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Old 02-08-2016, 02:30 PM   #1
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Knots vs MPH

A buddy of mine and I have this discussion all the time. In fact it has got to a point where we don't even discuss it anymore. He is not going to change the way I think and I am not going to change the way he thinks. His reasoning is that we boat mostly on the GICW. The mile markers on the GICW are in statute miles with a reference to the Harvey Locks in New Orleans. It is hard to argue his point there. But I still reference knots and nautical miles regardless. The mile markers are every 5 miles and whether you are ref nautical or statute does not really matter in such a small increment.

I have to admit, whenever I see or hear someone refer to MPH when talking about boats, I cringe. If I see a listing on Yachtworld and the broker lists MPH in cruise speed/max speed, it cheapens the listing in my eyes. Am I overreacting?

So what do you use and why?
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Old 02-08-2016, 02:37 PM   #2
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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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Old 02-08-2016, 02:41 PM   #3
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"Am I overreacting?"


Probably not, but I do use statute miles and MPH when in the ICW. Just makes more sense.
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Old 02-08-2016, 02:46 PM   #4
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Quote:
Originally Posted by Tom.B View Post
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.
Knots implies "nautical miles per hour". So you could just as easily use knots for that reason.

MPH is something we use in our cars all of our lives. It is familiar. Which is probably why boaters are likely drawn to it.
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Old 02-08-2016, 02:49 PM   #5
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Quote:
Originally Posted by Tom.B View Post
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.
Not sure I understand. Knots and nautical miles do relate distance to time in just the same way as statue miles and mph. If you had to go 60 nm and avearged 20kt you have the same simple process (60/20 = 3 hours).

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Old 02-08-2016, 02:50 PM   #6
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Quote:
Originally Posted by Baker View Post
Knots implies "nautical miles per hour". So you could just as easily use knots for that reason.
Good point. I never really put those together

Quote:
Originally Posted by Baker View Post
MPH is something we use in our cars all of our lives. It is familiar. Which is probably why boaters are likely drawn to it.
TBH... Maybe that has more to do with it than anything.
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Old 02-08-2016, 02:50 PM   #7
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Am I overreacting?

So what do you use and why?
Overreacting - possibly!

That's why all chart plotters give you the option - different strokes...

I use statute almost exclusively - inland lakes, and NY / Canadian canals & Great Lakes lend themselves to statute mi & MPH.

I'd never try to convince anyone either way and when I figure waypoints & routes I record both in my log
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Old 02-08-2016, 03:00 PM   #8
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Every place I go uses nautical miles and since that is what I've always used (both in boating and flying) it seems a lot more natural than statute. Now if I could just find a car that used knots...

What bugs me are self-proclaimed knowledgeable folks who talk about about "knots per hour" as a speed measure. Nope.
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Old 02-08-2016, 03:27 PM   #9
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What bugs me are self-proclaimed knowledgeable folks who talk about abouts "knots per hour" as a speed measure. Nope.
So "knots" are not a measure of speed?
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Old 02-08-2016, 03:33 PM   #10
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When sailing the Caribbean we used nautical miles. When we transitioned to the trawler we continued using nautical miles and would swag to figure time to travel a distance marked on the chart in statute miles. I soon became apparent that it was silly not to use the info readily available and changed to statute miles. An added benefit is the psychological boost when looking at a larger number for our boat speed.
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Old 02-08-2016, 03:34 PM   #11
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I'm like you, Baker....knots and NM all the way. I started in aviation with mph, then they all went knots in the 70s and I never looked back. When I segwayed into boating, nautical measurements seemed natural. But then, we don't have statute mile markers out here, either.

On a long day of boating, a 15% difference in calculated boat speed can make a big difference in time, but the current, lock times and speed-restricted zone variables could have a much larger impact.

For the newbies to nautical measurements, a simple rule of thumb to convert in your head if using nautical speed on a statue mile measured course, add 3 miles for every 20 SM segment, then divide by your kts.
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Old 02-08-2016, 03:35 PM   #12
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I normally use knots and nautical miles as my "normal" cruising grounds are LI Sound and that's what the charts use.
However, when I cruised in the Erie system I used statute miles, that's what the charts used and when I got to Canada I used km for that same reason.

Quote:
So "knots" are not a measure of speed
Yes. A knot is a nautical mile per hour. So to say "knots per hour" is improper.


(A nautical mile is 6076 feet, a statute mile is 5280 feet)
statute miles x .87 = nautical miles
nautical miles x 1.15 = statute miles
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Old 02-08-2016, 03:39 PM   #13
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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.

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Old 02-08-2016, 03:40 PM   #14
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Yes. A knot is a nautical mile per hour. So to say "knots per hour" is improper
Ok...thanks for the clarification. Wasn't sure what he was getting at.
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Old 02-08-2016, 03:43 PM   #15
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A nautical mile is exactly (well maybe not quite, since the earth isn't perfectly round) one minute of longitude.
Isn't a nautical mile "one minute of longitude"....at the equator???
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Old 02-08-2016, 03:46 PM   #16
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If you use a paper chart, then why wouldn't you use knots.
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Old 02-08-2016, 04:00 PM   #17
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I am also normally on LI sound and usually use mph and statue miles but when heading up the Hudson or Ct river I switch over to knots and nautical miles per hour.
I see little difference between the two other than I want to match the markers and charts that are most available.
Now when I am communicating with some boaters in Europe or the PI islands they sometimes send data in KPH and that can get me really off base until I translate it to something I feel comfortable with.
Any measure that you are comfortable with will work equally well.
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Old 02-08-2016, 04:28 PM   #18
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Isn't a nautical mile "one minute of longitude"....at the equator???
Yes - also a minute of latitude at any location.

I use knots since I started with paper charts.

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Old 02-08-2016, 04:32 PM   #19
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If you use a paper chart, then why wouldn't you use knots.
On ICW charts it's easier to just use the mile marks.

Latitude is used as the measuring side of the chart.

I flip flop all the time as for me Jersey to Norfolk is all NM, Norfolk to FL is all SM unless I run outside which so far has been rare.

I went back to using my aviation circular slide rule because I can make the conversion and speed/distance/time calculation in about 1/5 the time of punching it into a calculator.
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Old 02-08-2016, 05:10 PM   #20
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So I don't guess anyone is using cables. 1 nm = 10 cables
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