NavList:

By his own report, at least, Joshua Slocum seemed to be able to dead reckon at this level of precision.

Still, he seemed aware of the limitations in navigation precision.  The last sentence of this quote has stuck with me.

"It lacks now three minutes of the half-hour," shouted the captain, as he gave me the longitude and the time. I admired the businesslike air of the Olympia; but I have the feeling still that the captain was just a little too precise in his reckoning. That may be all well enough, however, where there is plenty of sea-room. But over-confidence, I believe, was the cause of the disaster to the liner Atlantic, and many more like her. The captain knew too well where he was. 

Bob