True vs Magnetic course to a mark

[jdazey]

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]

Never seen a true bearing from a race committee.

and even their magnetic bearings are suspect

 

[ryley]

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.

 

[Talchotali]

Addition and subtraction are hard?

I was told there would be no math.

 

[Tax Man]

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]

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.

 

[Howler]

Addition and subtraction are hard?

I dunno, I've been sailing for a long time and I have a graduate degree in a quantitative field, and I still sometimes screw up bearing conversion when I try to do it fast in my head.

 

[fan]

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]

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).

 

[The Q]

Just tell em where the buoy is, let them work out how to get there..

 

[DRDNA]

The only time I used, and converted, true courses to magnetic (and the opposite) was to pass navigation tests.

 

[Howler]

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]

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.

 

[ExOmo]

CDMVT?