km/hr or kn?

[whichway]

Hi

Do you use km/hr or knots when talking about teh speed of you boat.

I use km/hr, but a mate who has a bay cruiser always insists on knots.

Does it matter which you use.

Whichway.

[Spaniard_King]

Knots on sounder and KLm/hr on GPS

Garry

[Tony_N]

My Furuno GPS is set on kliks. The mental conversion from knots to kph to give the speed some benchmark/relevance to me is too hard. #I think REAL navigators use knots

[seatime]

Doesn't matter so much for general small boat use, local fishing, that kinda thing. Depending where your GPS or sounder was manufactured, they'll read in kts or klms.

If travelling large distances and using Admiralty type charts, you'll need to use knots (nautical miles/hour).
Distances on paper charts or Raster scanned charts are measured in nautical miles, in the case of paper charts, off the latitude scale, Raster calculate automatically.
This is because the earth's curved surface is measured in nautical miles, e.g. 1nm = 1 minute on the latitude scale, 60nm = 1 degree of latitude.

Kilometers don't really allow for curvature, more for flat surfaces, so for short journeys and it's OK to use klm/hr.

regards
Steve.

[triman]

A knot is 1 nautical mile per hour or about 1850 metres an hour, give or take.
So 1 knot is just a tad under 2 km/hr.
Note that a kilometre is a fixed distance of 1000 metres while a nautical mile is slightly variable, depending upon the latitude. At the Equator a nautical mile is 1842 metres, at the pole its about 1861.

[Kerry]

....Note that a kilometre is a fixed distance of 1000 metres while a nautical mile is slightly variable, depending upon the latitude. At the Equator a nautical mile is 1842 metres, at the pole its about 1861.

The nautical mile is defined as (exactly) 1852 metres and all nations have adopted this value since 1929.

[seatime]

whichway,

You'll notice the wind speed predictions on the more reputable sites, bom, seabreeze, buoyweather, use knots not klms, as knots are a nautical measurement.

Whereas some media like TV Ch 10 use klm/hr, mostly for landbased observations and predictions, but some marine forecasts. They're more for landlubbers, as they can relate to kms in their cars more easily.

Jetskis have klm/hr on the speedo, as they're a motorbike on water ;)
although the speed restrictions are legislated as knots :-/

When talking boat speed, I prefer knots (old school) so I can refer to a chart and calculate fuel consumption if needed, tho klm/hr does sound faster.
At the end of the day, use whatever you're most comfortable with :)

[eugah]

My speedo & trip computer are set up for knots, but i have my GPS set up KM as i fing it easier to visualize 100 metres rather than .048 knot when looking for my spots.


Adrian

[Cheech]

I am obviously not a real navigator then because I use kph and also metres in depth.

I understand all 3 units. My first car was actually in mph, and I did a lot of sailing in my youth with knots. But for smallcraft boating, it seems a lot easier and readily understood by everyone when you talk metric. Same with fuel economy, I talk k per litre rather than litres per hour.

[mirage]

Hi Guys,

I'm sorry but I have to make a comment here and correct some of the cr@p that's coming out.

1 Nautical Mile is 1 minute of longitude (not latitude) when measured at the equator and ONLY at the equator, as a minute in longitude decreases in distance as you head North or South. Therefore 1 degree of longitude at the equator is 60 Nautical Miles.

A Nautical Mile has never been and will never be a variable figure.

As to the original question, if you are happy talking about how fast your pride and joy goes in km/h then go for it. Knots just makes it far easier for navigating off nautical charts etc. Personally I wish boat magazines would use knots not km/h when doing boat tests. 53km/h means nothing to me when I'm thinking about boats or aircraft for that matter.

Any GPS that I know can be changed between km/h, mp/h or knots!

Cheers, Scotty.

[StevenM]

Doesnt matter you are still doing the same speed at the end of the day

but for the record

knotts

[dogsbody]

Just keep it in knots saves confusion. Whats next do you weigh your fish in kg or lbs. :)
Dave.

[seatime]

mirage

where's the cr@p, maybe you should check your facts.

Latitude is an arc, measured from the centre of the earth, North and South of the equator.
Longitude is an arc measured at the poles, East and West of Greenwich.
Lines of Longitude are great circles, that is, their planes pass through the centre of the earth.
The only great circle line of latitude is the equator, as it's the only one who's plane passes through the centre of the earth. Parallels of latitude are then measured as arcs N and S of the equator. These are small circles.

Navigation calculations of position are made in units of arc, degrees and minutes. It is convenient for navigators to use as a unit of distance, the length of a minute of arc of a great circle upon the earth's surface.
So, a nautical mile is taken as the length of a meridian which subtends an angle of 1 minute at the centre of the earth.
As the earth is not a perfect circle, it's modified so that 1 minute of geographical latitude is equal to 1 nautical mile (1853 metres) at any given latitude.

What you're refering to is a geographical mile, which is 1 minute of arc of the equator, or the length of the equator which subtends an angle of 1 minute at the centre of the earth.

At the equator, 1 minute of longitude is equal to 1 GEOGRAPHICAL MILE or 1855.3 metres.

You can apologise anytime now, thanks.

regards
Steve

[Kerry]

Hi Guys,

I'm sorry but I have to make a comment here and correct some of the cr@p that's coming out.

1 Nautical Mile is 1 minute of longitude (not latitude) when measured at the equator and ONLY at the equator, as a minute in longitude decreases in distance as you head North or South. Therefore 1 degree of longitude at the equator is 60 Nautical Miles.

A Nautical Mile has never been and will never be a variable figure.

Cheers, Scotty.

Cr@p coming out #needing correcting ;) Well then lets correct this cr@p!

What you have desicribed is not a Nautical mile but a Geographical mile this being 1/60th of a degree (in Longitude) of the earth's equatorial circumference.

A Nautical mile is one minute of arc in latitude of a great circle.

Regards, Kerry.

Edit: PS Gelsec :) Ah you already said that :)
# #

[Brissyguy]

Well our old Caribbean has a speedo in mph, a sounder set on km/h and the GPS reads in knots lmfao...

[seatime]

cheers Kerry,

#I don't have a problem with someone disagreeing with my posts, opinions and debate are what forums are all about.

It's the calling of solid facts "cr@p" that irks.

[IcemanII]

Interesting ........

If in a boat should a nautical mile be defined as a sea mile by default ?

An alternative term - sea mile - is still used for the distance travelled in transversing one minute of latitude (or, alternatively, one minute of longitude at the equator).

Current editions of the Collins English Dictionary have separate entries for nautical mile and sea mile. According to the entry the latter is defined as 6000 feet (1828.8 m).

http://en.wikipedia.org/wiki/Mile

[seatime]

IcemanII

there's no default or alternative for a nautical mile.

in navigational terms, which is what the topic is about, knots are nautical miles per hour.
a nautical mile is a nautical mile.

a sea mile is possibly an alternative name for a geographical mile, though never heard the term before, it's not a nautical mile by default, not in any-one's book!

regards

[dogsbody]

A sea mile is a former British unit of length once used in navigation; equivalent to 1828.8 meters.

Dave.

[mirage]

Gelsec, Kerry and others, I do apologise for being so abrupt.

When I did my navigation course (aviation navigation), many years ago, we were specifically taught 1Nm is 1 minute of longitude at the equator. I realise 1 minute of latitude is close to the same distance as at the equator but as you know differs slightly because the earth is not a perfect sphere.

So I have gone back to school and it seems there is two common definitions for a nautical mile on the internet. One is using 1 minute of latitude the other is using longitude at the equator. I have copied a couple of definitions below.

1. "A nautical mile is based on the circumference of the planet Earth. If you were to cut the Earth in half at the equator, you could pick up one of the halves and look at the equator as a circle. You could divide that circle into 360 degrees. You could then divide a degree into 60 minutes. A minute of arc on the planet Earth is 1 nautical mile. This unit of measurement is used by all nations for air and sea travel. "

2. "Knots and Nautical miles are good old navy terms. The nautical mile was based on the circumference of the earth at the equator. Since the earth is 360 degrees of longitude around, and degrees are broken into 60 so-called "minutes", that means there are 360 * 60 = 21,600 "minutes" of longitude around the earth. This was taken as the basis for the nautical mile; thus, by definition, 1 minute of longitude at the equator is equal to 1 nautical mile. So the earth is ideally, by definition, 21,600 nautical miles (and 21,600 "minutes" of longitude) in circumference at the equator. If anyone ever asks you how far is it around the earth, you can quickly do the math in your head (360 degrees * 60 minutes per degree) and answer "about 21,600 nautical miles!"

In fact, even modern navigators use the "minute of latitude" on charts to measure distance; this is what you see them doing when they use their compass spreaders while they are hovering over their nautical charts (maps). [For geometrical reasons, we use the minute of latitude on charts to correspond to a nautical mile rather than the minute of longitude. Minutes of longitude shrink as they move away from the equator and towards the poles; minutes of latitude do not shrink. Take a look at a globe with longitude and latitude lines marked on it to understand why."

[seatime]

mirage

you can't navigate by sea or air using google #::)
even those definitions don't fully explain how a nautical mile is measured.

as you rely on google so much, try googling latitude, longitude and nautical mile, then compare them #:-?

as stated in an earlier post;
latitude is an arc (angle) measured at the centre of the earth.
longitude is an arc (angle) measured at the poles.
a nautical mile is an arc (angle) measured at the centre of the earth.

see the relationship between latitude and a nautical mile. just because it's measured on the equator doesn't make it longitude, it's measured on the equator because the equator is a great circle #;)

a nautical mile is a measurement of distance used at sea.

you really need to understand the principles of navigation before jumping in and calling a post cr@p.
you weren't only abrupt, you were also wrong.

BTW, longitude can be used to measure time, and vice -versa,(longitude by chronometer, a nav position calculation)

15 degrees of longitude = 1 hour

e.g. Australian EST zone = UTC + 10hrs
longitude around SEQ is approx 153 degrees East
i.e. 1 hour for every 15 degrees east of Greenwich.

TAFE on the Sunshine Coast conducts navigation courses, why don't you drop in and confirm your definitions with the lecturers there. #;)

regards
Steve #;)

[mirage]

Hi Gelsec,

Not to be put off by your Google comments, ;) I did a search for the Royal Observatory Greenwich, which would have to be one of the worlds top authorities on such matters. Based on their definition I apologise to you. It is copied below.

"A mile on land (statute mile) is 1760 yards (1609.3m): a sea or nautical mile is one-sixtieth of a degree of latitude, and varies from 6046 feet on the Equator to 6092 feet at latitude 60°. This difference is due to the Earth not being a perfect sphere, but slightly flattened at the poles – an oblate spheroid.

The ‘standard’ nautical mile is taken as 6080 feet (1.151 statute miles or 1853m)."

As for my "need to understand the principles of navigation", I do. I use navigation every day around the world in my job. It just seems I was taught a slightly different definition, the same as many sources still quote on the net, including reputable dictionaries.

I don't know why this is so but as I said based on the Royal Observatory, you are correct. :)

Cheers.

[wessel]

The first time I was allowed onto the bridge of a warship, the first mistake I made was to ask how many kilometers we still had to go to our destination.
I have never been smacked so hard in my life without retaliating. It was the captain, who is a bit of an old seadog and you spoke in British Naval terms on his ship. (Never ever call it a boat either, that was the second time the man clipped me up the back of the skull.)

So, you learn to speak in naval terms and I found out what a poop deck is.

A while later I ended up in the back of helicopters for a few years. Would they not go and change the bloody terms again. Before getting clipped up the back of the skull again, I changed my vocabulary. There is a few differences between naval and aviation terms again.

Now that I am in charge of my own boat, I will answer questions in terms that the person is familiar with.
The wife - km/h
Some of my friends - knots
My other friends - Dont really care because they are never envited offshore. They are the chardonnay drinking types that deplore any kind of violance. They just get invited to dinner. We were just over the horison is the standard answer.

I can mentally convert between the two rather acurately by now.

Wessel

[Kerry]

And just to clarify things further, the standard nautical is 1852 metres (exactly) as defined in 1929. The distance of 1852 metres as defined is basically very close to the average length of 1 minute of latitude between the poles.

Traditionally the length of a nautical mile did vary with latitude, which is a reflection of the chart projection and for most purposes is not important for practical purposes.

Effectively the equator is a one off special case of a great circle but in nautical terms it is the great circles (meridians) running from pole to pole on which 1 minute of latitude "used to" determine a nautical mile.

Today the nautical mile is 1852 metres exactly and all nations adopt this standard.

The figure of 6080 feet is purely an old and outdated British definition and is generally referred to as the admiralty mile, which was replaced by the international nautical mile almost 40 years ago. The admiraltly mile was not based on any scientific reasoning, simply 800 feet longer then the statue mile.

Regards, Kerry.

[seatime]

mirage

accepted, no worries. :)

being a public forum, I'm not fond of seeing misinformation posted.
someone reading it may believe it and put it into practice.

it goes to show, don't believe at face value, info posted on the net, look deeper to find the facts.

I've navigated ships worldwide for many years, weather permitting I'd switch off the electronic aids to navigation and do it by hand, sextant, air tables or Norries, nautical almanac, etc, to keep the skills up.

no malice intended :)

safe boating
regards

[mirage]

OK Kerry and Gelsec,

Maybe the RAAF was still using outdated British stuff years ago when they taught me. Points all taken. :)

[Shanoss]

So.......can someone run that by me again? ;D ;D

[Roughasguts]

So.......can someone run that by me again? ;D ;D

Yep 1NM is 1852 metres.

And yes it plays a part in navigation, but consider the other variables.

Like wind strengh.
Wind direction or tide.
Indicated speed or true airspeed adjusted to pressure height, or dencity height.
Magnetic direction and true heading.
Adjust that to how long it will take you to travel a given distance.

Then try and maintain some degree of accuracy considering the variables in navigating.

Would a couple of metres either way make much differance, I dought it.

Of course you could use a GPS instead provided it's working.

[Shanoss]

Yep. Will stick with the GPS. Though i do take a compass heading on the way out, just in case. And a spare compass. You never know........

Just took the hard top off my V213c, so if anyone is interested, they are welcome to it.

[Black_Rat]

I have the sounder/GPS combo set to km/hr on the Humminbird Matrix15 and have both km/hr and I think MPH on the motor guages (Mercury)

I really don't know which is the most accurate though ??? #:-/

I bought my sounder/GPS combo a few weeks before the boat was ready for fitting so one day I hooked it all up via a 12v battery and duct taped the GPS antenna to roof of the Falcon ute and went for a spin ;D

The GPS was showing 62km/hr while the speedo was showing 60km/hr #:-/ Not that you could trust a Ford #(not my choice, it was a company car)#:P ;D ;D ;D

Damo

[Roughasguts]

I don't think gps is that accurate, mine tells me the height = 26 foot.
How can that be when the boat is at sea level.

[predator]

Have a read of your Gps/sounder setup instructions, some where you will be able to fix that.. ;)

[Roughasguts]

Thanks Predator, think I will go find me manual.

[Kerry]

I don't think gps is that accurate, mine tells me the height = 26 foot.
How can that be when the boat is at sea level.

GPS vertical accuracy is quite different to horizontal accuracy and the height a GPS determines/computes is effectively not directly related to sea level. SPS vertical accuracy is in the order of 2-2.5 times less accurate than horizontal.

A GPS height initially relates to the ellipsoid, which is a mathematically defined shape of the earth. To determine heights related to sea level then an additional geoid model has to be used. Some GPS receivers might only display ellipsoid heights, others might also display orthometric (seal-level type) heights but the sea-level height is only as good as the accuracy of the geoid model and since most GPS receivers that do this have a world wide geoid model it's not all that accurate in itself.

Regards, Kerry.

[seatime]

GPS vertical accuracy is quite different to horizontal accuracy and the height a GPS determines/computes is effectively not directly related to sea level. SPS vertical accuracy is in the order of 2-2.5 times less accurate than horizontal.

A GPS height initially relates to the ellipsoid, which is a mathematically defined shape of the earth. To determine heights related to sea level then an additional geoid model has to be used. Some GPS receivers might only display ellipsoid heights, others might also display orthometric (seal-level type) heights but the sea-level height is only as good as the accuracy of the geoid model and since most GPS receivers that do this have a world wide geoid model it's not all that accurate in itself. #

Regards, Kerry.
#


wow, try saying that after a couple of bourbons :)

for interest sake, in the world of offshore navigation, spherical trigonometry and celestial bodies, the earth is referred to as an 'oblate spheroid'.
just thought I'd throw that in :D

regards

[Roughasguts]

Thanks Kerry, Electronic gadgets aren't my thing, I get lost after congratulations on your purchase.

It's then I realise I will never use the gadget to it's full potential.

[mirage]

I'm staying away from this one. ;)

[Kerry]

for interest sake, in the world of offshore navigation, spherical trigonometry and celestial bodies, the earth is referred to as an 'oblate spheroid'.
just thought I'd throw that in #:D

:) "Oblate Spheroid"? Yes, but which one? :D

[seatime]

for interest sake, in the world of offshore navigation, spherical trigonometry and celestial bodies, the earth is referred to as an 'oblate spheroid'.
just thought I'd throw that in #:D

:) "Oblate Spheroid"? Yes, but which one? #:D


the kind the earth is? flattened at the poles.

not so apparent if using mercator projection charts in latitudes less than 60 degrees.
if you wish to navigate in higher latitudes, not advised unless you have a good reason (some great circle routes do go to 65-70 tho), gnomic projection charts are used, and the earth's shape becomes more apparent.

this is for maritime navigation, not familiar with aviation navigation, so can't comment, and not a wizz on GPS techs.

[ahoj]

Next question :are knotts faster then Km/h?



Ahoj---its not me)

[Roughasguts]

Use kilometers they tick over faster, and whats this oblate hemoroid stuff!! the earth is flat.
Well at least it soon will be, just got to speed it up a bit and get that centrifical force going harder and she will flatten out even more at the poles.
Gonna have to watch that sharp ridge at the equator, in a boat though.

[seatime]

Use kilometers they tick over faster, and whats this oblate hemoroid stuff!! the earth is flat.
Well at least it soon will be, just got to speed it up a bit and get that centrifucal force going harder and she will flatten out even more at the poles.
Gonna have to watch that sharp ridge at the equator, in a boat though.

;D ;D ;D

actually you might be closer than you think, it's not centrifugal force we have to worry about!
it's the magnetic poles reversing, North moves to South, or no poles, no lines of magnetic force.
the earth's core is running out of magnetism, and the scientists aren't quite sure what will happen when it does.

of course it's a lot more complicated, but it could happen in our lifetime or maybe the next, a big grey area.

now that I brought it up, I'm sure someone will enlighten us or debunk the theory.

what's the website of the flat earth society? ;)

[Roughasguts]

Hmmm interesting losing our magnatism.
You think it's from mining all those billions of tonnes of iron ore, then re-distributing it in other parts of the world.

Think we better start looking for another planet to live, this ones........ just about, F***** ah, finished,. We could then sap the earth of all it's goods and when were done, dump all our waste here.
Bloody hell even killing off the magnetic field, were pretty good.

[Kerry]

Oh yeah the North Pole has been on the move for many years, gradually and progressively heading west/north-west. Even on a daily basis the north pole dances around quite a bit.

As for the poles switching there is evidence that this has occured?


....and whats this oblate hemoroid stuff!! the earth is flat.

That is slightly flat, top and bottom :D Oblate Spheroid? flash sounding name for what most simply call the datum :o

Flat :-X but it can't be flat? Like where does all the water go when the tides out :)

Regards, Kerry.

[Roughasguts]

Oh yeah thanks Kerry, I remember reading the magnetic variation changes on me charts, they don't stay fixed any more they do move over time.

I think since I started flying the magnetic variation has moved 0.5 degree which sound like nothing. But if you use the 1 in 60 dead rechoning stuff thats 0.5 Nm off track or course, in a 60 mile trip.

Recipe for disaster with old charts.

[wayne_cook]

The first time I was allowed onto the bridge of a warship, the first mistake I made was to ask how many kilometers we still had to go to our destination.
I have never been smacked so hard in my life without retaliating. It was the captain, who is a bit of an old seadog and you spoke in British Naval terms on his ship. (Never ever call it a boat either, that was the second time the man clipped me up the back of the skull.)

So, you learn to speak in naval terms and I found out what a poop deck is.

A while later I ended up in the back of helicopters for a few years. Would they not go and change the bloody terms again. Before getting clipped up the back of the skull again, I changed my vocabulary. #There is a few differences between naval and aviation terms again.

Now that I am in charge of my own boat, I will answer questions in terms that the person is familiar with.
The wife - km/h
Some of my friends - knots
My other friends - Dont really care because they are never envited offshore. They are the chardonnay drinking types that deplore any kind of violance. They just get invited to dinner. We were just over the horison is the standard answer.

I can mentally convert between the two rather acurately by now.

Wessel




luv it

[madman1]

I use both

[Kerry]

According to the people, mainly the Canadians who think they own the north pole, it has moved 1,100km towards Sibera in the past 100 years. On any given day it wanders and elliptical path some 80km across.

Recent wanderings have supposedly resulted in homing pidgeons getting lost and crashing?

And supposedly the poles flipping isn't somthing new, been going on for millions of years, last time was 780,000 years ago. They say it takes 1000 years to reverse the poles so charts should well and truely out of date by then ;D

[Mad_Barry]

Poles flipping ??

No,, don't let it happen, throw the anchor out quick !!

I like North Qld, I don't want to be down in South Australia :-[
http://www.thesavvytraveller.com/agraphics/world_views/maps/odt/upside_down_475h.jpg

[Roughasguts]

My god what would happen to daylight saving then.

[seatime]

Hmmm interesting losing our magnatism.
You think it's from mining all those billions of tonnes of iron ore, then re-distributing it in other parts of the world.

Think we better start looking for another planet to live, this ones........ just about, F***** ah, finished,. We could then sap the earth of all it's goods and when were done, dump all our waste here.
Bloody hell even killing off the magnetic field, were pretty good.


not really from mining iron ore, though it does have permanent magnetism, as it's a 'hard iron' as opposed to 'soft iron' which has induced magnetism like the correction magnets in a compass. even the huge amounts of iron ore mined wouldn't effect local magnetic variations. they would most likely take on the values of the surrounding magnetism.

the earth's magnetic field comes from it's molten core, that really hot wet sh!t in the middle. the lines of force flow out of the North seeking pole (red pole, South) and into the South seeking pole (blue pole, North).
there is also a magnetic equator, it's a wiggly line running around the earth near the geographic equator, where dip is zero.
variation is the angle between the true and magnetic meridian, then there's local disturbances and anomalies caused by what's just under the surface. places like Magnetic Island and in the Spencer Gulf have an abnormal effect on variation.