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Old 06 September 2001, 07:38   #1
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Isn't it confusing ?

Just wondering, why all the engine manufacturers offer speedometers scaled in miles/hour and not knots? We all know the difference between land mile and nautical mile. Why then, even they sell marine accesories, they scale them to land measuring units ?
Wouldn't it be less confusing if these instruments had nautical scale ?

Michael
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Old 06 September 2001, 11:06   #2
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AHA, methinks I know the answer to this one!
A knot is NOT a fixed constant. It is a nautical mile per hour and the length of a nautical mile depends where you are! Around wher I live it is about 2080 yards, but it is longer or shorter depending on whether you go South or North.
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Old 06 September 2001, 11:18   #3
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Really ?

Does that mean boats go faster in the north ?

And does it explain why speed records are set on Coniston rather than the Thames ?
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Old 06 September 2001, 11:18   #4
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I think it's more to do with manufacturers (and quite a lot of boat owners) preferring to have bigger numbers to talk about when they brag about the top speed.

Speedo's fitted by boat manufacturers may have an indicated scale in MPH or knots, but when compared with the real speed from a GPS they often seem to be calibrated in Km/h to make the top speed look better!

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Old 06 September 2001, 11:19   #5
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Hmmm. Surely a nautical mile is the same wherever you are. A nauticle mile is 1 minute of latitude - 1 minute of longitude is different distance depending where you are.
Always measure distance on the side of the chart - have I got that right?!

Ah.... Here we go......


Knot (Nauticle Mile)
A knot is equal to one nauticle mile per hour. A nautical mile differs from the statute mile - nautical mile = 6080 feet while a statute mile = 5280 feet (learned it in imperial, don't have the metric conversion). A cable is equal to 1/10th of a nautical mile - i.e. 608 feet, but is generally rounded off to 600 feet. nautical miles are indicated using the (') symbol - the same symbol used for minutes. Makes sense, because one nautical mile = one minute of latitude! Therefore, 60 minutes of latitude (one degree) = 60' - whether your near the equator, the tropic of Cancer or Alert! But you have to be careful when measuring on a chart - Mercator charts are made by wrapping a piece of paper around the earth in a cylinder shape (as opposed to conic or polyconic which use a cone shape or a series of cones) However, Mercator are the most popular chart projection for navigating because rumb lines or course lines are straight, and they are easy to measure distances, bearings and lat and long off. Like any chart, there is distortion - the distance between the latitude lines change and you must be careful to measure distances off the latitude scale closest to where you are transitting. And make sure you know what projection your chart is - you can't use the latitude scale on a Polyconic chart for measuring distances, you have to use the 'sea mile' scale located at the top and the bottom of the chart! (if you have a polyconic chart look at the latitude lines - if you put a set of parallel rulers along them you'll see they aren't straight! As well, the lines of longitude aren't parallel - they converge towards some point off the chart - the point of the cone).
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Old 06 September 2001, 11:21   #6
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Am I wrong then chaps.
Who will give us the definative answer??
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Old 06 September 2001, 11:33   #7
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Well MY book says:
"A nautical mile at any place is the length of one minute of arc measured along the meridian through the place. It varies in diferent latitudes owing to the irregular shape of the earth. In practice it is taken as 6080 feet which is its value at latitude 48 degrees."
I am still no clearer!!
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Old 06 September 2001, 11:48   #8
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Blimey....

I think other people are confused - check out this thread!!

http://www2b.abc.net.au/science/k2/s...pic209136.shtm

It even goes on to explain different types of nauticle mile British, American and Monaco!!!

Another defination is

nautical mile (nmi, naut mi or NM)
a unit of distance used primarily at sea. The nautical mile is defined to be the average distance on the Earth's surface represented by one minute of latitude. This may seem odd to landlubbers, but it makes good sense at sea, where there are no mile markers but latitude can be measured. Because the Earth is not a perfect sphere, it is not easy to measure the length of the nautical mile in terms of the statute mile used on land. For many years the British set the nautical mile at 6080 feet (1853.18 meters), exactly 800 feet longer than a statute mile; this unit was called the Admiralty mile. Until 1954 the U.S. nautical mile was equal to 6080.20 feet (1853.24 meters). In 1929 an international conference in Monaco redefined the nautical mile to be exactly 1852 meters or 6076.115 49 feet, a distance known as the international nautical mile. The international nautical mile equals about 1.1508 statute miles. There are usually 3 nautical miles in a league. The unit is designed to equal 1/60 degree [2], although actual degrees of latitude vary from about 59.7 to 60.3 nautical miles.
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Old 06 September 2001, 11:55   #9
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Well, here's how they come to the "international" nautical mile:

The arc between a pole and the equator is 90 degrees x 60 minutes which gives 5400 minutes. The length of the arc was approximated at 10000 km. Dividing the two gives that a nautical mile is 1.852 km.

I would say that it is this "international" nautical mile that everyone (OK, almost everyone) means when they speak about nautical miles...

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Old 06 September 2001, 12:04   #10
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Well this all explains one thing. I can see why the manufacturers callibrate the speedo in MILES PER HOUR....nobody really knows what a Nautical Mile is!

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