I was reading the website What is the difference between a nautical mile and a knot?, and it said

The common log was a rope with knots at regular intervals, attached to a piece of wood shaped like a slice of pie. Mariners would lower the wood piece into the water and allow it to float freely behind the ship for a specific amount of time (often measured with an hourglass). When the time was up, they would count the knots between the ship and the piece of wood, and that number estimated their speed.

Why did the piece of wood need to be shaped like a slice of pie? Wouldn't a circle have more surface area, and thus more drag? Or was it because they couldn't have too much drag so they could haul it back to ship to count? In that case, why not use a smaller circle, to me it seems more intuitive.

  • $\begingroup$ Too much drag gives the wrong result. My intuition. $\endgroup$
    – Solar Mike
    Commented Nov 21, 2022 at 6:13
  • $\begingroup$ Apparently the curved edge of the pie segment shaped log was weighed with lead to cause "it to float upright and resist towing". The distance between knots on the rope was 47 ft 3 inches (14.4018 m) & the timing interval was 28 seconds. For a picture of the system see this. $\endgroup$
    – Fred
    Commented Nov 21, 2022 at 7:37
  • $\begingroup$ @SolarMike but then why not simply use a smaller circle? $\endgroup$ Commented Nov 21, 2022 at 13:11
  • $\begingroup$ @Fred but why does it need to do pie-segment-shaped? Why can't it be a circle with lead on the lower half? $\endgroup$ Commented Nov 21, 2022 at 13:12
  • $\begingroup$ So, instead of guessing, sorry relying on "intuition" you could do some experiments. Some time in a water tank which is a controlled environment would help. $\endgroup$
    – Solar Mike
    Commented Nov 21, 2022 at 13:21


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.