Measuring speed in knots started with tying knots in rope
**Secret Revealed**
Sailors have it easy these days — an inexpensive GPS will tell you how fast your ship is traveling. Heck, even your smartphone can do it if you have the right app. That’s how I clocked the car ferry on Lake Michigan at 35 mph (56 km/h) last summer.
Photo by hans s on Flickr under Creative Commons License
However, a blackberry could not measure your speed 450 years ago. That required a “chip log” (aka “ship log” or “log”). This was a spool of rope attached to a small piece of wood. The sailors would place the wood in the water where it would drag in the water, unspooling the knotted rope. One sailor counted the knots passing over the haul and another would use a 30 second sandglass to measure the time. They had a table to look up the speed (“knots”) based on the number of knots that passed by.
Although the method has changed significantly, the units are still called “knots.” To put that in a way that might have more meaning, one nautical mile translates to 1.151 miles or 1.852 km.
Chad Upton is the editor-in-chief of Broken Secrets and an official Yahoo Answers contributor.
Sources: wikipedia (knots, chip log)
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Was that the Milwaukee-Muskegon car ferry, or the Manitowoc-Ludington boat?
Actually, no table was required. The knots were spaced in such a way that the number of knots counted equaled the speed in knots directly.
I read an article concerning knots, and from what I understand, they used a knotted rope to measure water depth long before they began using the measurement for speed.
Nautical miles have to do with terrestial navigation, knowing where you are on earth. Earth locations given by Latitude and Longitude are measured in Degrees, Minutes and Seconds. Where one Nautical mile (6,000 ft) equals one Minute of Latitude and one Minute of Longitude at the Equator.
The distinction between a mile and a nautical mile is in the curvature of the earth.
Saw this in the movie, “Master and Commander”. It’s been featured in a few other movies as well.
what was the distance between the knots
Wow, for once I actually learned something.
Actually, no table was required. The knots were spaced in such a way that the number of knots counted equaled the speed in knots directly.
me too!!!
Joe is correct. If he hadn’t mentioned it, I was just about to.
A knot is one nautical mile per hour. A nautical mile is equal to one minute of latitude, so distance can easily be measured from any latitude scale. If you travel directly north from the equator for example to 1 degree north, you have traveled a distance of 60 nautical miles. If you traveled this distance over a time period of 6 hours, you traveled at an average speed of 10 knots.
It makes much more sense than using kilometers or statute miles which do not have the same 1:1 relationship.
Having spent many years at sea and sailed many nautical miles around the world’s oceans, may I offer a difinitive explanation of te subject.
Approximately 60 years ago Knots were accepted as the international way of expressing speed at sea and also in the air — based on an internationally agreed length of the nautical mile. Therefore:-
1 knot = 1 nautical mile per hour (or exactly 1.852km/hr.
So because
• they counted the knots of rope in a set time of 1/2 min = 1/120 hr, and
• at some point, a speed of 1 knot became equal to 1 nautical mile per hour, and
• the definition of the nautical mile is based on the circumference of a longitude circle of the Earth (C=360*60 n.mi.; neglecting Earth’s oblateness)
this all means that they either had to measure out somewhat carefully, the spacing of knots in the rope at (1/120) n.mi. ≈ 50 1/2 ft, or if some other spacing was more convenient, calculate knots from whatever that spacing was.
Looks like the wiki article has a spacing of 47′ 3″ with a timing of 28 s, revised from the earlier 30 s.
Hi It’s very intersting! can you give our the knots tables?
> what was the distance between the knots?
Well, if 1 knot = 1 nautical mile/hour = 6000 ft/hr, then:
1 knot = 6000 ft/hr * 1 hr / 60 min =
1 knot = 100 ft/min * 1 min / (2 * 30 sec) =
1 knot = 50 ft/(30 sec)
The knots would have to be 50 ft apart for a 30 second sand glass.
However, that only gives precision to the nearest knot. You could increase the number of knots to get higher precision. For instance, having knots every 25 ft will give precision to the 1/2 knot, and so forth. But this would require some simple division to convert the knot count into the actual knot rate.
“Sailors have it easy these days — an inexpensive GPS will tell you how fast your ship is traveling. Heck, even your smartphone can do it if you have the right app.”
Sorry, Chad. Consumer grade GPSs don’t get used in ships or airplanes. They are notorious for errors. The GPSs they use are thousands of dollars, have extensive databases which require continual subscription and updates, and they have to be certified. Lives depend on it. They also may require as many as 4 satellites to get a reliable position fix.
All these miles per hour, kilometers an hour, its no wonder we are all getting tied up in knots and, I do hope that is not the reason for some accidents.Tom
Nice and informative but I’m wondering how accurate knots were back then. The conditions of the water would’ve made a huge difference. I like the standardised version posted by James Kay
thanks!
it had something to do with the sailors intestines when removed and knoted rightt? it a sailor disagreed with the captain he (the captain))had him disemboweled and they told his mother/wife/children he attempted mutiny.
so it was the measure of the captains arm while wrapping the guts around and “knotting”.
Ships did not know where they were untill a good chronometer was invented. Knowing your speed through the water is not much knowledge when you do not know the velocity of the water.That is why there was so little open ocean sailing. The Vikings were shoreline sailors. Crossing to North America was island to island and along shore lines in day trips.
Samuel Clemence – got his pen name from this, as they were sailing the sailors shouted out ‘Mark Twain’ which I think meant 2 knots or it could be inbetween a knot.
Excellent question with very good answers. I just wanted to add that aviation uses these terms as welll having adopted much of its terminology from naval activities. Aircraft speeds are in knots, position lights are green on right (or starboard) and red on left (or port side) and white lights on the tail. Even the titles of Captain and First Officer are the same.
Being a Master Mariner I can answer this one!
A knot of speed is equal to one nautical mile per hour which is 6080 feet. Which in turn is equal to the measurement of one minute of latitude on the surface of the earth.
A knot is a miles per hour. Nautical mile is equal to the latitude, so distance can easily measure any latitude. If you directly from the North such as the 1 degree North of the equator, you have to go to a distance of 60 nautical miles. If you came to this distance in a 6 hour
http://www.xsweddingdress.info time period, your average speed is 10 knots.
inshore fishing boats on strange grounds still use the rope for depth with a block of margerine on the end to find sandy bottoms when searching for flatties
How they did it was they got a rope with a piece of lead at the end of it. Then they tied regular intervals at around seven feet. Then they would throw it off the back of the ship and by the time they brought it back into the boat they copuld mesaure how many knots there were.
The term “knot” refers to a speed, specifically one nautical mile per hour
1 nautical mile is 6072 feet,a statute mile 5028 feet straight through the curve of the earth —1.15 distance farther 50280×1.15= 6072 feet , one degree of latitude
5280 x 1.15=6072feet typed wrong numerals in above comment
one minute of longitude&latitude at the equator = 1kn mile 1 degree= 60 kn miles –
i think this is good idea of measuring speed.
It would have been better to have a photo of the ferry (or an other car ferry) rather than a sailing ship that would be hard put to even dream of 35mph (or about 30.4 knots)
Secret reveled? I’ve known that for a while. It played as a commercial all the time on AFN when my family and I were stationed overseas.
This made me think about the knot spacing on the rope. Given that 1 NM = 1852 m and a knot (speed) = 1NM/hr, if they were using a 30 sec timer, I calculate that the knot spacing should be 15.432 meters.
(1 knot speed = 15.432 m/30sec)
1 nautical mile = 6080 feet
Depth of water was measured with a lead line (it has a lead sinker) which has markings every 6 feet (1 fathom). “By the mark twain” called out by the sailor told Sam Clemens that the river boat he was piloting was in water 12 feet deep. The lead sinker has a cavity which could be packed with wax to bring up a sample of the sandy or muddy or rocky bottom.
The term knot (speed) derives from counting the number of knots in the line that unspooled from the reel of a chip log in a specific time, and this is how a ship’s speed came to be measured in knots.
You idiot, try using your smartphone on the open waters of the Atlantic.
I am a Boater – therefore I know that 35 MPH is very respectable on the water. Are you sure ?
Just to add to what has been said, the wave action disrupts what we would use on the land.
The earth turns about 1 degree every four minutes, so the nautical mile is not as easy to figure as someone stated above. Without GPS It takes a good mathematician to figure it out and mostly is done at daybreak and nightfall.
Not all sailing is east to west and north to south. If one takes a sailing ship and tacking and such all day. A lot can happen in a day and a lot may not have happened in a day between readings.
I buy the log & knot explanation. In tidal waters it will nearly always give a different answer to GPS. This is because the log reads speed through the water whereas the GPS reads speed over the earth’s surface (put simply).
The modern log on most yachts is a paddle wheel (impeller) and most cruising sailors will compare a log reading against the GPS and the difference (provided everything has been calibrated and the GPS has nautical units selected) will be the speed of the tidal stream or current.
35 MPH in a ferry? I knot thinking so. Knots in a rope. How about a bobbing thing from the bow and count how long it takes to get to the stern. Ship 50 feet long , 4 seconds to get from stem to stern. ~12.16 feet per second. ~12.16 X 60 X 60= FPH \ 5280 = ~8MPH. The knots in the rope thingy seems to me a little arduous. And still doesnt achieve distance. What about current and side slippage of the boat. The best way is with a sextant. That will actually give your location latitude wise + a Chronometer set to GMT. Take settings every hour and plot difference to previous hour. I haven’t been cruising in the middle of the ocean. but if I ever do I will take a sextant and a serious marine radio as backup to my 4 GPS’s , 1 primary and 3 backup’s. Marine radio’s have a frequency to get the current GMT.
1st Calculation:
Take any Length: i.e. 30 Feet
This could be the towed chip log mentioned earlier with the ‘knots’ distance apart of i.e. 30ft or an object in the water thrown from the bow and timed to the stern. (the loa of the vessel)
Divide this by the TIME: i.e. 5 Seconds
Use the result….
i.e. 6
2nd Calculation:
60 x 60 = Number of seconds in One Hour
i.e. 3600
Divide this by the Length of a Nautical Mile.
i.e. 6076.117 adjusted for Latitude at 50 Deg N
or use 6080.000 (number of feet in 1 nm.)
30/5 x 3600/6076.117 = 3.554 kts
A table of time against distance could be made up for various vessel lengths and times in seconds.
or:
Use this quick calculation…
3 x Length / 5 x Time = Kts of vessel.
Sailing Instructor for 35 yrs.
I think it was because some captain “roped” in some guys to join his crew
Jimmy and Terry make good points. Also don’t forget that a GPS doesn’t measure the speed of the water you’re travelling through (resistance) if you’re heading directly into a current, and it also doesn’t measure the distance and speed as you go up and down curvature of the waves. So you could be travelling at 20 miles per hour but because the current is 12 miles per hour your gps may say that you’re only travelling at 8 miles per hour even if you’re doing 20 because it appears that you’re treading water or not moving away from the same spot due to the current and the greater disatance due to the curvature of the waves and current.
CORRECTED ERROR;
ACCORDINGLY TO THE ENCYCLOPEDIA FOR THE PAST 100 YEARS OR MORE A KNOT IS 1 1/4 MILES PER HOUR
i live on a boat and am looking to do an experiment to see if we can work out what speed weare going when er leave Istanbul going to Greece. we have a GPS that tells you how fast your going but just for fun we are going to see if we get it right. so i’m thinking 1820m/120 is about 150m, 15m per 30sec is 1 mile per hour, let the rope go for 1 min that’s 150m for 10 knots. this is the idea. will it work? i think my math is right.