Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 26-07-2012, 10:53   #31
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,182
Re: Compass Heading - True vs Magnetic

Unless the compass is in a proper binnacle and properly corrected, deviation will change if the boats latitude is changed significantly. The more "iron" on the boat, the more likelihood of this happening.
On a big scale, say a steel ship, going from Europe to round Cape of Good Hope, if the compass is not corrected , deviation can change maybe 30 degrees on the passage, most especially on east/west headings.
Change in variation is no big deal, you'll know the variation, its marked on chart. Deviation, unless you can check compass errors is anyone's guess
__________________

__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is online now   Reply With Quote
Old 26-07-2012, 10:54   #32
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,182
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by Adelie View Post
Oddly enough recently go an article related to this topic.

Separation and Redundancy

Same thing last year, turned out that the girl friend had stowed the baking sheets and trays in the same locker as the fluxgate compass
__________________

__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is online now   Reply With Quote
Old 26-07-2012, 10:55   #33
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,920
Re: Compass Heading - true vs magnetic

Quote:
Originally Posted by Exile View Post
... I suppose if I only managed to create ONE additional issue by fixing another I'm doing well!
Typical boat project!
__________________
belizesailor is online now   Reply With Quote
Old 26-07-2012, 11:00   #34
Registered User
 
Exile's Avatar

Join Date: Sep 2010
Location: Land of Disenchantment
Boat: Bristol 47.7
Posts: 2,961
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by nigel1 View Post
Unless the compass is in a proper binnacle and properly corrected, deviation will change if the boats latitude is changed significantly. The more "iron" on the boat, the more likelihood of this happening.
On a big scale, say a steel ship, going from Europe to round Cape of Good Hope, if the compass is not corrected , deviation can change maybe 30 degrees on the passage, most especially on east/west headings.
Change in variation is no big deal, you'll know the variation, its marked on chart. Deviation, unless you can check compass errors is anyone's guess
__________________
Exile is offline   Reply With Quote
Old 26-07-2012, 11:03   #35
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,920
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by nigel1 View Post
Same thing last year, turned out that the girl friend had stowed the baking sheets and trays in the same locker as the fluxgate compass
And another: GF has a nifty fold up wide brimmed hat. She unfolded it and plopped it on her head while sitting near the compass. Fortunately, I was looking at the compass and noticed it make a rather dramatic 90 degree turn and point straight at the hat!
__________________
belizesailor is online now   Reply With Quote
Old 26-07-2012, 11:14   #36
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,920
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by nigel1 View Post
Unless the compass is in a proper binnacle and properly corrected, deviation will change if the boats latitude is changed significantly. The more "iron" on the boat, the more likelihood of this happening.
On a big scale, say a steel ship, going from Europe to round Cape of Good Hope, if the compass is not corrected , deviation can change maybe 30 degrees on the passage, most especially on east/west headings.
Change in variation is no big deal, you'll know the variation, its marked on chart. Deviation, unless you can check compass errors is anyone's guess
And what causes this change in deviation?
__________________
belizesailor is online now   Reply With Quote
Old 26-07-2012, 11:16   #37
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,920
Re: Compass Heading - True vs Magnetic

Hey this is cool (and relevant). An animated isogonic chart showing change in variation (declination) over time.

http://en.wikipedia.org/wiki/File:Ea...90_to_1990.gif
__________________
belizesailor is online now   Reply With Quote
Old 26-07-2012, 11:25   #38
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,920
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by belizesailor View Post
And what causes this change in deviation?
Found a good explanation of this phenomena in "The American Practical Navigator" chapter 6. It is "induced magnetism". I'm familiar with this from electronics, but never thought of it as applying to navigation -- learn something new every day! Only applies to larger metal vessels -- especially those with much iron in their construction (older traditional sailing vessels for example -- like the ELISSA which I used to crew on).


"In addition to its permanent magnetism, a ship acquires
induced magnetism when placed in the Earth’s magnetic
field. The magnetism induced in any given piece of soft
iron is a function of the field intensity, the alignment of the
soft iron in that field, and the physical properties and
dimensions of the iron. This induced magnetism may add
to, or subtract from, the permanent magnetism already
present in the ship, depending on how the ship is aligned in
the magnetic field. The softer the iron, the more readily it
will be magnetized by the Earth’s magnetic field, and the
more readily it will give up its magnetism when removed
from that field."


So, deviation can in fact change in such circumstances with out physical modification to the vessel, but this is not likely to effect small boat navigators.

The ELISSA...over 200' of iron in the water!


__________________
belizesailor is online now   Reply With Quote
Old 26-07-2012, 11:28   #39
Moderator
 
Adelie's Avatar

Join Date: Feb 2010
Location: La Ciudad de la Misión Didacus de Alcalá en Alta California, Virreinato de Nueva España
Boat: Cal 20
Posts: 4,595
Re: Compass Heading - true vs magnetic

Quote:
Originally Posted by belizesailor View Post
True, but as I understand it, dip is more of an issue near the poles. Have you ever heard of it being a practical issue anywhere other than extreme N/S latitudes?
L&L Pardey discuss this in 'The Capable Cruiser'. They took their compasses, dip compensated for the US (lets say 34*N, LA where they started) to Tonga (lets say 20*S) and had problems bad enough to almost run them up on coral heads and a reef. Ignoring that True & Magnetic north differ considerably meaning dip does too, that amounts to about a 55* difference in dip. When they got to NZ the ships compass card was tilted 10-15* and the hockey puck hand bearing compass was unusable.
__________________
A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
SailboatData
Adelie is online now   Reply With Quote
Old 26-07-2012, 11:29   #40
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,182
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by belizesailor View Post
And what causes this change in deviation?
Biggest cause is magentic dip, the further from the magnetic equator, the greater the dip. At the magentic equator, dip is zero, induced magnetism in "vertical soft iron" is zero. Move away from the magentic equator, dip increases, and induced magnetism in vertical soft iron will increase.

Below is taken from a US publication
Quote:
211. Resultant induced magnetism from earth's magnetic field. The above discussion of induced magnetism and
terrestrial magnetism leads to the following facts. A long thin rod of soft iron in a plane parallel to the earth's horizontal
magnetic field, H, will have a red (north) pole induced in the end toward the north geographic pole and a blue (south) pole
induced in the end toward the south geographic pole. This same rod in a horizontal plane but at right angles to the horizontal
earth's field would have no magnetism induced in it, because its alignment in the magnetic field is such that there will be no
tendency toward linear magnetization and the rod is of negligible cross section. Should the rod be aligned in some horizontal
direction between those headings that create maximum and zero induction, it would be induced by an amount that is a
function of the angle of alignment. If a similar rod is placed in a vertical position in northern latitudes so as to be aligned with
the vertical earth's field Z, it will have a blue (south) pole induced at the upper end and a red (north) pole induced at the lower
end. These polarities of vertical induced magnetization will be reversed in southern latitudes. The amount of horizontal or
vertical induction in such rods, or in ships whose construction is equivalent to combinations of such rods, will vary with the
intensity of H and Z, heading, and heel of the ship.

Unquote

For the complete text, and a pretty good explanation of magentic compass correcting, see
Maritime Safety Information

When I was doing my Masters ticket at Liverpool I was self funded, no sponsership, and money was tight. I did my Compass Adjusters ticket and earned a bit of money adjusting ship's compasses. Was not that many really, and I had to resort to delivering lube oil from the Shell depot at Stanlow to varous ports around the UK to make ends meet.
__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is online now   Reply With Quote
Old 26-07-2012, 11:48   #41
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,920
Re: Compass Heading - true vs magnetic

Quote:
Originally Posted by Adelie View Post
L&L Pardey discuss this in 'The Capable Cruiser'. They took their compasses, dip compensated for the US (lets say 34*N, LA where they started) to Tonga (lets say 20*S) and had problems bad enough to almost run them up on coral heads and a reef. Ignoring that True & Magnetic north differ considerably meaning dip does too, that amounts to about a 55* difference in dip. When they got to NZ the ships compass card was tilted 10-15* and the hockey puck hand bearing compass was unusable.
Interesting stuff. I've done most of my sailing between about 35N and 09N (boat's in Panama now). Will look up dip angle diffs for these lat's just for grins.

So, hey I'm not just wasting time on-line today -- actually learned more about "dip" and "induced magnetism" !

Thanks!
__________________
belizesailor is online now   Reply With Quote
Old 26-07-2012, 11:51   #42
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 7,920
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by nigel1 View Post
Biggest cause is magentic dip, the further from the magnetic equator, the greater the dip. At the magentic equator, dip is zero, induced magnetism in "vertical soft iron" is zero. Move away from the magentic equator, dip increases, and induced magnetism in vertical soft iron will increase.
So, given that most modern ships are not built of iron (like the good ol' ELISSA), I assume this is less of an issue on modern ships?
__________________
belizesailor is online now   Reply With Quote
Old 26-07-2012, 11:59   #43
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,182
Re: Compass Heading - True vs Magnetic

Quote:
Originally Posted by belizesailor View Post
So, given that most modern ships are not built of iron (like the good ol' ELISSA), I assume this is less of an issue on modern ships?

"Iron" is a term used in compass correcting and causes of deviation, in fact, in the theory bit, they talk of iron rods.
OK, ships are mainly made of steel, but that includes iron, so is still an issue on ships. Any thing which can become magnetic on a ship, is referred to as iron
What I imagine will happen, is that as gyro's are so much mor dependable than they were, and with the introduction of GPS compasses, that some time in the future, ships will bhe excempted from carrying a magnetic compass.
My own ship (large tug), as two gyro's, a GPS compass, a magentic compass in a binnacle, and a spare magentic compass.
__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is online now   Reply With Quote
Old 26-07-2012, 23:14   #44
Senior Cruiser
 
Jim Cate's Avatar

Cruisers Forum Supporter

Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 11,447
Re: Compass Heading - true vs magnetic

Quote:
Originally Posted by belizesailor View Post
True, but as I understand it, dip is more of an issue near the poles. Have you ever heard of it being a practical issue anywhere other than extreme N/S latitudes?
Yes, it can be a problem even in moderate latitudes. Way back in 1989 we left San Francisco with a Danforth brand binnacle compass, purchased from a local vendor and supposedly compensated for the local dip angles. About a year later as we were heading for New Zealand we noted the card in that compass becoming more and more tilted. By the time we reached 30 degrees south it had tilted so far that it hung up and effectively stopped working. OUr "hockey puck" hand bearing compasses (two of them) still worked, sorta, but on some headings the very tightly focused optics became so blurry as to be unreadable... again due to the card tilting. Fortunately we had a bulkhead compass that continued to work ok, for in those pre-GPS days we depended on our compasses a good deal more than we do now.

Once we reached Auckland we found a chap who could rebalance the card in the binnacle compass and it started working again. In that GRP hulled boat with little ferrous material near the binnacle we did not note any changes in deviation.

Interestingly enough, a fellow cruiser from SF who had been in loose company with us and who had a Ritchie compass did not have much trouble from dip. I guess there is some difference in the geometry of the compass card and hence its sensitivity to dip changes between models. One of the many things they didn't tell us at the boat show!

Cheers,

Jim
__________________
Jim and Ann s/v Insatiable II , lying Port Cygnet, Tasmania once again
Jim Cate is online now   Reply With Quote
Old 26-07-2012, 23:49   #45
Marine Service Provider

Join Date: Oct 2011
Posts: 1,205
Re: Compass Heading - true vs magnetic

Quote:
Originally Posted by Jim Cate View Post
One of the many things they didn't tell us at the boat show!

Cheers,

Jim
Boat shows are much like dancing, one wants to lead and the other must follow.

Boat owners go to get knowledge and advice on the latest and greatest.

Sellers go to find new latest and greatest customer.

Go Prepared.

Lloyd
__________________

__________________
FlyingCloud1937 is offline   Reply With Quote
Reply

Tags
compass

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




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 15:38.


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.