0

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.

Fred
  • 9,562
  • 11
  • 32
  • 44
MeltedStatementRecognizing
  • 153
  • 6
  • Too much drag gives the wrong result. My intuition. – Solar Mike Nov 21 '22 at 06:13
  • Apparently the curved edge of the pie segment shaped [log](https://www.britannica.com/technology/log-nautical-instrument) was weighed with lead to cause "it to float upright and resist towing". The distance between [knots](https://www.britannica.com/science/knot-measurement) on the rope was [47 ft 3 inches (14.4018 m)](https://en.wikipedia.org/wiki/Knot_(unit)) & the timing interval was 28 seconds. For a picture of the system see [this](https://medium.com/knowledge-stew/why-is-a-ships-speed-measured-in-knots-15bf3368313c). – Fred Nov 21 '22 at 07:37
  • @SolarMike but then why not simply use a smaller circle? – MeltedStatementRecognizing Nov 21 '22 at 13:11
  • @Fred but why does it need to do pie-segment-shaped? Why can't it be a circle with lead on the lower half? – MeltedStatementRecognizing Nov 21 '22 at 13:12
  • 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. – Solar Mike Nov 21 '22 at 13:21
  • @SolarMike I'm not saying that a circle would produce a better amount of drag. I'm simply asking why a circle with the right area wouldn't do as well as a pie-slice? By intuitive I meant it would be more natural for sailors to have used a disk instead of a pie-slice, right? – MeltedStatementRecognizing Nov 21 '22 at 13:54
  • My understanding of drag is that it depends on surface area, not shape, please correct me if I am wrong – MeltedStatementRecognizing Nov 21 '22 at 14:00
  • I thought drag depends on frontal area and surface friction... – Solar Mike Nov 21 '22 at 14:42
  • @SolarMike Yes, that is what I meant, I apologize. So why a disk work instead of a pie-slice? Or can it – MeltedStatementRecognizing Nov 21 '22 at 14:48
  • How it behaves in the water? – Solar Mike Nov 21 '22 at 15:06
  • @SolarMike, I think that something is wrong with my communication, or perhaps I am overthinking everything. Either way, I think I should ask this question in person . I'll ask one of my teachers in school. Thank you for your time! – MeltedStatementRecognizing Nov 21 '22 at 15:09
  • [Chip log](https://en.wikipedia.org/wiki/Chip_log). The log is supposed to act as a chute, standing vertically, weighted down by lead, staying stationary as ship sails away. After use sailor sharply pulls on it making it go flat too *easily* pull it in. Hence the triangular shape. – StainlessSteelRat Nov 21 '22 at 17:37
  • @StainlessSteelRat Thank you! – MeltedStatementRecognizing Nov 21 '22 at 17:57

0 Answers0