Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 30-04-2013, 15:04   #31
Registered User
 
S/V Alchemy's Avatar

Join Date: May 2008
Location: Toronto
Boat: Custom 41' Steel Pilothouse Cutter
Posts: 4,575
Re: Handbook of Magnetic Compass (H.O. 226)

Anyone here ever sailed over a nice big iron deposit and watched what it did to the compass? Good times!
__________________

__________________
Can't sleep? Read www.alchemy2009.blogspot.com for fast relief. Can't read? Avoid www.volumesofsalt.blogspot.com, because it's just personal reviews of sea books.
S/V Alchemy is offline   Reply With Quote
Old 30-04-2013, 15:53   #32
Registered User
 
Capt Phil's Avatar

Join Date: Jun 2010
Location: Stateline NV
Boat: Prior boats: Transpac 49; DeFever 54
Posts: 2,749
Re: Handbook of Magnetic Compass (H.O. 226)

There are quite a few oddball compass inflicted deposits in the PNW but if your moving it doesn't seem to bother your course too much although those were the days when the autopilots were mostly Wood Freemans or a short lashing in the wheelhouse.
There is an island off the west coast of Australia called Magnetic Island that gave mariners fits until they sorted it but there are lots that give mariners fits 'down under', mostly the cost of marine and aviation equipment, I understand! Phil
__________________

__________________
Capt Phil is offline   Reply With Quote
Old 30-04-2013, 16:50   #33
Moderator
 
noelex 77's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Living on dirt waiting for our new yacht to be built.
Boat: Half built Bestevaer.
Posts: 10,618
Re: Handbook of Magnetic Compass (H.O. 226)

In most non steel boats the errors in a reasonably mounted compass are small.
By far the most common problems are siting something magnetic close by. Headphones, even some glasses cases have magnets installed.

Two steering compasses in separate locations are good insurance. One of these can be an electronic flux gate compass. Comparing the results will indicate if a problem has developed as a local magnetic anomaly will not effect two disparate compasses equally. An error between the readings indicates a problem.

The GPS and COG provides a further check, but COG will not match heading unless tide and leeway are insignificant, so take these factors into account before condemning the compass.
__________________
noelex 77 is offline   Reply With Quote
Old 30-04-2013, 16:58   #34
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,150
Re: Handbook of Magnetic Compass (H.O. 226)

There also used to be a degausing range on the east side of Angel Island in the SF Bay.
__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 30-04-2013, 18:07   #35
Registered User
 
S/V Alchemy's Avatar

Join Date: May 2008
Location: Toronto
Boat: Custom 41' Steel Pilothouse Cutter
Posts: 4,575
Re: Handbook of Magnetic Compass (H.O. 226)

Quote:
Originally Posted by noelex 77 View Post
In most non steel boats the errors in a reasonably mounted compass are small.
By far the most common problems are siting something magnetic close by. Headphones, even some glasses cases have magnets installed.

Two steering compasses in separate locations are good insurance. One of these can be an electronic flux gate compass. Comparing the results will indicate if a problem has developed as a local magnetic anomaly will not effect two disparate compasses equally. An error between the readings indicates a problem.

The GPS and COG provides a further check, but COG will not match heading unless tide and leeway are insignificant, so take these factors into account before condemning the compass.
Indeed. I have an old but reliable KVH AC103 fluxgate at the helm, with the sensor mounted far from the steel in the center of the aluminum pilothouse roof. I compare a known bearing (derived from visual daymarks lining up) with the fluxgate, the GPS and the Ritchie to confirm they are in the same ballpark. The Ritchie has a little delay in finding its course, but it is usually within two degrees of the fluxgate and the GPS and seemingly on all points, accounting, of course, for my local variation.
__________________
Can't sleep? Read www.alchemy2009.blogspot.com for fast relief. Can't read? Avoid www.volumesofsalt.blogspot.com, because it's just personal reviews of sea books.
S/V Alchemy is offline   Reply With Quote
Old 30-04-2013, 18:23   #36
Senior Cruiser

Cruisers Forum Supporter

Join Date: Nov 2005
Location: Out there doin' it
Boat: 47' Olympic Adventure
Posts: 2,634
Re: Handbook of Magnetic Compass (H.O. 226)

Quote:
Originally Posted by Richard5 View Post
Thank you for your comments. But while you state that deperming and degausing are not the same and you state how deperming is accomplished you do not state why deperming is done. Please elucidate.
I thought I had made that clear; I guess not

When a warship is brought into service the DG equipment is set-up to counter the permanent magnetism, and it is "tuned" by the adjustable settings to counter the induced magnetism - these are explained in your original posts. The subpermanent magnetism happens over time - caused by thermal cycling, flexing, pounding, added/removed equipment, etc., etc. This causes a change in the ship's magnetic signature that can't be easily measured or accounted for, and is generally dismissed as insignificant for the purpose of degaussing. This is the reason ships run the DG ranges in various ports - solely to measure the effectiveness of the DG equipment and give an indication that the subpermanent field is below limits. When there is so much subpermanent magnetism created that the DG equipment no longer satisfactorily negates the ship's magnetic field, then it has to be depermed. Deperming erases the subpermanent magnetism, effectively resetting the ship back to its original permanent signature, for which the DG equipment has been optimised. I suppose a ship could be depermed also to aid compass adjustment, but I don't imagine anyone would go to the expense of it.
__________________
Lodesman is offline   Reply With Quote
Old 02-05-2013, 06:14   #37
Moderator
 
nigel1's Avatar

Cruisers Forum Supporter

Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,182
Re: Handbook of Magnetic Compass (H.O. 226)

Quote:
Originally Posted by S/V Alchemy View Post
I would have thought there was absolute zero chance of that being true...

Might be a grain of truth to it, here's on example
http://www.texasnavy.com/Newsletter/...Correction.pdf

Google Kelvins Balls Magnetic Compass, lots of stuff out there
__________________
Nigel
Beneteau 473
Manchester, UK
nigel1 is offline   Reply With Quote
Old 02-05-2013, 07:49   #38
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: Handbook of Magnetic Compass (H.O. 226)

Quote:
Originally Posted by Capt Phil View Post
There are quite a few oddball compass inflicted deposits in the PNW but if your moving it doesn't seem to bother your course too much although those were the days when the autopilots were mostly Wood Freemans or a short lashing in the wheelhouse.
There is an island off the west coast of Australia called Magnetic Island that gave mariners fits until they sorted it but there are lots that give mariners fits 'down under', mostly the cost of marine and aviation equipment, I understand! Phil
Maybe your compass needs adjusting or maybe you not used to how we do things downunder but we call this part of the continent the east coast
__________________

__________________
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
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 19:31.


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.