Isn't it confusing ?

[batfalcon]

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

[Brian]

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.

[Allen]

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 ?

[John Kennett]

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!

John

[jools]

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).

[Brian]

Am I wrong then chaps.
Who will give us the definative answer??

[Brian]

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!!

[jools]

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.

[sasa]

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...

Sasa

[Keith Hart]

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!

Keith Hart

[Keith Hart]

Here is what Encyclopaedia Britanica says about speed at sea:

In navigation, measure of speed at sea, equal to one nautical mile (6,080 feet in the British admiralty mile; 1,852 m, or 6,076.115 feet, in the international mile) per hour. This is approximately 1.15 statute miles per hour. Thus, a ship moving at 20 knots is traveling as fast as a land vehicle at about 23 miles (37 km) per hour. The term knot probably results from its former use as a length measure on ships' log lines, which were used to measure the speed of a ship through the water (see log). Such a line was marked off at intervals by knots tied in the rope. Each interval, or knot, was about 47 feet (14.3 m) long. When the log was tossed overboard, it remained more or less stationary while its attached log line trailed out from the vessel as the latter moved forward. After 28 seconds had elapsed, the number of knots that had passed overboard was counted. The number of knots that ran out in 28 seconds was roughly the speed of the ship in nautical miles per hour.

SO THERE YOU ARE THEN.

Keith Hart

[batfalcon]

Well, all the opinions said, I still don't get the answer to my question. I suppose the reply from John Kennett is the most logical, but still it is based on guesswork.
To make the things even more confusing I have to place side by side 2 indications from two major manufacturers.
It was last July, when 2 ribs sailed side by side on flat sea. The one was an Oceanic (Greek manufacturer) with a Yamaha 150 provided with the original Yamaha multi meters and the other one was the "SUNKISS II" (Mercury 115 ) provided with the old fashioned analog meters.
My GPS (Garmin GPS III) was indicating 32 knots sharp, my speedo 38 to 39 and the Yamaha's digital speedo 32 !!!
So far we thought that the Yamaha multi meter does a better job, BUT when the Yamaha ran at WOT the indication was 53 !!
As I was watching the Oceanic overtaking me, I judged that in no way we had a difference of 16 knots. (At the moment I ran 36,8 flat out according to the GPS and the speedo was telling around 44.)
Who is telling the truth here ?????

Any comments ?

[Brian]

I think I am with you on this one Keith.
The answer is "it depends"
depends on what?
"Ah, that depends again!"

[jools]

I would say the GPS is correct. Obviously measuring speed through water is difficult.

Its bad enough on land! Car speedos are allowed by law to be upto 10% wrong!

e.g. travelling along the motorway with the speedo reading 70MPH - you could be doing anywhere between 63 and 77 MPH!

In a car as your tires wear the distance arround the circumferance gets less, so the wheel turns more times for the same distance and the speed shown on the speedo will increase.

So you go faster with worn tires

If you have a handheld GPS you can see how acurate your speedo is (My speedo reads 85 when I'm doing 80!) Most say that you are going faster than you are to be on the safe side.

So back to the original thread... I would say that a speedo on a RIB / Speedboat is in MPH because the majority of people using speedboats probably refer to speed in MPH and don't know what a knot is! (see above)

Bigger boats that are used for navigation would normaly have speedos measuring knots.

[Alan]

as fitted with Optimax digital engines, you can recalibrate the speedo in mph, nautical miles and kph! The gauge has a analogue and digital readout. The whizzy software in the gauge drives the analogue pointery bit (technical terms here!) so you can set whatever.

The speedo reads from a paddlewheel sensor at low speeds and from the engine ecu at higher speeds. Compared with the GPS reading it is pretty accurate, although of course the gps gives speed over ground whilst the speedo gives speed through water.

Alan

[seajay]

A really informative thread appeared on uk.rec.sailing recently.

Without wishing to steal the excellent responses from a guy who produces charts of the Antartica (he convinced me anyway!), but a summary would be:

Traditional Nautical mile = 1 minute of latitude
Minute of latitude varies from 1855m at the pole to 1848 m at the equator. Took me a while to get my head around why it ends up longer at the poles. But if you follow the explanations it works.

Often however for contractual purposes etc, and to avoid ambiguity, the nautical mile can been 'locally' defined and is often around 1852m

The article really was fascinating - if you like testing the grey matter for a few minutes that is.

Chris H

[Nick]

Another definition! The definition of a nauticle mile I was taught while I was studying for my 2nd mates ticket is that it is the distance of 1 minute of longitude measured at the equator. The reason 1 minute of latitude varies is because the earth is not exactly ball shaped. It is an 'oblate spheroid' ie it is flattened towards the poles. For most of us though, a nauticle mile measured off the side of the chart will be perfectly adequate, as this will be the unit of measurement the cartographer will have used to draw the chart with.(see notes above regarding different chart projections though)

[Alan Priddy]

An interesting thread but does it really matter what any instrument is telling you? The object of the exercise is for you to get from point to point without any problems. No one seems to have mentioned leagues or cables as referance lines yet.
Alan P

[Brian]

They will, Alan P. They will!!

[jools]

So what about leagues and cables then?!