Knots vs MPH

Maybe inland waters, lakes and rivers, is where folks stay with different units? My brothers boat mainly in those conditions and always use kmh.

A similar pattern seems to emerge from posts above. Land units for inland waters. But if your boating is primarily ocean-based then knots and nautical miles are normal use.

When brokers quote mph I think of them as either lazy and incompetent or sneaky: trying fool people into thinking the boat can cruise faster than it really can.

Fortunately the nautical mile is determined by the size/shape of the earth so that it is the same distance throughout the world. Different languages may use different terms for the nautical mile, and for knots, but they are the same distances as in the English language.

Miles per hour / statute miles mean nothing to an European, or Asian. In another generation statute miles will mean nothing to a Canadian as they are slowly thinking in terms of kilometers for their land based measurements.

For a cruiser who will travel outside of the US inland rivers familiarity with knots and nautical miles is essential. Much of the written and electronic data is based on nautical miles.

Both are so easy to use in the right venue, not confusing if said correctly and easy enough to convert between....


Use whatever you want...don't assume the next guy is and really don't assume they are a lesser boater for using one or the other or both....

Insequent said:

When brokers quote mph I think of them as either lazy and incompetent or sneaky: trying fool people into thinking the boat can cruise faster than it really can.

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Brokers, sellers, people in general. I think of it the same way. The builders quote knots and quote range in nautical miles. I just wonder how many are fooled and subsequently disappointed by sellers or brokers quoting mph.

Inland river boater and I use SM/MPH, the day marks are in SM so it easy to figure when we pass one how many miles and how much time we have left to travel. When I call a lock and tell them I'am a couple of mile out and I'll be there in 10 minutes or so they know what I'am talking about. I set all my displays to show SM and radar rings the same.


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I may be a fanatic when it comes to things like windlass/windless (recent thread), and yes, I do own a navy blue double breasted blazer with gold buttons, but unless I'm crossing the ocean or making some other long hike and it matters, even I use miles per hour most of the time out of habit. It's kind of like a meter or a yard. Unless I'm building furniture, the difference doesn't make enough difference to matter 99% of the time. What do I really care about that 0.15 knot/mile per hour conversion difference. And a centimeter is about half an inch, a kilogram is about a couple pounds, a 10K race is about six miles... There's a time and place for precision in life, but messing around on boats -- measuring the moving around anyway -- doesn't make enough of a difference to matter.

Ok, I always thought that a nautical mile was 2000 yards, or 6000 feet vs a statute mile of 5280 feet. Therefore a knot = 1.14 mph. Is that incorrect? Seems to me my old paper charts were marked that way........btw, I use knots/nautical miles except on the ICW.

Weather forecasts are now problematic, as you must listen carefully to the units, if mentioned, to determine whether the weather will be too much for your comfort. I am of course referring to the Canadian weather habit of referring to the winds on the waters of Georgia Strait in kilometers, not knots. It makes a difference of almost 2 to 1. So when my wife is listening to the forecast and repeats to me that the winds are predicted to blow 70, so we shouldn't go out, I need to know if there is a typhoon predicted, or if it just a breezy day. There is really that much difference between the winds of 70 kmh: ferries still run, waves get to 4' with enough fetch, or 70 knots: nothing goes out, damage is done. I know, the last time we had sustained 70 was in 1962, but this is just an illustration.

wyoboater said:

Ok, I always thought that a nautical mile was 2000 yards, or 6000 feet vs a statute mile of 5280 feet. Therefore a knot = 1.14 mph. Is that incorrect? Seems to me my old paper charts were marked that way........btw, I use knots/nautical miles except on the ICW.

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Yes, that is incorrect. Nautical mile is 6076.12 feet. A knot is 1.15078 mph. Rounded down to the nearest thousand you're right though.

koliver said:

Weather forecasts are now problematic, as you must listen carefully to the units... I am of course referring to the Canadian weather habit of referring to the winds on the waters of Georgia Strait in kilometers, not knots. It makes a difference of almost 2 to 1. So when my wife is listening to the forecast and repeats to me that the winds are predicted to blow 70, so we shouldn't go out, I need to know if there is a typhoon predicted, or if it just a breezy day...

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Well of course I'd agree with you there, in some scenarios different units do indeed matter, like that one. But if we get a weather report about wind gusts to 40 mph or 40 knots, either way I'm going to stay tied to the dock.

I have the chart plotter to set to show knots/NM and Coastal Explorer set to show mph/SM. If called I respond with the units that are appropriate. It's also a good training tool for visitors on the boat. Now all I need is a calibrated line with knots in it to measure speed.

It's simple.
Knots on the water and statute miles on land.

When I see mph in boat adds I assume it's nautical mph unless it's an add by a private owner .. then it's questionable.

As an aside Wm Atkin lists mph for his boat designs. I think it's statute mph.

I don't much care what other people do. For me it's knots on the water and statute miles on land. And it's silly to make an issue out of it in the pleasure boating world.

djmarchand said:

Knots is a nautical term and of course it is nautical miles per hour. A nautical mile is exactly (well maybe not quite, since the earth isn't perfectly round) one minute of longitude.

A good but maybe trivial use is measuring vertical distances on Active Captain. You can use the Path feature, but it takes time. I put the cursor on one feature and then note the longitude and scroll up or down to the next. The difference can easily be calculated: degrees * 60 + minutes equals nautical miles.

David

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David, I believe that would be one minute of latitude. Longitude does not work because the circles get smaller as you move north or south of the equator. Concentric circles so to speak. I always go the the latitude scale at the side of the chart to measure distances. I'll bet that you do too.

Baker is correct that a nautical mile is one minute of longitude at the equator. That is the definition of a NM. However, one minute of latitude is not exactly one NM since the diameter of the Earth is slightly smaller through the poles as compared to the equator. For practical purposes, to measure the distance on a paper chart, put your dividers between the two points you want to measure and then compare it to the latitude scale on the edge of the chart. The number of minutes of latitude is the distance in NM.

I am also always disturbed by the use of the improper term “knots per hour”. I am also disturbed that one of the posters used time=miles/mph but didn’t appreciate that time = NM/knots.

How many know the origin of the term “knots”?

Paul

Paul Swanson said:

Baker is correct that a nautical mile is one minute of longitude at the equator. That is the definition of a NM. However, one minute of latitude is not exactly one NM since the diameter of the Earth is slightly smaller through the poles as compared to the equator. For practical purposes, to measure the distance on a paper chart, put your dividers between the two points you want to measure and then compare it to the latitude scale on the edge of the chart. The number of minutes of latitude is the distance in NM.

I am also always disturbed by the use of the improper term “knots per hour”. I am also disturbed that one of the posters used time=miles/mph but didn’t appreciate that time = NM/knots.

How many know the origin of the term “knots”?

Paul

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You, sir, and David are correct. My error. I shall be dusting off my parallel rules and dividers.

Paul Swanson said:

How many know the origin of the term “knots”?

Paul

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Wifey B: I do...I do...

It's also where we get the term "log." They would throw a log over the transom with a rope attached. The rope had knots all along the way, evenly spaced. You let the log pull the rope out for a fixed period of time and then how many knots went through your hand doing it was your speed.

That's all I know about it.

You get an A+! It's also called a taffrail log since that is where is was thrown overboard.

Paul

Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out.[6] Knots placed at a distance of 8 fathoms - 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation.[7] The knot count would be reported and used in the sailing master's dead reckoning

So says Wiki.

Tom.B said:

I use MPH and Statute Miles 100% of the time because Knots and Nautical Miles don't relate the distance to time. If I need to go 60 miles and I go 20 MPH the entire way (-ish), I can very easily figure out the time it will take to make the journey. It just takes a step out of the process of estimating time over distance.

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Knots and nautical miles work the same way as MPH and statute miles. It takes 1 hour to travel 1 nautical mile at 1 knot.

Paul Swanson said:

Baker is correct that a nautical mile is one minute of longitude at the equator. That is the definition of a NM. However, one minute of latitude is not exactly one NM since the diameter of the Earth is slightly smaller through the poles as compared to the equator.

Paul

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360 degrees x 60nm/degree = 21,600nm (24 856.836 077 mile [statute]) is closer to the polar circumference (24,860) than the equatorial circumference (24,902) so even at the equator, using latitude is more accurate.

kthoennes said:

Well of course I'd agree with you there, in some scenarios different units do indeed matter, like that one. But if we get a weather report about wind gusts to 40 mph or 40 knots, either way I'm going to stay tied to the dock.

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For others that don't understand, koliver is not referring to knots per hour vs miles per hour. He's referring to kilometers per hour vs knots per hour and miles per hour. Big difference.

If you're in Canadian waters and listening to certain weather broadcasts, it's imperative to know the conversion factor.

And hence the term, log book or captains log

ulysses said:

So I don't guess anyone is using cables. 1 nm = 10 cables

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Yes I do. Ive always used cable as a description of distance at sea. Its a handy distance. Traditionally it about 100 fathoms or 200 yards but if you want to get more precise its 608 ft for the imperial measurement or 720 ft in the US navy. You'd have to wonder why we could never standardize such a measurement.

Thank god for KPH

gaston said:

Thank god for KPH

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As in Kilometers Per Hour!

Offshore where Lat & Lon are used K makes the most sense.

Inshore where lubber miles are the norm MPH works best.

Selling the boat MPH gives the best numbers for speed and MPG,

although counting in Klicks would give even better numbers !

Gdavid said:

360 degrees x 60nm/degree = 21,600nm (24 856.836 077 mile [statute]) is closer to the polar circumference (24,860) than the equatorial circumference (24,902) so even at the equator, using latitude is more accurate.

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I agree, I have never hear NM related to longitude. It's not how a NM was originally calculated according to these and dozens of other references.

What is the difference between a nautical mile and a knot?

A nautical mile is based on the circumference of the earth, and is equal to one minute of latitude. It is slightly more than a statute (land measured) mile (1 nautical mile = 1.1508 statute miles). Nautical miles are used for charting and navigating.

https://en.wikipedia.org/wiki/Nautical_mile
A nautical mile (symbol M, NM or nmi) is a unit of distance, set by international agreement as being exactly 1,852 meters (about 6,076 feet). Historically, it was defined as the distance spanned by one minute of arc along a meridian of the Earth (north-south), and developed from the sea mile and the related geographical mile.




I also believe the name "taffrail log" didn't become popular till logs that were left overboard for periods of time....previously know by the naes previously described.




http://educators.mysticseaport.org/artifacts/taffrail_log/


This type of log is known as a taffrail log. The taffrail is the aftermost railing around the stern of a ship, to which a log could be mounted with a clamping mechanism (thus resulting in the name taffrail log).


https://en.wikipedia.org/wiki/Chip_log




More modern logs and replacements[edit]

https://en.wikipedia.org/wiki/File:Negus_ship_log.jpg
A patent or taffrail log


Mechanical speed logs called patent logs or taffrail logs,

Donsan said:

As in Kilometers Per Hour!

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Yes Kilometers Per Hour!
Metric makes life so east

Baker said:

So "knots" are not a measure of speed?

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What he means is, a knot is one nautical mile per hr, so if you say knots per hr, it is saying nautical miles per hour per hour...but you knew that, right..?

Coming back to why you like knots etc, it's probably because in aviation you also use those terms, and the raison d'être of the nautical mile is that it is the distance the subtends one minute of longitude at the equator, so has a navigation validity no other measurement has. A lot of folk don't know that. Also, although you do drive to mph in the US, being in the aviation field, you would see more of other countries, and are probably more aware than most USians, that apart from the US, maybe Canada, and the UK, the rest of the world use kph..?

https://en.m.wikipedia.org/wiki/Nautical_mile

PS. Sorry I just repeated a lot that had been said already. I didn't realise so much traffic on this had occurred while we southern hemispherians were kipping...

Knots were used by navigators with a "chip log".

A triangular piece of wood was dropped overboard , and a sand glass was started.

When the sand ran out the number of knots that had gone overboard was logged as boat speed.

The line slipped out of one corner of the chip log so hauling aboard was eased.

If you want to make one the Navy Chip Log Pattern has good info.

Chip log - Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Chip_log


Wikipedia


A chip log, also called common log, ship log, or just log, is a navigation tool ... All nautical instruments that measure the speed of a ship through water are known ...



Chip Log pattern - Navy & Marine Living History Association

www.navyandmarine.org/planspatterns/speedlog.htm




The Speed Log --History, Construction and Use. by Allen Mordica, TMLHA. Introduction- One of the three main aspects of navigation is dead reckoning. Simply ...



How do sea navigators measure their ships speed?

www.physlink.com/Education/AskExperts/ae400.cfm




Chip Log Rope had knots tied at equal distances along the reel. Sailors would throw the wood ... In fact that is the origin of the nautical speed unit: the knot.



Chip Log Explained - An Ancient Navigation System

www.brighthubengineering.com › Marine Engineering › Seafaring




Dec 31, 2009 - Chip log is a navigational tool that was used during the ancient time to ... Marine Navigation: Understanding basic maritime terminology · Ship ...