True vs Magnetic course to a mark

jdazey

Member
457
141
Kingston, WA
From Round the County NOR 2022:

2.2 Sunday: Start off the east side of Battleship Island between the race committee boat and
a temporary orange pyramid buoy. Leave Danger Shoal buoy, Stuart Island, Waldron
Island, Skipjack Island and Patos Island to starboard. Short course finish line is from Patos
Lighthouse at 48° 47.340` N, 122° 58.280 W (aka 48° 47’ 20” N, 122° 58’ 17” W) on a
bearing line pointing true north on longitude 122° 58.280 W (aka 122° 58’ 17” W). Leave
Clements Reef Buoy, Clements Reef, Matia Island, Clark Island, Sisters and Orcas Island to
starboard. Finish between the race committee boat and Lydia Shoal buoy.
 

ryley

Super Anarchist
5,506
662
Boston, MA
From Round the County NOR 2022:

2.2 Sunday: Start off the east side of Battleship Island between the race committee boat and
a temporary orange pyramid buoy. Leave Danger Shoal buoy, Stuart Island, Waldron
Island, Skipjack Island and Patos Island to starboard. Short course finish line is from Patos
Lighthouse at 48° 47.340` N, 122° 58.280 W (aka 48° 47’ 20” N, 122° 58’ 17” W) on a
bearing line pointing true north on longitude 122° 58.280 W (aka 122° 58’ 17” W). Leave
Clements Reef Buoy, Clements Reef, Matia Island, Clark Island, Sisters and Orcas Island to
starboard. Finish between the race committee boat and Lydia Shoal buoy.
the exception that proves that rule. also, not a bearing to a mark but an imaginary finish line following a line of longitude.
 

Tax Man

Super Anarchist
1,979
333
Toronto
The finish line will be a virtual GPS finish line in the vicinity of the southeast tip of Mackinac
Island. The north end of the finish line is located onshore at latitude N 45° 51.065’, longitude W
084° 36.315’. The south end of the finish line is located at latitude N 45° 50.379’, longitude W
084° 35.921’. The approximate compass bearing from the north end finish line to the south end
of the finish line is 165°.

Does not say whether it is true or magnetic.
 

JohnMB

Super Anarchist
2,837
609
Evanston
The finish line will be a virtual GPS finish line in the vicinity of the southeast tip of Mackinac
Island. The north end of the finish line is located onshore at latitude N 45° 51.065’, longitude W
084° 36.315’. The south end of the finish line is located at latitude N 45° 50.379’, longitude W
084° 35.921’. The approximate compass bearing from the north end finish line to the south end
of the finish line is 165°.

Does not say whether it is true or magnetic.
It says compass bearing, which is magnetic by definition.
 

fan

Super Anarchist
1,873
104
San Diego
Latitude: 45° 51' 30.1" N
Longitude: 84° 37' 23.4" W
FORT HOLMES
Magnetic Declination: -7° 2'
Declination is NEGATIVE (WEST)
Inclination: 71° 26'
Magnetic field strength: 54770.4 nT
 

Monkey

Super Anarchist
10,867
2,493
I can’t think of a single good reason to set any sort of buoy course to anything but magnetic. That’s all I’ve ever used, and all that’s ever been used when I worked for others (going all the way up to guys who PRO’ed the America’s Cup).
 

Howler

Member
172
169
The only time I used, and converted, true courses to magnetic (and the opposite) was to pass navigation tests.
Last I checked, the iPhone version of Navionics insists on using True, so if you use that and a steering compass you still need to correct.
 

El Borracho

Verified User
6,664
2,642
Pacific Rim
Last I checked, the iPhone version of Navionics insists on using True, so if you use that and a steering compass you still need to correct.
Navionics can be configured for magnetic using the instrument menu under the (i) symbol to the right of the instrument bar. Uses the current boat position for variation. Uses the current chart’s variation for plotting. Can be a bit confusing. The Deviation control is documented as correcting the devices compass but does not do anything on my iPhone.
 

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