Compass deviation

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Coast Rider's picture
Coast Rider
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Joined: 10/17/14
Posts: 43
Compass deviation

I'm in the Coast Guard Auxiliary and need the magnetic compass deviation for my 1986 Catalina 36. Of course almost all of us use our auto pilots and almost never use the compass on the helm. Hovever, I need to sail manually on a true course to train on real navigating. This requires adding or subtracting the deviation from the compass heading to get the correct magnetic heading. Anybody got this figure...a deviation is caused when boat material..metals etc affect the true magnetic heading. It's usually about 5 degrees or so.

Joe Lucido
1986 Catalina 36
Hull # 0625
M-25 Universal diesel
Oceanside, CA

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Sojourn
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Posts: 163

Hi Joe,

This old data I found in my files.  The GPS was an old Raymarine unit the Auto Pilot was ST4000.  All have been replace long ago.  You may find this data interesting.  No Guarantees.  This may be more confusing than helpful.  I will send a second file, just the work sheet to use to get you own data.

Good Luck! 

Lou Bruska
Sojourn
1985 C-36 Mk-I TR #495
Eldean Shipyard
Lake Macatawa (Holland, MI) Lake Michigan
Rallyback@comcast.net

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Office spreadsheet icon Comp-GPS-AP data 2007.xls43 KB
Sojourn's picture
Sojourn
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Hi Joe second file.

Lou Bruska
Sojourn
1985 C-36 Mk-I TR #495
Eldean Shipyard
Lake Macatawa (Holland, MI) Lake Michigan
Rallyback@comcast.net

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Office spreadsheet icon Comp-GPS-AP data 2015.xls16.5 KB
EUREKA's picture
EUREKA
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Posts: 115

Hi Joe,

If I understood correctly you would like to know the deviation of your compass, well there is no one set rule. The deviation of the compass should be individually calculated for each compass and each boat, same compass might have different deviation on different boats.

In Spain, it is mandatory that you keep onboard the compass deviation card. Normally it is calculated by a certificated official that sails with you and made the necessary calculations, otherwise, you can try to made your own deviation card.

I use, from time to time, to verify my fix compass, I do it against a hand bearing compass away from any magnetic object, I also use an old Autohelm electronic hand bearing compass to check my fix one.

On a calm day, I turn 360 degrees my boat checking the readings from the different source, each 90 degrees, you should be able to plot your own deviation card.

Hope it helps, and excuses for my English.
 

Eladio Vallina

C-36 TR EUREKA II
Hull 1122 (1991)
Home port Barcelona (Spain).

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Coast Rider
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Thanks for the deviation card. !

Joe Lucido
1986 Catalina 36
Hull # 0625
M-25 Universal diesel
Oceanside, CA

deliveryman
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Joined: 10/20/08
Posts: 9
The deviation is different on every boat, so you have to create your own deviation table. Have fun.
deliveryman
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Joined: 10/20/08
Posts: 9
The deviation is different on every boat, so you have to create your own deviation table. Have fun.
pierview
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Joined: 9/27/09
Posts: 582

I concur with DeliveryMan. The deviation is determined by what electronics and metal you have on each boat so it varies.

An interesting point; years ago the Coast Guard stopped using True on the OUPV  Captain's test and the plotting is all magnetic since uninspected vessels like ours don't normally have Deviation Tables.  They do require True on the Masters upgrade for inspected vessels. Can Dead Men Vote Twice (+east) or True Virgins Make Dull Companions At Weddings... depending on how "woke" you are. The first comes from my Navy navigation training , the second from Sea School.

Chuck Parker
HelenRita 2072 Mk II
2002 Tall Rig - Winged Keel
Atlantic Highlands, NJ

SeaSparrow
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Joined: 12/15/22
Posts: 7

Hi Joe

only you can determine deviation on your compass. It varies for every compass on any given boat depending on the ferrous material around the compass such as a secondary anchor in the lazzarette etc. it will also vary depending on your heading as the various items tug on the compass card deviating it from magnetic north either east or west. You need to make your own deviation card for your boat by "swinging the ship", a process well described in Bowditch or any navigational text. Once done, you won't have to do it again unless you place new ferrous items in new locations. 

best of luck. 

ken passeri
Sea Sparrow
Catalina 36 Mk II
#2270

USCG Master 25Grt
US Sailing Certified Instructor Coastal and Celestial Club Nautique SF Bay

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