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Knot vs MPH
In another posts someone asked questions concerning nautical terms. I don't
know if anyone has had this problem, but when I first started boating I noticed that NOAA weather charts used knots for wind speed instead of mph. Since I was not familiar with knots (and actually asked someone what a knot per hour was), I wanted to know how windy "X knots" really was. I found out that 1 knot = 1.15 mph, so 10 knots is equal to 11.5 mph. Since I can not tell the difference between 10 mph or 11.5 mph or even 20 mph and 23 mph (I will never intentionally go out in anything over 20 knots) in my mind a knot and a mph were the exact same thing. The question I have is, why does NOAA use knots on their weather charts instead using mph? Is there a historical reason for this (i.e. that is the way it has always been done) or a scientific reason for using knots? For some reason I suspect Gould will field this question. |
The real ME writes:
The question I have is, why does NOAA use knots on their weather charts instead using mph? Is there a historical reason for this (i.e. that is the way it has always been done) or a scientific reason for using knots? 1 knot is one minute of latitude per hour, which simplifies navigational estimates. Under sail, one estimates potential speeds from wind speed, and thus the knots of wind translate with simple calculation to chart divisions you can potentially cross with your boat. |
The nautical mile is the standard unit of measurement
on a chart. Every minute of latitude equals a nautical mile- (not longitude, for reasons that are apparent when considering a globe). Of course we are all aware that a knot equals 1nm/hr. Sailing vessels and aircraft measure speed in knots rather than mph. If you were sailing or flying and you wanted to consider the effect of a certain wind on your course or speed, it would be needlessly awkward to convert between mph and knots. Vessel speeds are more commonly measured in mph on inland lakes or other waters which may be uncharted. Certain motorboats frequently measure speed in MPH, offshore racers and hydroplanes being common examples. Take my trawler, for example. I always consider my speed as a certain number of knots. Usually somewhere in the 8-9 knot range. It would be silly to talk about a trawler doing "10 miles an hour", especially since among other inconveneinces one would have to switch to the statute mile scale on a chart to time,speed, distance calculations. What a pain. On the other hand, a boater with a speedy planing hull doing 35 mph or about 30 kt, can use mph or knots almost interchangeably and both would be considered acceptable for purposes of general discussion. |
It makes so much sense now that someone explained it to me.
Why doesn't everyone just use GPS . ; ) wrote in message oups.com... The nautical mile is the standard unit of measurement on a chart. Every minute of latitude equals a nautical mile- (not longitude, for reasons that are apparent when considering a globe). Of course we are all aware that a knot equals 1nm/hr. Sailing vessels and aircraft measure speed in knots rather than mph. If you were sailing or flying and you wanted to consider the effect of a certain wind on your course or speed, it would be needlessly awkward to convert between mph and knots. Vessel speeds are more commonly measured in mph on inland lakes or other waters which may be uncharted. Certain motorboats frequently measure speed in MPH, offshore racers and hydroplanes being common examples. Take my trawler, for example. I always consider my speed as a certain number of knots. Usually somewhere in the 8-9 knot range. It would be silly to talk about a trawler doing "10 miles an hour", especially since among other inconveneinces one would have to switch to the statute mile scale on a chart to time,speed, distance calculations. What a pain. On the other hand, a boater with a speedy planing hull doing 35 mph or about 30 kt, can use mph or knots almost interchangeably and both would be considered acceptable for purposes of general discussion. |
On Wed, 13 Apr 2005 16:20:46 -0400, "The real ME"
wrote: ~~ snippage ~~ Why doesn't everyone just use GPS . ; ) Almost everybody does and they still manage to screw it up. :) Later, Tom |
The nautical mile is the standard unit of measurement on a chart. Every
minute of latitude equals a nautical mile- (not longitude, for reasons that are apparent when considering a globe). Of course we are all aware that a knot equals 1nm/hr. -================================= Not necessarilly. Great Lake charts are in statute miles and use Poly Conic projection, rather than the Mercator that the off shore charts use. |
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Not necessarilly. Great Lake charts are in statute miles and use Poly
Conic projection, rather than the Mercator that the off shore charts use. Reply ******************** An exception somewhat proving the rule. I'm not familiar with charts for the Great Lakes, of course, but it's surprising they wouldn't follow the convention of the majority of the world. Do Great Lakes sailors discuss vessel speed or wind speeds in knots or MPH? |
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Not necessarilly. Great Lake charts are in statute miles and use Poly
Conic projection, rather than the Mercator that the off shore charts use. Reply ******************** An exception somewhat proving the rule. I'm not familiar with charts for the Great Lakes, of course, but it's surprising they wouldn't follow the convention of the majority of the world. Do Great Lakes sailors discuss vessel speed or wind speeds in knots or MPH? ================================== I'm told that the Lakes use statute miles because of the land mass involved in the charts. However, had the St. Lawrence Seaway came into being sooner, bringing "salties" to the Lakes, the chartmakers might have decided to use the "off-shore" type of chart and nautical miles to avoid confusion. As far as speed terminology, you hear both. |
On Fri, 15 Apr 2005 10:33:48 -0400, Black Dog
wrote: and occasionally some drunken lubber will throw in KPH too ========================== Personally I've always had a preference for furlongs per fortnignt. |
On Fri, 15 Apr 2005 15:05:11 -0400, Wayne.B
wrote: On Fri, 15 Apr 2005 10:33:48 -0400, Black Dog wrote: and occasionally some drunken lubber will throw in KPH too ========================== Personally I've always had a preference for furlongs per fortnignt. I always thought cubits per parsec was a good method. Later, Tom |
On Fri, 15 Apr 2005 21:21:22 GMT, Gene Kearns
wrote: On Fri, 15 Apr 2005 20:15:02 GMT, Short Wave Sportfishing wrote: On Fri, 15 Apr 2005 15:05:11 -0400, Wayne.B wrote: On Fri, 15 Apr 2005 10:33:48 -0400, Black Dog wrote: and occasionally some drunken lubber will throw in KPH too ========================== Personally I've always had a preference for furlongs per fortnignt. I always thought cubits per parsec was a good method. Let it go, guys.... this is getting too close to how long you taught your wife a foot was... Um... ¿Que? |
On Fri, 15 Apr 2005 19:12:36 GMT, Red Cloud©
wrote: The windspeed during the storm was increasing by roughly 3 KPH! ============================================ In Europe it is very common to measure wind speed in meters per second. There is a handy conversion however taught to me by a Swede who used to crew on my former racing boat. If you double meters per second, it is almost exactly equal to knots. Try it, it works. |
On Wed, 13 Apr 2005 13:51:46 -0400, "The real ME"
wrote: In another posts someone asked questions concerning nautical terms. A knot is what you put in a line (not a rope), a MPH is a speed measurment. |
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My only point was that there IS a correct way to use the term "KPH",
although it is frequently used incorrectly. "Knots" includes an implied "per hour", so the only time you would add "Per Hour" after knots, (KPH) would be to describe by how much speed something (wind, boat, dolphins) was increasing or decreasing on average referenced to an elapsed hour. rusty redcloud *********** "Knots per hour" is an incorrect usage. Since a 1 knot speed is equal to a nautical mile per hour, some people might wonder why they are spelled differently. "Knot" isn't a misspelling of "naut". Using knots to measure speed dates back to the days of sail. A midshipman or the quartermaster would be required to measure the speed of a vessel at regular intervals, make a note of it in a log, and make report to the master and/or navigator. To measure speed, a wooden, wedge-shaped board (speed log) was tossed overboard. A spooled line would be attached to the board, and the line would be allowed to pay out as the board was left in the vessel's wake. The line payed out for a short but precisely measurable period of time. (A short interval sand glass would have been used) A series of knots were tied in the line at known intervals, and the seaman measuring the speed would count the number of knots that slipped through his fingers during the measuring period. Speed would be reported as "eight knots" if eight knots slipped though the fingers of the party doing the measuring. The math is simple, but ingenious. The time interval will be a specific portion of an hour. Say for example the measuring exercise took place for 30 seconds after the "log" splashed down- that would be 1/120th of an hour. If the knots were tied in the line 1/120th of a nautical mile (about 47 and 1/4 feet) apart then at 1 nautical mile per exactly 1 "knot" would pay out during the measuring. If 6 X approx 47 1/4 feet payed out in 30 seconds, then the vessel would cover six nautical miles in an hour, or be making a speed of six knots. |
A boat was traveling at 5 knots. It speeds up gradually at a rate of 2
Knots per Hour. ******************** A vessel's speed may be recorded at specific periods of time, but nobody familiar with the movement of a boat that is effected by winds, currents, etc would presume that the change had been, or even likely would be, evenly graduated over a period of time. If you boat is making 5 knots at 0500 and 11 knots at 0800, that doesn't mean that it sped up at a rate of 2 knots per hour. It is entirely possible that the boat didn't speed up until 0759, when the wind shifted. |
Here's a new term to keep you confused: AVERAGE
sheesh! rusty redcloud ********* Thank you. Here's one for you, as well. NAVIGATION There is no navigational use for determining the average hourly change in knots during a previous period of time. It cannot be used to forecast future changes, will almost certainly be non-linear, and could lead to some wildly erroneous conclusions about position. It is common to project present speed, particularly for short intervals, to predict future position. How fast you were running yesterday, or even several hours ago becomes meaningless. The average rate of hourly change in knots made would be useful for evaluating tits on a boar. |
BTW - If you are going to pretend to be some sort of "writer", you
really should learn the difference between "effect" and "affect". rusty redcloud *********** I don't pretend, or claim, to be anything. But one sure sign of a losing argument is the switch of focus from substance to spelling and syntax. If you're going to pretend to discuss speed within the context of navigation, you would do well to avoid an attempt to prop up an incorrect usage with the invention of a new concept. |
I thought it was bring in Hitler and Nazi's into the discussion was a sure
sign you were losing an argument. wrote in message oups.com... BTW - If you are going to pretend to be some sort of "writer", you really should learn the difference between "effect" and "affect". rusty redcloud *********** I don't pretend, or claim, to be anything. But one sure sign of a losing argument is the switch of focus from substance to spelling and syntax. If you're going to pretend to discuss speed within the context of navigation, you would do well to avoid an attempt to prop up an incorrect usage with the invention of a new concept. |
BTW - If you are going to pretend to be some sort of "writer", you
really should learn the difference between "effect" and "affect". rusty redcloud *********** I don't pretend, or claim, to be anything. But one sure sign of a losing argument is the switch of focus from substance to spelling and syntax. If you're going to pretend to discuss speed within the context of navigation, you would do well to avoid an attempt to prop up an incorrect usage with the invention of a new concept. ??? You are even more lost and confused than previous estimates indicated. rusty redcloud *************** In addition to nit picking spelling and grammar, another sure sign that a poster's argument is full of crap is the introduction of personal insult. Thanks for the classic example, twice in a row. |
On Tue, 19 Apr 2005 11:06:18 GMT, Red Cloud®
wrote: You were wrong from the start, and you remain wrong. There is in fact a proper usage of the term KPH, and I illustrated it clearly with several examples. =================================== You are as full of misdirected certitude as you are of biological waste products. Now go away. |
On Tue, 19 Apr 2005 14:02:13 GMT, Red Cloud©
wrote: your arguments are invalid, and classical examples of someone who cannot win an argument with facts. ===================================== See previous post on this topic... |
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