Why do boats use nautical miles and why are they different than normal miles? Couldn't you just convert it to normal miles or km?

[Gurrig]

Let's not even start on Irish Miles....

Or if more than 1 mile it may be described as a mile and a bit!

 

[lustyd]

As a practical matter, yes, I agree. Many good reasons. But is there a reference for this statement? I swear to God, I am not trying to be argumentative, but I am a writer and I like to know what I know is supportable fact. The below (from Wiki, but the same information is many places) is pretty weird, but there is nothing I have found that says navigation uses minutes instead.

Of course it uses minutes of arc, where do you measure distance against on a chart when navigating? There's no distance scale on a nautical chart, there doesn't need to be, and you certainly don't have a little ruler with distances on it. When navigating at sea, you use dividers to measure distance and set them on the scale at the side of the chart which measures minutes of arc.

 

[Daydream believer]

A radar mile is a unit of time.

In my experience-Radar Love does not last long either

 

[sfellows]

It really doesn't matter, whether you use Radians, KM, NM, Miles, light years, London Buses, Metres or Cables you are always going to end up X (where X is a large/small) number of Radians, KM, NM, Miles, light years, London Buses, Metres or Cables down-wind and down-tide of your intended destination when using DR.

 

[mattonthesea]

Members of an advanced age will remember the back of old school exercise books:

View attachment 185157


Not a good image but nice to recall that 36lbs of Old Hay = 1 Truss. 36 Trusses = 1 Load

.

I stopped using these when a cup of coffee cost 50 bob!

 

[zoidberg]

It really doesn't matter..... you are always going to end upwind and down-tide of your intended destination when using DR.

There remain scores of RN (retd) and RAF (retd) master navigators who'll be smiling indulgently at that slice of b***s.

HMQueen spent a whole lot of money having us 'trained to a hair' in using DR to the n'th degree so as to navigate HM ships through tight wee channels and fly HM aircraft to precise points in the sky, come hail, rain and shine.... largely 'cos HMQueen's more senior officers didn't choose to spend all that money on shiny high-falutin' gadgets which used expensive 'Murricain SatNav until the kit had matured, everyone else had bought some, and the price came down.

 

[AntarcticPilot]

Of course it uses minutes of arc, where do you measure distance against on a chart when navigating? There's no distance scale on a nautical chart, there doesn't need to be, and you certainly don't have a little ruler with distances on it. When navigating at sea, you use dividers to measure distance and set them on the scale at the side of the chart which measures minutes of arc.

Also, on most charts, the scale varies noticeably from north to south of the chart. This is a property of the Mercator projection; scale varies according to the secant of the latitude. So, taking a chart extending 1 degree in latitude at a latitude of (say) 52°N, if the scale at the south of the chart is 1:10000, the scale at the north of the chart is 1:10230. The rate of change increases rapidly as you go north - it is negligible in tropical latitudes, increasing to infinity at the Pole!

 

[Refueler]

I feel that the flow changes to quickly there, especially at my speeds !

C'est la vie mate !!

There remain scores of RN (retd) and RAF (retd) master navigators who'll be smiling indulgently at that slice of b***s.

HMQueen spent a whole lot of money having us 'trained to a hair' in using DR to the n'th degree so as to navigate HM ships through tight wee channels and fly HM aircraft to precise points in the sky, come hail, rain and shine.... largely 'cos HMQueen's more senior officers didn't choose to spend all that money on shiny high-falutin' gadgets which used expensive 'Murricain SatNav until the kit had matured, everyone else had bought some, and the price came down.

So we'll ignore INS ... Omega .... Loran A and C and dear old RDF ... and here's one for you ... my Father navigating a B17 to UK :

Pictures a bit distorted as they are clips from the Cine 8mm ..... it was one of the B17's used in the movie "War Lover".

 

[Sandro]

It is very likely that the subdivision of the circle in 360 degrees instead of a more round figure was originated by the number of days in one year, one degree being the arc travelled by earth in one day.
By the way, following what AntarcticPilot said in post #22, it's a pity that we were not born with sex fingers instead of five. The decimal system would have been duodecimal, there would have been 11 figures instead of 9, "twelve" would have been written "10" and the possibility of more factors would have been convenient in many instances.

 

[mattonthesea]

Also, on most charts, the scale varies noticeably from north to south of the chart. This is a property of the Mercator projection; scale varies according to the secant of the latitude. So, taking a chart extending 1 degree in latitude at a latitude of (say) 52°N, if the scale at the south of the chart is 1:10000, the scale at the north of the chart is 1:10230. The rate of change increases rapidly as you go north - it is negligible in tropical latitudes, increasing to infinity at the Pole!

When I learnt a bit about celestial nav (adapted sun run sun - in practice, accurate to about 30x1850m) I came to the conclusion that Mercator grids were a cosine ratio IE the horizontal dimension is cosine of the latitude degrees x the vertical dimension of the grid. This would be equivalent to the lines of longitude starting at 90 degrees to latitude at the equator and becoming ludicrously too close to zero when nearing the poles for a sensible Mercator chart.

Have I got this right?

 

[AntarcticPilot]

When I learnt a bit about celestial nav (adapted sun run sun - in practice, accurate to about 30x1850m) I came to the conclusion that Mercator grids were a cosine ratio IE the horizontal dimension is cosine of the latitude degrees x the vertical dimension of the grid. This would be equivalent to the lines of longitude starting at 90 degrees to latitude at the equator and becoming ludicrously too close to zero when nearing the poles for a sensible Mercator chart.

Have I got this right?

No - it's secant (1/cosine). And of course that's only for a spherical earth - it's MUCH nastier taking into account the ellipsoid.

 

[thinwater]

When was last time you took a sight ... run the calculation ... plotted ?? Until you move the position lines to intersect - all calculations are based on angular ..
Actually even the movement of position lines to cross at desired 'clock time' are based on the minute of arc Nm ...

This is ad hominem. I'll answer anyway.

When I started sailing 45 years ago GPS didn't exist and I learned both coastal nav and celestial nav. I'm and engineer and consider the math both obvious and straightforward. Yes, I understand the practical usefulness of nautical miles. It's a great system and using kilometers would not be helpful.

The question about standards was intentionally pendantic and I said as much. I'm simply curious if there is a STATED practice, in any standard, of using a different value for nautical miles when doing nav. It seems clear at this point there is not, only a matter of custom and obvious practicality. The difference between the two is generally a matter of inches, so it makes little difference.

 

[Gurrig]

It is very likely that the subdivision of the circle in 360 degrees instead of a more round figure was originated by the number of days in one year, one degree being the arc travelled by earth in one day.
By the way, following what AntarcticPilot said in post #22, it's a pity that we were not born with sex fingers instead of five. The decimal system would have been duodecimal, there would have been 11 figures instead of 9, "twelve" would have been written "10" and the possibility of more factors would have been convenient in many instances.

And instead of 'Gimme Five' we could have 'Gimme Six' Not sure if this does not happen in some parts.

 

[Daydream believer]

I stopped using these when a cup of coffee cost 50 bob!

Yes and half of it was froth

 

[capnsensible]

It is very likely that the subdivision of the circle in 360 degrees instead of a more round figure was originated by the number of days in one year, one degree being the arc travelled by earth in one day.
By the way, following what AntarcticPilot said in post #22, it's a pity that we were not born with sex fingers instead of five. The decimal system would have been duodecimal, there would have been 11 figures instead of 9, "twelve" would have been written "10" and the possibility of more factors would have been convenient in many instances.

If you mean six, visit Norfolk.

Oh, beaten to it!!

 

[sfellows]

There remain scores of RN (retd) and RAF (retd) master navigators who'll be smiling indulgently at that slice of b***s.

HMQueen spent a whole lot of money having us 'trained to a hair' in using DR to the n'th degree so as to navigate HM ships through tight wee channels and fly HM aircraft to precise points in the sky, come hail, rain and shine.... largely 'cos HMQueen's more senior officers didn't choose to spend all that money on shiny high-falutin' gadgets which used expensive 'Murricain SatNav until the kit had matured, everyone else had bought some, and the price came down.

Someone’s had a sense of humour failure….

…maybe they’re just disappointed that Trump got re-elected. Or maybe not.

 

[Refueler]

This is ad hominem. I'll answer anyway.

When I started sailing 45 years ago GPS didn't exist and I learned both coastal nav and celestial nav. I'm and engineer and consider the math both obvious and straightforward. Yes, I understand the practical usefulness of nautical miles. It's a great system and using kilometers would not be helpful.

The question about standards was intentionally pendantic and I said as much. I'm simply curious if there is a STATED practice, in any standard, of using a different value for nautical miles when doing nav. It seems clear at this point there is not, only a matter of custom and obvious practicality. The difference between the two is generally a matter of inches, so it makes little difference.

There isn't use of different value when doing nav. The difference only comes about if someone mistakenly tries to look at nav in terms of physical length units instead of minutes of arc.

 

[zoidberg]

Someone’s had a sense of humour failure….

.....Or maybe not.

Not so much an SOH Failure, more a 'weak joke' penchant!

You needed an SOH when burning long holes in the sky, 8 miles up over the East Atlantic, using up the squadron's monthly allocation of Flying Hours..... and doing manual DR for hour after hour with Fix-Monitored Azimuth Astro 'cos half the 'war-fighting kit' was knackered with no available spares and the other half was 'not to be used' in case there WAS a war.

"Can't take a joke, shouldn't have joined!"

 

[Yorkshire Exile]

Have I reached the end of this thread?

 

[zoidberg]

Have I reached the end of this thread?

I wish....