NavList:

Robin,

I'm swamped with a dozen small projects this weekend, including preparing that talk I'm doing in Princeton on Tuesday, so I can't give a detailed reply right now. But there is one part that I think I can address quickly...

You wrote:
"I wonder to what extent the navigator got to choose the procedures that were followed. If I were a ship’s owner I think I’d probably want some qualified party to take a look at the log and check that the navigator is following best practices to protect my valuable asset."

No such thing happened on any regular basis, and yes, the navigator had complete authority, because the "navigator" was usually the master of the vessel --king of his maritime castle-- or someone that he personally trusted, like his wife (when aboard) or the first mate if he was good at sums. You can tell by simple inspection of the logbooks that no one was checking the math and the detailed procedures because there was no math and no evidence of detailed procedures except in rare cases. They recorded daily latitude and longitude for reference. They recorded activities on-board. They kept track of who was sick, and they recorded crimes aboard. The logbook fulfilled a minimal legal requirement, and that was standard operating procedure. Perhaps one in a dozen voyages included a few scribbled calculations in page margins and maybe one in a hundred included extensive calculations. They were not required. Owners did not "audit" the navigation of their vessels. 

When calculations are included in a logbook (or other accompanying notes), we can try to learn from them, but of course we have to keep in mind the problem of sampling effect. Is there some reason why calculations were occasionally preserved? In some cases, I think we can see that there were reasons that might distort the value of the recorded calculation as evidence of practice, at least to some extent. We might see it today as "portfolio building". When I see a lunar calculation carefully ruled and written up in perfect manuscript, I suspect that the navigator is preserving the calculations in order to show them off to the backers before the next voyage. On the other hand, if the calculations are barely legible and organized in a way that only a mother could love, then we're probably looking at raw calculations, preserved by sheer luck. It's not at all uncommon to see a statement like "got our longitude by observing the distance of the Moon from the Sun this afternoon", for example, but it's rare to find the observed distances and altitudes and rarer still to find the worked numbers. The owners of the vessels did not expect to see such information.

Frank Reed