About speed

[Colin Morley]

Thank you Julian,

It is most important I eventually get things right.

I have now also found that the correct abbreviation of knot is kn although common usage is kt although in scientific notation this means a kilotonne.

By the way the editor in me cant help pointing out that accelerationn is spelt with one n,

[Julian Swindell]

The world needs us pedants and editors! I don't think ISO recognises knots as theoretically they vary depending on latitude, just as nautical miles do.

[Jeremy]

The world needs us pedants and editors! I don't think ISO recognises knots as theoretically they vary depending on latitude, just as nautical miles do.

Pedants are indeed useful!  I was taught many years ago that a nautical mile is defined as a minute of arc of a Great Circle.  As a Great Circle isn't dependent on latitude, the distance of a nautical mile is fixed.

I believe the official nautical mile is a minute of arc of an averaged Great Circle, as the Earth has since been found to be a bit uneven, but the principle still holds good.

As a nautical mile is fixed, and as time is also fixed, then the knot as a unit of speed is also fixed.

[Julian Swindell]

I think a true nautical mile is officially 1 minute of longitude, which is why  you should always measure it off the edge of a chart which is local to your current position. The length of a minute of longitude varies from the Equator to the pole as the Earth is a bit flattened. For practical purposes a nautical mile has been set at 1,852m but all of us pedants know that that isn't strictly true. The only circular Great Circle is the equator itself. All others are a bit squashed because the Earth is a bit squashed. The official international designation of the shape of the Earth is that it is not a spheroid (sphere shaped) but a geoid (Earth shaped). So the Earth is shaped like...the Earth. One of the more useful scientific designations.

[Jeremy]

Julian, I don't disagree, what you've said is exactly as I recall being taught during the yachtmaster stuff 30 odd years ago, but my (rather tongue in cheek) point was that nautical miles (or knots) don't vary with latitude.............

[Johan Ellingsen]

Mon Dieu..

1 nautical mile=

1 degree of latitude at any meridian

or

1 degree of longitude at the equator,

Thus spake Wikipedia.

[Jeremy]

Mon Dieu..

1 nautical mile=

1 degree of latitude at any meridian

or

1 degree of longitude at the equator,

Thus spake Wikipedia.

If that is what Wikipedia says then it proves how wrong it can be.

A nautical mile is one MINUTE of an arc of an average Great Circle, so there are 60 nautical miles in a degree.  Generally using the approximation 1 nm = 1 minute of latitude works well enough, certainly better in terms of absolute accuracy than even a GPS will give.

[Matt Newland]

I've just read this thread from start to finish, and not wanting to blow my own trumpet, the competitor in me is pleased I might still have the record, at 10.8 knots? This was during a PBO magazine test, when David Harding was at the helm (so I cant actually take credit in any case), and measured on his GPS.
Today I went out with a potential customer and a friend and we hit 8.6knots at the top of the tide, in winds gusting to 32mph. We had ballast tanks full and board down (should have pulled it up but we had very little channel available). I wanted to empty the tank but my crew suggested otherwise...
In terms of how she feels, well I am biased but to me she feels great. she fizzes along with no instability. I think the fastest point is about 90degress to the apparent wind. I have no windex on the mast though so am just guessing this last point.

Matt

[Julian Swindell]

I won't (wouldn't dare) challenge Matt's speeds. But on the previous points, the exact length of a nautical mile is a bit trivial in practice, but please don't use a minute of latitude (along the bottom/top edge of a chart)as a measure of distance. A minute of latitude is 1nm at the equator, and precisely 0 nm at either pole. The UK is more than half way to the pole, so a minute of latitude in these waters will be much less than 1 nm. Use the left/right edge of the chart for a minute of longitude which is as near a measure of a nm as anyone needs.

BTW I did record 18kts max on my GPS, but I only checked it after I had towed the boat up the road to the car park, so there may be some doubt cast on that as a legitimate record...

[Jeremy]

I won't (wouldn't dare) challenge Matt's speeds. But on the previous points, the exact length of a nautical mile is a bit trivial in practice, but please don't use a minute of latitude (along the bottom/top edge of a chart)as a measure of distance. A minute of latitude is 1nm at the equator, and precisely 0 nm at either pole. The UK is more than half way to the pole, so a minute of latitude in these waters will be much less than 1 nm. Use the left/right edge of the chart for a minute of longitude which is as near a measure of a nm as anyone needs.

BTW I did record 18kts max on my GPS, but I only checked it after I had towed the boat up the road to the car park, so there may be some doubt cast on that as a legitimate record...

Perhaps worth checking the above, Julian.  It is the distance of minutes of longitude that vary with latitude, or at least it was back when I did my Yachtmasters ticket many years ago.

The angle of latitude is measured up the sides of the chart, longitude along the top and bottom, unless the hydrographic office have started printing them sideways now..................

[Julian Swindell]

You may well be right there! I can never get them right.