ASA 105: Skill 10, Calculating Deviation

 


10.Check compass deviation by a transit bearing or other means.


Compass deviation is how we address the magnetic field of a vessel.  Deviation may change over time as the magnetic field of the yacht changes.  This is more common for steel or wooden hulls fastened with iron than for fiberglass yachts.  

There are two basic ways to check deviation.  The first and oldest is by using a table of the sun's true bearing at sunrise and sunset for a given latitude.  The other is by using a charted range (transit bearing).  We will look at the range method.


Nantucket Harbor Channel Range
This is similar to the range used in the example, Cleveland Ledge Channel Range.
The yacht should move right to get the ranges aligned


The TVMDC AW formula is used.  When D is unknown, the arithmetic result is D using TVMDC.  In such a case, TVM and C are knowns so we solve for D.

Turning to Chart 1210 Tr, observe the charted "Cleveland Ledge Channel" range formed by two lights.  The northern most light is on Great Neck.  It is "F G 74ft."  The southern end is a lighted structure at the end of Stoney Point Dike.  It is "Qk Fl G 27ft."  The bearing of the range is 015 degrees True.

Use your plotting tools to confirm the bearing of the range.

Let us assume we are steering northerly in the channel, are on the range and headed 027 degrees PSC along the range.  Calculate the deviation.

T015 degreesFrom the chart
V015 deg WFrom the chart compass rose
M030 deg M015 T + 15 W = 030 d M
Dxunknown value
C27 deg PSCSolve for x: 30 - x = 27  D = -3 deg or 3 deg E
AWAdd Westerly values
SESubtract Easterly values




Chart 1210Tr: Detail showing Cape Cod Canal and Cleveland Ledge Channel

The image is of the charted range with a length of yarn stretched along pins.  The upper pin is on the Fixed Green, the middle pin on the Quick Flashing Green, and the lower pin on the Red "14" Flashing Red 2.5 seconds at the entrance to Cleveland Ledge Channel.

Using a Nautical Almanac with Eldridge, page 232, one can calculate D at sea.  The procedure is beyond the scope of this work. 

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