NavList:

A few points to consider,

First, there were no intermediate markings on a log line, only the integral knots.  When the line was pinched as the sand glass ran out, the fractional 'fathom' reading was simply an estimate of the excess line past the previous knot.  And what better way to 'measure' this fractional excess than by the number of arm spans.  And these arm to arm spans roughly equate to one fathom.

Second, there was no standard inter-knot distance of log lines.  It generally varied from about 40 to 50 feet, depending upon the sand glass used for timing.  Some captains used a full 30 second glass, but 28 seconds or even 24 seconds were also common choices.  And finally, it was common practice to deliberately make the inter-knot spacing a little shorter than the mathematically computed spacing so that the estimated distance made was always a bit farther than the actual distance travelled.  This deliberate error was considered to be a safety factor, so that dead reckoning would not underestimate the distance to land or other hazards.

So, depending upon the personal choice of knot length and sand glass of the captain, there could be 8 fathoms per knot or less.

Don Seltzer