But what is the requirement for a highly precise sextant when due to motion of the boat (combined with the steadiness and eye of the navigator) the best you can expect is +/- 5 minutes of arc? You can read minutes worth of accuracy to a precision of 12 (or 9) seconds -- so what.
I haven't personally tried working with a sextant, but I've long been interested in various aspects of navigation. Note that 1 minute of Latitude translates to 1 nautical mile of position, so +/- 5 minutes means you are at best within 5 nautical miles north or south of your true position. This agrees with what I've been told by experienced navigators. What they've said is that on a large ship (relatively stable), a good hand with a sextant might get within 1 nautical mile of their true position. On a small boat (a much more "lively" platform), you will do considerably worse.
I think one of the most amazing feats of small boat navigation ever done was by
Frank Worsley on Shackelton's 1914 expedition. He successfully navigated the James M. Caird, a 22 foot lifeboat, 800 miles across Drake Passage and the South Atlantic to South Georgia Island.
Worsley was faced with the task of navigating the Southern Ocean to South Georgia. There was no margin for error as the James Caird would sail into the South Atlantic if he missed the island; this would mean almost certain death for those in the lifeboat but also those remaining on Elephant Island. Fortuitously, the weather was fine on the day of departure from the island and this allowed Worsley to obtain a sun sighting to ensure that his chronometer was rated.
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For most of the voyage, the weather proved to be stormy and so overcast Worsley was unable to take more than a few sightings with his sextant. He described one sighting as "...cuddling the mast with one arm and swinging fore and aft round the mast, sextant and all..." and he would "...catch the sun when the boat leaped her highest on the crest of a sea...". At times the sea conditions were so rough he was braced by the other crew members when taking his sightings. On occasion, the temperature was bitter and each man would spend one minute shifts chipping away ice that coated the top surfaces of the James Caird, affecting its buoyancy. The heavy seas meant there was considerable risk that a man could go overboard.
After two weeks, Worsley began to worry about the lack of sightings and advised Shackleton he could not calculate their position to less than 10 miles (16 km) accuracy. As a result, Shackleton opted to aim for the western side of South Georgia which meant, given the prevailing winds, that if they missed their target they would be carried onto the east coast of the island. The following day, they began to observe drifting seaweed and seabirds circulating overhead, indicating the presence of land ahead. On 8 May, through mists and squalls, the crew sighted South Georgia's Cape Demidov, precisely in line with the course calculated by Worsley. He saw a "...towering black crag, with a lacework of snow around its flanks. One glimpse, and it was hidden again. We looked at each other with cheerful, foolish grins."
For a fascinating biography of Worsley, which explains how he became the amazing seaman he was, see: Thomson, John (2000).
Shackleton's Captain: A Biography of Frank Worsley. Christchurch, New Zealand: Hazard Press Publishers. ISBN 1-877161-40-3.
Come on, just admit it!