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.

On Thu, 14 Apr 2005 19:08:32 -0400, (N.L. Eckert)
wrote:

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.

Learn something new everyday.

Later,

Tom

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?

On 14 Apr 2005 17:50:34 -0700, wrote:

Do Great Lakes sailors discuss vessel speed or wind speeds in knots or
MPH?

=====================================

It depends on whether or not they are writing power boat reviews for a
magazine. Any marketing guy worth his salt will always quote speed in
mph because: 1) It sounds faster; and 2) Probably doesn't know any
better. :-)

My experience on the Great Lakes is that sailors use knots but smaller
power boats use mph.