NavList:

What you need depends on where you are.  Assuming you are on a boat in the middle of the ocean where the nearest hazard is 50 or 100nm away then 10nm accuracy is more than acceptable.

If you are approaching a high coastline during the daytime or with a lot of on-shore lights with no outlying hazards 5-10nm accuracy is also probably acceptable.

The Tuamotus is another story.  The are sparsely populated, low and have a lot of outlying reefs and occasionally unexpected currents.  Approaching this archipelago I would want 1-2nm accuracy, I would very carefully pick my first landfall to be relatively clear of outlying hazards, time it for daylight at landfall and I would start cross referencing visual sights as soon as objects became identifiable.  i would always assume that I was only accurate to 2nm with the celestial and would change plans if the weather was such that I needed better than 2nm.

In between there are a spectrum of conditions and 'needed' accuracy varies.  To determine what is needed you will have to research each desired landfall to determine if you have the capabilities.

 

From: John D. Howard
Date: 2015 Jul 21, 08:33 -0700

Since I first started this thread by asking what precision is needed the topic has morphed to what precision can be achieved.  I am still left to wonder what is required.  I, John, get .7', and Frank gets 2.5' then is Frank in danger?

Reading the posts it seems that if you go a day or two without a fix because of weather then get a great 3 star fix you will update your position and start your DR from your new fix.  Was the boat in danger or in violation of some law during the day of no fix?

Much discussion about compact sight reduction tables and slide rules and Bygraves and Rust diagrams.  My main question is still this - is using a Bygrave and Rust diagram at sea instead of table 229 unsafe?  From what I have learned reading the posts the table 229 will get you a much better (precise) fix than a Bygrave.  If you NEED the best possible how can you accept the use of lesser methods

 

It is only unsafe if you need to go somewhere that requires better accuracy.  If you are out for a pleasure cruise then you don't NEED to go somewhere that requires better accuracy than you can obtain.  As previously mentioned you need to research your desired destination to determine relevant conditions.

Deciding what sight reduction method to bring and practice is a function of risk assessment.

If I was going to leave GPS at home and rely on CN then I would bring 229 or another high precision method.  I would be dedicating space and weight on the boat for this because it is a primary system of the boat.  Also I would be using it regularly and would be constantly in practice.

If the CN is being used as a backup to GPS then you have to make judgements not just about the accuracy of the system but also the space and weight penalty of carrying it and how likely are you to practice.  If it is complex and time-consuming then it is less likely you will practice and you WILL practice less in any event.  You will be less proficient with the method which means greater accuracy may be for naught.  Also what are the odds you will have a breakdown that forces you to use CN in a location where the need for high accuracy can not be avoided?  That depends on where you want to go.

Another tradeoff to consider is that dedicating the money, space and weight to get the best navigation backup system may preclude the carriage of other emergency equipement that you might also want.

I figured out that for about $200 I can get a minimal CN setup that delivers accuracy around 2-5nm and would occupy about 1/2-2/3 cubic foot.  For $50 more it would work even after a lightning strike that destroyed all electronic/electrical equipment aboard.  With this I should be able to get home.

If I only accepted the very best in backup systems I probably wouldn't have enough room left on the boat to enjoy it and I know I wouldn't have the money to go anywhere.