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?

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.

"Am I overreacting?"


Probably not, but I do use statute miles and MPH when in the ICW. Just makes more sense.

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.

Click to expand...

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.

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.

Click to expand...

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).

Richard

Baker said:

Knots implies "nautical miles per hour". So you could just as easily use knots for that reason.

Click to expand...

Good point. I never really put those together

Baker said:

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.

Click to expand...

TBH... Maybe that has more to do with it than anything.

Baker said:

Am I overreacting?

So what do you use and why?

Click to expand...

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

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.

danderer said:

What bugs me are self-proclaimed knowledgeable folks who talk about abouts "knots per hour" as a speed measure. Nope.

Click to expand...

So "knots" are not a measure of speed?

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.

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.

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.

So "knots" are not a measure of speed

Click to expand...

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

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

jleonard said:

Yes. A knot is a nautical mile per hour. So to say "knots per hour" is improper

Click to expand...

Ok...thanks for the clarification. Wasn't sure what he was getting at.

djmarchand said:

A nautical mile is exactly (well maybe not quite, since the earth isn't perfectly round) one minute of longitude.

Click to expand...

Isn't a nautical mile "one minute of longitude"....at the equator???

If you use a paper chart, then why wouldn't you use knots.

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.

Baker said:

Isn't a nautical mile "one minute of longitude"....at the equator???

Click to expand...

Yes - also a minute of latitude at any location.

I use knots since I started with paper charts.

Richard

Rebel112r said:

If you use a paper chart, then why wouldn't you use knots.

Click to expand...

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.

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

I use knots and nautical miles. It's just more nautical.

Baker said:

So "knots" are not a measure of speed?

Click to expand...

Sorry for being a bit flippant in my wording. Here is the longer version:

In common usage (Speed Limit, Maximum Speed, etc), "speed" equates with "velocity". Some would argue more strictly that speed is a scalar while velocity is a vector but I don't think that matters here.

Velocity is the first derivative of position and time--the change in position divided by the change in time. Distance/time (eg. miles/hour). A knot is a measure of velocity: 1 nautical mile/hour.

Acceleration is the second derivative of position and time--the change in velocity (first derivative) divided by the change in time. Distance/time/time (eg. miles/hour/hour).

So when someone says "knots per hour" they're talking about an acceleration not a velocity. If they say "we were going 20 knots per hour" that says nothing about their speed (velocity) but rather that they were accelerating. What they likely meant to say was "we were going 20 nautical miles per hour" or "we were doing 20 knots".

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

Click to expand...

You mean Latitude, right? Latitude lines are evenly spaced (within the constraints you noted) 60nm apart. Spacing between longitude lines gets smaller as you approach the poles. Or am I totally mixed up here?

1 min. of longitude at the equator as noted in several posts above or 1 min of latitude anywhere.

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.

Click to expand...

If I need to go 60 miles and I go 20 knots the entire way (-ish), I can very easily figure out the time it will take to make the journey. I don't have any additional step.

Knots are to do with boats and airplanes and MPH and KPH is to do with cars and nautical appliances, like Sea Rays. 8^)

We use knots always and nautical miles always, even when on the GICW. They are just our standard unit of measure. You give me statute and I'm going to immediately convert it to nautical in my mind. I've just conditioned myself that way.

Now, on the lake we used mph and statute miles as that was the standard there.

I admit that knots and nm can become a bit confusing when talking to a non-boater. Still it's our standard, the way we plan, the way all our equipment is set. At this point when it comes to boating, knots and nm is our first language and mph and statute miles requires us to translate.

When I'm referring to distance, I use whatever measurement the applicable charts use. When in the ICW, I use statute miles.

When I'm referring to speed, I always use knots.

BandB: OK how long to make the journey ? How long if they were 60 nautical miles and not statue miles ?

ulysses said:

BandB: OK how long to make the journey ? How long if they were 60 nautical miles and not statue miles ?

Click to expand...

In my thought they were nm so 3 hours. If they were statute, I'd convert to nautical, round to 52 in my head, and it would take me 2.6 hours. Oh, typically I will leave that at 2.6 hours and not translate it to 2 hours 36 minutes.

Oh and my trick for conversion rather than multiplying by 8.689876. I deduct 10% and then I deduct 1/3 of that amount again. So 60 statute miles I subtract 6 and then subtract 2 and I'm at 52 nm. Close enough for that purpose.

I just think knots and nm on the water and assume most I'm talking to do, which may be a bad assumption. I do make the point to always note. I was copying in the above statement but I try to never say miles, always nm. To me, I think of the word miles as defaulting to statute.