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Old 06 September 2001, 12:10   #11
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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
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Old 07 September 2001, 01:37   #12
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Miles, miles, miles....

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 ?
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Old 07 September 2001, 09:19   #13
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I think I am with you on this one Keith.
The answer is "it depends"
depends on what?
"Ah, that depends again!"
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Old 07 September 2001, 12:43   #14
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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.
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Old 07 September 2001, 14:23   #15
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If you have Mercury Smartgauges.....

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
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Old 07 September 2001, 16:22   #16
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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.

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Old 07 September 2001, 17:29   #17
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Nauticle Mile

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)
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Old 08 September 2001, 04:40   #18
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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.
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Old 09 September 2001, 17:15   #19
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They will, Alan P. They will!!
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Old 09 September 2001, 18:28   #20
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So what about leagues and cables then?!

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