Graphical Calculation of Deviation

[jbenagh]

I came across an interesting website that discuses graphical methods (pre-calculator) for determining course with deviation and variation. I thought it might interest some of you -- it reminded me of the navigating without electronics thread from a while back.

http://myreckonings.com/wordpress/2009/ ... i/#more-41

Jeff

[GLutzow]

Shucks. Ifn I can't figger it out by takkin mah shoes off to count I sho nuff caint figger thisn stuff out! Its enuf to confuse a po country boy lak me!

[tartansailor]

Now that is a rigorous analysis that must have caused a bit of
banter in the British Admiralty among the contenders for "The
First Lord"

Thanks much for the link.

H.O. Pub. No. 260 - Azimuths of the Sun is the way the pro's do it.

This might sound heretical to the Old Salts among us, but using a gps should work, although I've never tried that.

Dick

[Jim Davis]

The method referred to by Jeff is a good one that does not really apply to wood or fiberglass. It can be done, but.........

As Dick said the better method is azimuths of the Sun and the Navy has made it too simple for us. They have a site that computes them for us. All that is needed is to correct the azimuths to Magnetic and time to DST. Just select a city closest to you and hit enter. Note you only need the nearest degree for this and try to work early/mid morning (or late afternoon) as the azimuth's rate of change is slower. Use a peloris, or the shadow pin on the compass for your readings.
http://aa.usno.navy.mil/data/docs/AltAz.php

If you are also going to adjust the compass - do it first - then make the deviation table. Work on a calm day with minimal traffic and it should take about twenty minutes.

As to using a GPS, I have found it really isn't that good. There are some methods that come close, but involve more effort to apply. Also the GPS is telling you the direction the boat is moving, not necessarily pointing and their averaging is too slow. It is close enough for quick checking and alerting you there may be a need to do a deviation table.

There is also a "down and dirty" method that comes close to the accuracy of using Sun Azimuths. It involves standing on the transom with a hand bearing compass and comparing its results with the steering compass. This assumes that the person with the hand bearing compass is not standing next to a RADAR antenna or other magnitic/electronic device.

Of course if you have speakers mounted near the steering compass (up 6 feet) the above probably does not apply since their magnets are going to have interesting affects on the compass.

The above was learned years ago when I did compass adjusting as a way to increase my income.

[Carl Thunberg]

I dunno. The down and dirty comparison with a reliable handheld compass sounds a LOT more accurate to me. I've taken sun azimuth readings on a moving boat. It ain't easy. There's a big difference between accuracy and precision. Precision is over-rated. I'll take an accurate reading over a precise one any day.

Ask yourself this question first. How well can you actually steer to a compass? That's roughly the degree of precision you need.

[The Patriot]

... The down and dirty comparison with a reliable handheld compass sounds a LOT more accurate to me ... Ask yourself this question first. How well can you actually steer to a compass? That's roughly the degree of precision you need.

The Eldridge used to have a small article called "The Magic Spot" that described what Jim and Carl mentioned. "Magic Spot" refers to a place on board free from magnetic interference. From there one uses a handbearing compass to swing the ship's compass. Often this spot is located behind the backstay from where it's possible to use the headstay for fore-and-aft alignment.

Like Carl and Jim I've used this method many times and at present the autopilot display, ship's compass, handbearing compass, and derived GPS information agree within a degree. Of course there's a bit of circular reasoning involved here since the autopilot calibration is based on the ship's compass. Independent readings are still available from the handbearing compass, corrected ship's compass, and GPS derivations. However it's worthwhile calibrating the A/P using the manufacturer's instructions if an autopilot remote is used.

In our particular case there is no deviation table because correcting the bulkhead-mounted ship's compass removed measurable error. Each spring it's a simple task to check these numbers.

[Jim Davis]

For boats in the size of CD's the down and dirty is usually fine. As to ability to steer, you are right. In boats our size it there is any sea, or significant wind, we can only approximate a course and the compass is too small for precision. For my own boat I use the "down and dirty approach".

However when you are charging real money for the service you need to work to a different standard. I made it a rule to only work during the week and tried to only use mornings when the wind was less and there were no kids out on speed boats. The owner was also on board and drove. I won't admit some "showmanship" may be involved. For larger vessels the ability to hold a course is better, but the electronics near the wheel add to the problem. I should note that the new pedestal displays with RADAR/chart plotter/ET AL right over the compass open a whole new bucket of worms. Also the remote microphone and speaker have magnets.

Short side story. When I bought my Alberg 30 the surveyor, who was a friend noted on the survey that the boat needed a deviation table. That turned out to be the most important gig the insurance company wanted corrected.