NavList:

Lu,

I practice CN every chance I get whether on my 18,000 pound 38 ft. ketch or when crewing on another vessel. Results underway going to weather are the least accurate at 4' or better. This is caused by motion of the boat beating into seas and swells. As you stated it is quite a trick to keep the sextant perpendicular to the horizon. Under these conditions observations with a sight tube are generally just as good as with a scope. When on a run down wind and swell I get 3' or better accuracy and have no trouble using a 4 x 40 scope. At anchor 2' or better. From the beach 1.5' or better. Artificial horizon from land 1' or better. Lunars on land with a 7 x 35 scope 0.5' or better. Bubble octant from land 5' or better.
 

When using a plastic sextant I expect a couple more minutes of inaccuracy vs. a metal sextant.

Greg Rudzinski

From: Lu Abel
Date: 2015 Jul 18, 13:43 -0700

One thing I haven't seen discussed (apologies if I missed it) is achievable accuracy.   We debate the accuracy of various sight reduction methods.   But the achievable accuracy of an LOP or a fix starts with the is achievable accuracy of sights themselves.  And I don't believe we've discussed the latter.

It's all well and good to discuss the quality of sights while standing on solid land.   But how about on board a vessel offshore?   Since most of us are amateurs, it's unlikely we're navigation officers aboard commercial ships (do those people even know how to do celestial any more?).  We're more likely to be navigating recreational boats.   So the question becomes "what sort of sight accuracy can one expect when taking sights aboard a 40~50 ft boat under 'reasonable' sea conditions?"

I own a 36' sailboat.  It is of medium displacement for a boat its size, almost 16,000 pounds.   I have done several passages (eg, Boston to Halifax, Providence to Baltimore) that I would characterize as "semi-offshore" -- out of sight of land but not out of VHF radio range if I ever needed assistance.   On each of these I brought my sextant, both to sharpen my celestial skills and to give my crew a chance to give celestial a try.

Even under relatively ideal conditions (winds 15kts or less, leading to sea swells of only a foot or two), my boat rolls and heaves.   Not a lot.  Not enough to make the voyage in any way uncomfortable.   But enough to make it hard to keep my sextant pointed at the horizon.   My grasp on it is rock-solid, my sextant is light, but my boat is rolling and so the horizon is moving up and down due to the boat's motion faster than I can move the sextant up and down.

I'd also point out that under many circumstances at sea the horizon is unclear due to humidity or moisture creating a "smoky" horizon.  

As a result of both of these phenomena, I found it very hard to get sights where I had confidence that my sight was accurate to two or three minutes.  

There's a basic rule of science and engineering -- if your measurements are off then your results will be off.  So I wonder if whether a sight reduction method is accurate to, say, 0.1' when a sight itself will likely have an accuracy of only 2 or 3 minutes is essential.

If others have actually tried celestial while offshore on a boat of, say, 30~50 ft LOA, I'd love to hear their opinions.

Lu