NavList:

You wrote:
"His DR seems amazingly good, +/- 10 nm, even considering his years of experience and math ability."

First, very little math competence was required to keep a good DR position. The required character trait was meticulous attention to detail. A navigator, typically a ship's captain or master, had to follow a religious schedule of tossing the log to check the speed. He had to yell at his helmsmen to maintain an exact course. But if these things were done, and if the results were recorded carefully, then a dead reckoning position could be quite reliable for many days. There's no need to speculate on this. We have mountains of logbooks from history.

The modern conception of dead reckoning as horribly dangerous and woefully inaccurate is partly founded on a misconception created by 20th century pedagogy and also from stories of extreme cases where a vessel sailed for many weeks entirely by dead reckoning in complex currents or with mediocre attention to the detail of the process. The errors grow as time passes, of course. The pedagogic side is basically just a sort of "cheerleading" of progress of the modern world. Things are better now because we're not so dependent on that "awful old dead reckoning".

As for Sumner, when you read his account of his "discovery", you're not reading an honest historical account. He told that story to convince his readers that the method worked, and you can be confident that he cleaned it up and modified the details to make the story more convincing. Unfortunately for him, by emphasizing various aspects of the method that we now see as secondary or irrelevant, he created an unconvincing story, and his method was an almost complete failure in the marketplace of navigation for decades. Unfortunately, most navigators are fed "just so" histories...

In the beginning, there was dead reckoning, and people did die, cuz DR sucked. Then John Harrison invented the chronometer! And there was much rejoicing. Then Nathaniel Bowditch invented all the math of navigation and taught it to everyone on his ship. And there was some rejoicing in America. Then Sumner invented the line of position! And there was a whole lot more rejoicing!

Most of us first learned a version of the history of navigation that reads more of less like this. Some of the words are in the right places, but it's not history. It's a cartoon.

Frank Reed