years, my LORAN, GPS and compass usually agree to within a degree or two. I got mine on sale from Skipper Marine Electronics at Christmas time - $175.
JimP
Louie Kokinis posted 04-02-2003 07:19 PM ET (US)
I bought and returned a KVH electronic compass because it would not set properly. After speaking with their customer service department I concluded that the remote sensor units are far superior but difficult to mount properly on my boat.
I’m back to my magnetic :(
Jim, do they make a steel ship (magnetic) that is small enough for our little consoles?
Perry posted 04-02-2003 08:59 PM ET (US)
I'm planning on installing a vhf radio on my center console next week. How far away should the speaker be from the compass to keep it from affecting its accuracy?
jimh posted 04-02-2003 09:52 PM ET (US)
Regarding the distance between a radio's permanent magnet speaker and a compass: as far as possible.
I am afraid I was in error previously when describing the length of the article on compass error in Nathaniel Bowditch's monumental work THE AMERICAN PRACTICAL NAVIGATOR.
The article discussing compass errors and how to correct for them runs about 55-pages.
By the way, Bowditch's book was always known for its brevity and its concise prose.
As a young fellow, a teenager in the 1780's, Bowditch taught himself French, Spanish, Latin, Greek, and German, in order that he might read the discoveries of others. At 16 he found an error in Newton's Principia after translating parts of it from Latin, but did not publish he correction until he was older.
Bowditch's study of navigation and the existing reference materials led him to discover over 8,000 errors in the then accepted work The Practical Navigator. When news of this reached the publisher, he asked Bowditch to prepare an entirely new work, which in 1802 was published as The American Practical Navigator. Bowditch was 29 years old at that time and considered the finest mathematician in the United States.
Bowditch's work presented everything in very practical terms, required little mathmatical knowledge, and was intended to be useful and understandable by an ordinary sea-going man.
These same qualities make it an excellent reference book today.
triblet posted 04-02-2003 11:19 PM ET (US)
BTW, even my simple plastic Ritchie has a
couple of compensating magnets built into
the base. There are instructions on how to
use them at
http://www.ritchienavigation.com/service/compensation.html

Chuck
triblet posted 04-03-2003 10:30 PM ET (US)
Ritchie's instructions missed one point:
if you use their method 2 (GPS) be sure your
GPS is configured to use magnetic north
rather than true north.

Chuck

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: Compass vs VHF Radio
Jimm	posted 04-02-2003 10:42 AM ET (US) I noticed in another topic that a member was asking where to mount a VHF radio in his new boat. Naturally the question of compass deviation came up. Is this deviation caused by the speaker magnet in the radio or the transmit power from the antenna? Would shielding the compass from the raqdio with sheet lead help?...Jim
triblet	posted 04-02-2003 11:07 AM ET (US) It's the speaker magnet. Opposite sides of the console is probably sufficient separation. Lead is nonmagnetic and won't help. Steel (not stainless, also nonmagnetic) might help. Also some radios have better speakers (stronger magnets), some have the speaker on one side, some on the other. Chuck
Jerry Townsend	posted 04-02-2003 01:15 PM ET (US) "Blocking" the VHF from the compass also requires "rebalancing, realigning, recalibrating - (what-ever the correct words are)" the compass. ---- Jerry/Idaho
jameso	posted 04-02-2003 02:05 PM ET (US) Jerry, It's called "swing the compass" you do it on a "compass rose". Not to be confused with "swinging the cat' Jim
jimh	posted 04-02-2003 05:21 PM ET (US) It is possible to have an accurate magnetic compass in the presence of many metalic objects, as for example on a large ship built of steel. In order to do this you must house the compass in a binnacle. You have many flinders bars to adjust, other compensating magnets (which look like big bowling balls on either side of the compass itself). In my older two-volume edition of Bowditch's Practical Navigator, there is a long section, perhaps 10-pages or more, devoted to describing the process of compensation of the compass. It is far more difficult that anything you could imagine. To assess compass accuracy one "swings ship" not the compass. Hopefully the compass never swings, that is, it remains pointed at magnetic north. The term compass "variation" refers to local geographic magnetic variation, which can be significant. In Nova Scotia their variation is something like 35-degrees west. Here in the Great Lakes we are blessed with much smaller variation, and in Lake Michigan it is often zero. Deviation is the error in the compass resulting from onboard effects, like improper mounting, presence of metallic objects, etc. This is often deduced by observation and noted on a chart kept near the compass.
jimp	posted 04-02-2003 05:31 PM ET (US) As another compass option, look at a "flux-gate" or electric compass. I installed one several years ago - Ritchie Mag 1. Its worked great. Compensation is very easy... make two slow 360 degree turns the first time you use it and all deviation errors are removed from the compass. For the last several years, my LORAN, GPS and compass usually agree to within a degree or two. I got mine on sale from Skipper Marine Electronics at Christmas time - $175. JimP
Louie Kokinis	posted 04-02-2003 07:19 PM ET (US) I bought and returned a KVH electronic compass because it would not set properly. After speaking with their customer service department I concluded that the remote sensor units are far superior but difficult to mount properly on my boat. I’m back to my magnetic :( Jim, do they make a steel ship (magnetic) that is small enough for our little consoles?
Perry	posted 04-02-2003 08:59 PM ET (US) I'm planning on installing a vhf radio on my center console next week. How far away should the speaker be from the compass to keep it from affecting its accuracy?
jimh	posted 04-02-2003 09:52 PM ET (US) Regarding the distance between a radio's permanent magnet speaker and a compass: as far as possible. I am afraid I was in error previously when describing the length of the article on compass error in Nathaniel Bowditch's monumental work THE AMERICAN PRACTICAL NAVIGATOR. The article discussing compass errors and how to correct for them runs about 55-pages. By the way, Bowditch's book was always known for its brevity and its concise prose. As a young fellow, a teenager in the 1780's, Bowditch taught himself French, Spanish, Latin, Greek, and German, in order that he might read the discoveries of others. At 16 he found an error in Newton's Principia after translating parts of it from Latin, but did not publish he correction until he was older. Bowditch's study of navigation and the existing reference materials led him to discover over 8,000 errors in the then accepted work The Practical Navigator. When news of this reached the publisher, he asked Bowditch to prepare an entirely new work, which in 1802 was published as The American Practical Navigator. Bowditch was 29 years old at that time and considered the finest mathematician in the United States. Bowditch's work presented everything in very practical terms, required little mathmatical knowledge, and was intended to be useful and understandable by an ordinary sea-going man. These same qualities make it an excellent reference book today.
triblet	posted 04-02-2003 11:19 PM ET (US) BTW, even my simple plastic Ritchie has a couple of compensating magnets built into the base. There are instructions on how to use them at http://www.ritchienavigation.com/service/compensation.html Chuck
triblet	posted 04-03-2003 10:30 PM ET (US) Ritchie's instructions missed one point: if you use their method 2 (GPS) be sure your GPS is configured to use magnetic north rather than true north. Chuck