So, what is a "knot"

[loubetti]

The moderator can add this to the nautical lexicon sticky above.

 

Yes, it is something you can tie in a rope or string, but in nautical and aviation parlance it is a measure of speed. It also relates to nautical miles, which aviation also uses.

 

So, you see that your cruise ship is doing 20 knots. What does that mean?

 

I'm going to leave the metric system out of this, as to "kilometers", but refer to "miles per hour".

 

A mile refers to a "statute mile", which is 5280 feet in length. A nautical mile is 6076 feet in length or 15+% longer. Thus, your ship is sailing at 23 miles per hour.

 

Now, where did we get "knot" from? Well, as I said above, it is something we tie in a rope!

 

In the old days of sailing vessels the only way to determine the vessel's speed was to take a long rope, tie equally spaced knots in it, attach it to a piece of wood (that provides flotation) toss it off the ship, then use an hour glass or perhaps a better timing device and count the knots that pass during the given time!

 

And that, my good people is where knot comes from. Also, NEVER say "knots per hour", it's just "knots". As to distance say "nautical miles" or just miles. "We're doing 20 knots and have 100 miles to go".

[pspercy]

I saw an alternative definition once for a knot - "A tangle with a name " :)

[luddite]

Sometimes we say 'knothead'.

[sparky-elpaso]

Now if we could just figure out who decided the difference between statute miles and nautical miles. Especially since they're both such odd numbers.

[TheOldBear]

Now if we could just figure out who decided the difference between statute miles and nautical miles. Especially since they're both such odd numbers.

 

The nautical mile is easy - it's defined as a minute of latitude, allowing for easy measurement on nautical charts. [Just use dividers on the latitude scale, instead of printing a distance scale on the chart]

 

Oh, and a minute is 1/60th of a degree when measuring angles - on earth, the equator is at zero degrees and Santa's Workshop is at 90 degrees North.

[navybankerteacher]

Now if we could just figure out who decided the difference between statute miles and nautical miles. Especially since they're both such odd numbers.

 

A statute mile is the distance a Roman Legionary would cover in a thousand paces (each pace being two steps) from the Latin word for thousand. A nautical mile is the distance represented by each minute of latitude between the equator and one of the poles.

 

Or, is this thread really about flaws in lumber?

[Aquahound]

And that, my good people is where knot comes from. Also, NEVER say "knots per hour", it's just "knots". As to distance say "nautical miles" or just miles. "We're doing 20 knots and have 100 miles to go".

 

Being a Coastie and a former navigator, I find nautical miles and knots very easy. One rule that stands out to me is the 3 Minute Rule. Your speed in knots divided by 10 is the distance you'll go in 3 minutes. For example, at 30 knots you'll go 3 nautical miles in 3 minutes.

[navybankerteacher]

Being a Coastie and a former navigator, I find nautical miles and knots very easy. One rule that stands out to me is the 3 Minute Rule. Your speed in knots divided by 10 is the distance you'll go in 3 minutes. For example, at 30 knots you'll go 3 nautical miles in 3 minutes.

 

I think you want to divide speed in knots by 20, not 10: covering 3 nautical miles in 3 minutes would mean making a speed of 60 knots

[luddite]

Or, is this thread really about flaws in lumber?

 

Naught for nothin' but knots in lumber add character.

Sometimes.

[Aquahound]

I think you want to divide speed in knots by 20, not 10: covering 3 nautical miles in 3 minutes would mean making a speed of 60 knots

 

Wow, I really messed that up. Thanks for catching it. I mixed the 3 minute rule with the 6 minute rule. Under the 6 minute rule, speed in knots divided by 10 = distance traveled in 6 minutes.

 

Under the 3 minute rule, you multiply your speed by 100, and that equals yards traveled in 3 minutes.

[sparky-elpaso]

Thanks for the explanation of nautical miles, etc. No Navy guys in my family - just Army and Air Force.

[LA_CA_GAL]

1 knot = 1.150 779 448 mile/hour (mph).

 

1 knot = 1.852 kilometer/hour

Edited December 22, 2015 by LA_CA_GAL

[flutepilot]

The nautical mile is easy - it's defined as a minute of latitude, allowing for easy measurement on nautical charts. [Just use dividers on the latitude scale, instead of printing a distance scale on the chart]

 

Oh, and a minute is 1/60th of a degree when measuring angles - on earth, the equator is at zero degrees and Santa's Workshop is at 90 degrees North.

 

You are correct to a point. It is one minute of latitude measured at the equator. Remember, it is different, the farther you are from the equator.

 

PS - Yes I am a long time flight instructor.

 

Dan

[Aquahound]

You are correct to a point. It is one minute of latitude measured at the equator. Remember, it is different, the farther you are from the equator.

 

PS - Yes I am a long time flight instructor.

 

Dan

 

Sorry, but I'm afraid it's you who is mistaken. It's the Longitudes that differ in distance. Latitudes remain constant. There is a very slight difference in distance at the equator than at the poles between latitudes, but it's way too small for any significance.

Edited December 23, 2015 by Aquahound

[navybankerteacher]

You are correct to a point. It is one minute of latitude measured at the equator. Remember, it is different, the farther you are from the equator.

 

PS - Yes I am a long time flight instructor.

 

Dan

 

Incorrect - minutes of latitude are consistently one nautical mile each all the way from the equator to either pole. You are thinking about minutes of longitude which are just over one nautical mile each at the equator and diminish in length as latitude (north or south) increases - until right at the pole all 21,600 minutes compress into nothing.

 

PS - But no so much a navigation instructor.

[Wol]

Incorrect - minutes of latitude are consistently one nautical mile each all the way from the equator to either pole. You are thinking about minutes of longitude which are just over one nautical mile each at the equator and diminish in length as latitude (north or south) increases - until right at the pole all 21,600 minutes compress into nothing.

 

PS - But no so much a navigation instructor.

 

Without researching it, I would think that you are both right - and wrong.

 

The earth's geographic poles are at exactly 90' North and South, and the equator is at 0' latitude. Those are exact and fixed numbers.

 

A chart is necessarily inexact, whichever projection is used, but each geographic location is mapped to a point on the chart. Since the earth is a modified oblate spheroid and not a sphere it seems to me that yes, actual precise distances as measured on the chart will necessarily be very slightly different to actual ground measurements since the chart will assume a sphere - if it didn't, the lines of latitude would vary very slightly on the paper in separation.

 

Shoot me down if you like but that's my take on it!

[chengkp75]

Without researching it, I would think that you are both right - and wrong.

 

The earth's geographic poles are at exactly 90' North and South, and the equator is at 0' latitude. Those are exact and fixed numbers.

 

A chart is necessarily inexact, whichever projection is used, but each geographic location is mapped to a point on the chart. Since the earth is a modified oblate spheroid and not a sphere it seems to me that yes, actual precise distances as measured on the chart will necessarily be very slightly different to actual ground measurements since the chart will assume a sphere - if it didn't, the lines of latitude would vary very slightly on the paper in separation.

 

Shoot me down if you like but that's my take on it!

 

No, charts assume a sphere and project onto a cylinder (for the most part). Lines of latitude are parallel and equally spaced, and measure 1/360th of the distance between the equator and the pole on a sphere. What is distorted are land mass shapes caused by not only the non-spheroidal shape of the planet, and the projection from a sphere to a flat cylinder.

 

I'm neither a flight navigation or marine navigation instructor, nor even a navigator, but this engineer knows enough that regardless of what chart is being used on the bridge, if I measure something to the scale on the right side of the chart (latitude), but not the scale on the bottom (longitude), I get nautical miles.

Edited December 24, 2015 by chengkp75

[loubetti]

Well, I'm happy I posted this for all the replies received!

 

At least with ships, we don't deal with "Indicated airspeed", "True Airspeed", Ground Speed", "Calibrated Airspeed" and "Mach number" as on aircraft! On ships I figure it is just sea speed, that will be affected by wind / ocean speed.

 

Otherwise, I like my knots!

Edited December 24, 2015 by loubetti

[chengkp75]

Well, I'm happy I posted this for all the replies received!

 

At least with ships, we don't deal with "Indicated airspeed", "True Airspeed", Ground Speed", "Calibrated Airspeed" and "Mach number" as on aircraft! On ships I figure it is just sea speed, that will be affected by wind / ocean speed.

 

Otherwise, I like my knots!

 

We still have "speed through the water" (from Doppler sonar) and "speed over ground" (from GPS).

[navybankerteacher]

We still have "speed through the water" (from Doppler sonar) and "speed over ground" (from GPS).

 

Right - a ship, say 200 miles east of the mouth of Chesapeake Bay making 20 knots on a southwesterly course will cover about 34 nautical miles over ground, while one heading northeast would cover about 46 nautical miles in the same two hour period.

[chengkp75]

Right - a ship, say 200 miles east of the mouth of Chesapeake Bay making 20 knots on a southwesterly course will cover about 34 nautical miles over ground, while one heading northeast would cover about 46 nautical miles in the same two hour period.

 

And regardless of direction, due to "slip", the engine room will get to port before the bridge does. :D

[Wol]

No, charts assume a sphere and project onto a cylinder (for the most part). Lines of latitude are parallel and equally spaced, and measure 1/360th of the distance between the equator and the pole on a sphere. What is distorted are land mass shapes caused by not only the non-spheroidal shape of the planet, and the projection from a sphere to a flat cylinder.

 

I'm neither a flight navigation or marine navigation instructor, nor even a navigator, but this engineer knows enough that regardless of what chart is being used on the bridge, if I measure something to the scale on the right side of the chart (latitude), but not the scale on the bottom (longitude), I get nautical miles.

 

Whichever projection one uses, polar stereographic, Lambert's conformal conic, mercator or whatever if the globe is assumed a perfect sphere and each ground position is mapped to the chart's lat/long I assume that a chart distance measurement would be fractionally - and I mean MINUTELY! - different from the actual ground measurement.

 

But, as I say, that's my take. It's a long time since I did spherical geometry and I can't say I found it easy then!

 

Most aircraft and ships seem to find their destinations despite, if I'm correct, being a couple of inches out :D

[luddite]

Right - a ship, say 200 miles east of the mouth of Chesapeake Bay making 20 knots on a southwesterly course will cover about 34 nautical miles over ground, while one heading northeast would cover about 46 nautical miles in the same two hour period.

 

That ship is doin' REAL good to make that kinda time over ground.

I would've expected the barnacles to be real grabby on the turf.

[chengkp75]

Right - a ship, say 200 miles east of the mouth of Chesapeake Bay making 20 knots on a southwesterly course will cover about 34 nautical miles over ground, while one heading northeast would cover about 46 nautical miles in the same two hour period.

 

That ship is doin' REAL good to make that kinda time over ground.

I would've expected the barnacles to be real grabby on the turf.

 

What he is saying is that the ship is making 20 knots through the water. Going northbound, it makes 46 nm in two hours, or 23 knots over the ground, benefiting from the 3 knot Gulf Stream. Going southbound, it makes 34 nm in two hours, or 17 knots over the ground, since the water is moving 3 knots against the ship. Given today's preponderant reliance on GPS for navigation, the ship's speed is always given in terms of speed over ground, while the speed through the water from the Doppler sonar is a "backup" reference, and a key to the exact current strength and direction at that time.

 

Not sure what the barnacles have to do with it. That affects speed through the water, but has nothing to do with current.

[Aquahound]

Not sure what the barnacles have to do with it.

 

Oh come on. You never had to careen your hull? ;)