A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2015 Mar 30, 22:07 -0700
What I do aboard my fiberglass ketch is to take a bearing of the Sun with the magnetic hand compass while standing at the helm. UT time of observation is recorded to calculate the true azimuth of the Sun. Variation is applied to the magnetic bearing of the Sun to get a true bearing. The difference between the observed true bearing and the calculated true azimuth is deviation for the hand compass on the current heading. The next step is to take a magnetic bearing of the bow to compare to the reading on the steering compass. There is now enough information to determine deviation of the steering compass on the course steered. Deviation is generally small for fiberglass boats.
From: David Pike
Date: 2015 Mar 30, 12:47 -0700
Once the hand compass is checked for deviation then it becomes the standard to check the steering compass or for taking bearings while piloting. Stand well above and behind the steering compass when doing this. 2° precision is obtainable which should be good enough for checking a small craft steering compass with 5° graduations.
That’s a great idea for getting a row of ghost images of the sun, but I’m not quite sure what you mean by ‘deviation’. If you’d said instrument error or lubber-line error, or even coefficient A, I’d have accepted that, but the deviation affecting a compass (i.e coefficients B,C,D, and E) is a function of its position on the ship and the ship’s heading, not a function of the compass itself. If you’re going to use the Sun to check a hand-bearing compass, you must stand in the same position when you check the steering compass and check the hand-bearing compass against the sun before each of the four check readings and the twelve calibrating readings. If you walk around or change heading, the deviation affecting it will alter depending upon where you’re standing and which way the ship is pointing (if your on the ship). That’s why before a portion of an airfield set aside as a compass swinging area can be used, it must be carefully surveyed for anomalies in the Earths field caused by forgotten iron piping, live electrical wiring, or pierced steel planking left over from WW2. There is then a strict routine for subsequent inspections. I can’t remember what the frequency is.
I was interested to watch an old style compass swinger brought on board a 72’ yacht I was on. He swung the steering compass by bringing his own pelorus on a tripod on board and made use of a few favourite transits off his home port to calculate the ships true heading. To convert to magnetic heading, he had a working knowledge of the local variation in his favourite spot, and if he’d wanted a more accurate figure, he could have rung up the Admiralty Compass Observatory for any diurnal changes affecting the UK. Dave