NavList:

Lars, you wrote:
"The original meaning of F is somewhat unclear."

That rings a bell. We may have discussed this some years ago... Like I say, it was "cultural" and fluid/flexible practice. You won't see this "H K F" data at all in logbooks from American vessels after about 1820 (that's not a "drop dead" date... there were always "elder statesmen" among the old navigators). The absence of the "H K F" data doesn't mean they stopped doing dead reckoning. They always recorded DR positions, which necessarily depended on the same data. They just stopped drawing up tables like this. Why? Styles change...

And you added:
"Bowditch, 20th edition, 1851, states on page 126 'Each of these knots [i.e. the knots on the log-line] is divided into 10 fathoms, of about 5 feet each.' "

So by 1851 they had decimalized?! Let's all sing the Marseillaise! What next, stinking meters?? [I'm kidding...]

A detail: was there a change in/by/near 1851, or is that just an example edition you have available at-hand? Anyone care to check earlier Bowditch editions to see what he claims "F" stands for (full list of Bowditch editions here: https://navlist.net/historical-navigation-texts). Since those tables were obsolescent by 1851, I wouldn't be surprised to find that this goes back much earlier, even to the very beginning of the century. The "New American Practical Navigtor" (that was the full title back then) was not new in 1851. Most of its chapters and many examples had not been updated in decades.

You also noted:
"According to a note in The Journal of Navigation, vol. 27, page 536, 1974, Duncan Henderson claims that F means fractions, i.e. tenth of a knot."

Who cares? Seriously. You quote this like Scripture... As if "F" means "fact".

Frank Reed