Knots (as in speed)?
I understand "how a knot works" and that
one knot is one nautical mile per hour
that the ratio of 47 feet 3 inches to 6076 feet is the same as 28 seconds is to 3600 seconds (ie one hour)
But i assume that the 28 second time period was calculated to make the calculation work and that the distance between the knots was already decided.
Does anyone know why?
Answers:
This is a wonderful question friend, i tried finding the answer as i definitely didnot know, but i caught hold of an article which talks about the origin, hope this is a bit of help though not much.
A knot is a non SI unit of speed equal to one nautical mile per hour. Knot is usually abbreviated kt.
The knot is now usually 1.852 kilometres per hour precisely (approximately 1.15 miles per hour or 0.514 metres per second). See nautical mile for various earlier definitions.
The knot is widely used in air and sea navigation, even though it is a non-metric unit. It is listed by the BIPM among the units "currently" accepted for use with SI (table (http://www1.bipm.org/en/si/si_brochure/c.
Because a knot is already a measure of speed, the expression "knots per hour" is a solecism. Taken literally (nautical mile/hour²), it would be a measure of acceleration.
Origin
In some sailing ships, speed was measured by casting the log from the stern. The log was relatively immobile, and attached by line to a reel. Knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor's fingers, while another used a 28 second sandglass to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1851.66 m/h. The difference from the accepted value today is a bit less than 0.02%.
Terms
KTAS is "knots true airspeed", a measure of an aircraft's true airspeed through the air
KIAS is "knots indicated airspeed", meaning the airspeed shown on the airspeed indicator
KCAS is "knots calibrated airspeed", or indicated airspeed corrected for position error
KEAS is "knots equivalent airspeed", which is calibrated airspeed corrected for compressibility effects.
Log & Knot
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Everyone konws that a "log" is a record of progress, or record of occurrences. Originally used by mariners, the log was used aboard ships to calculate speed. It was a piece of wood tied to a line which had knots tied every 47.25 feet. The mariner also had a 28 second sand glass, or sang a ditty, or recited a religious chant to consistenly measure 28 seconds. The log later shaped like an arc of a circle, was dropped overboard and would stay more or less where dropped if the mariner allowed the line to unwind from a reel. Using the 28 second glass, the mariner counted the number of "knots" passing his hands in the 28 seconds. The reasoning for this is that the ratio of 47.25 feet to 28 seconds is the same as 6080 feet (one nautical mile), to 3600 seconds (one hour).
Do we have the idea now??
47.25 feet / 28 seconds = 6080 feet / 3600 seconds = 1 nautical mile / 1 hour
So if three knots passed through his hands in the 28 seconds, then the ship was moving at a speed of three "knots", or three nautical miles per hour. Interpolation to a half knot, was easy, and even a quarter knot was probably possible.
So now you know that a knot is not a diminutive of nautical miles per hour. Rather, it was originally, an actual count of knots in a line. The speed, time, and new calculated position were recorded in a book. You guessed it, the "log book".
None of the articles I looked at explained why it was so. It may just be the length the inventor used and then manipulated his hourglass accordingly or vice versa. Sorry I couldn't be of any help.
Top marks for the question.
I have absolutely no idea as to the answer but await the replies eagerly.
Go to Wikipedia.com there's a great article on what you are looking for.
go onto the breitling watch website and download what they call a
world time converter and it can work out knots in mph or km
Apparently the original time factor was determined at 30 secs but therewas a time deduction allowed for the fact that the log had to be put out and the glass turned over during the measurement, check out for further reading
http://www.navyandmarine.org/planspatter.
I'm confused
Did you notice that the difference between 6076 x 28 and 47.25 x 3600 is 28? Irrelavent, but interesting.
Possibly the 47.25 feet was supposed to be 8 fathoms (48.00 feet) but the guy's arms were a little short.
Interesting question. why would they have a 28 second sand glass? or..
More than likely it was a function of the boat length.
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