Trojan Boat Forum

Knots vs. MPH

I think most of us know that 15 knots is faster then 15 mph, but may not know how much faster. You will see from the chart below that 15 knots is about 17 1/4 mph. You will also see that 20 knots is about 23 mph. I know you won’t remember every speed, but if you remember a few key speeds you can tell your guests those speeds and they will a good idea of what your speed is, and what a knot is. A knot is 1 nautical mile per hour.

7 knots is about 8 mph
13 knots is about 15 mph
20 knots is 23 mph

The difference between statue mile and a nautical mile is the distance of the mile. A statue mile is 5280 feet. A nautical mile is 6076.1 feet.

Knots X 6076.1/5280 = Miles per Hour ---- Miles per hour X 5280/6076.1 = Knots

Knots - Miles/Hour Conversion Chart

1 = 1.15 ** 6 = 6.91 ** 11 = 12.67 ** 16=18.42 ** 21 = 24.18
2 = 2.30 ** 7 = 8.06 ** 12 = 13.64 ** 17 = 19.58 ** 22 = 25.33
3 = 3.45 ** 8 = 9.21 ** 13 = 14.97 ** 18 = 20.73 ** 23 = 26.49
4 = 4.61 ** 9 = 10.36 ** 14 = 16.12 ** 19 = 21.88 ** 24 = 27.64
5 = 5.76 ** 10 = 11.52** 15 = 17.27 ** 20 = 23.03 ** 25 = 28.79

I always used the multiplication of 1.15 Kts. to a mile...so I looked it up again.

A nautical mile is 6076 feet and a statute mile is 5280 feet. Dividing 6076 by 5280 gives us 1.1507575 or...about 1.15.

Y'all are making my head hurt with all these numbers. I just set my GPS to read MPH and it fits in with speeds on cars, trucks, etc...something I can relate to.

I have been using the "15% rule" for years. Made feel a bit better when my old boat - a 26' Four Winns Liberator with twin 350 Magnums (overall weight about 6000 pounds, 1/3 of that was engines!!!) topped out at only 58 knots. But thanks to the 15% rule, I could tell people it did more than 65 miles an hour ! (just had to get past that magic number of 60!)

I'm sorry for the misinformation.
I was wrong when I stated that a nautical mile was 6000 feet.

A nautical mile is 6076 feet and a statute mile is 5280 feet. Dividing 6076 by 5280 gives us 1.1507575 or...about 1.15.

I also looked up whet the distance of a nautical mile was and found that it was slightly different then 6076 feet. The Werster's Dictionary defined it as 6080.20 feet. Not much of a difference and the 1.15 should be used.

I've made corrections to my earlier post.

Gerry, it's all good information, thanks for taking the time to post it!

BTW: I want to pass along a book for those who may be interested.

This book puts navigation into simple and easy to understand reading. I have had it for years and keep it as a reference. Great book! next time you at the book store, look it up and enjoy!

http://www.amazon.com/Boat-Navigation-R ... 865&sr=1-1

silly me... and all this time I thought a nautical mile is the distance between each minute of degrees latitude, [or longitude as measured at the equator.] The way I do the conversion, is to set the units on my GPS's SOG [speed over ground] to knots. [but then I never did like math]
When I first started boating on the Great Lakes, I used knots, but I think I was the only one at our marina, or certainly our side of Lake Erie, so I give up.

I checked Chapman, the boaters bible, and found that they agrees with Stripermann2 and defines a nautical mile as 6076.1 feet. Chapman states that "a nautical mile is essentially equal to one minute of latitude."

From Wikipedia, the free encyclopedia:

The historical definition differs from the length-based standard in that a minute of arc, and hence a nautical mile, is not a constant length at the surface of the Earth but gradually lengthens with increasing distance from the equator.

Prior to the international agreement the U.S. defined the nautical mile to be 6080.2 feet. The British definition of the nautical mile originally related to the length on the surface of the Earth just south of Great Britain to be 6080 feet. The U.S. navy used 6000 feet, as a simpler approximation, for a nautical mile when they designed radar systems for ballistic and cruise missiles in the 1950s.

This definition was agreed to by international agreement.

1 nautical mile = SI units
1.8520 km 1,852.0 m
US customary / Imperial units
1.1508 mi 6,076.1 ft

A nautical mile or sea mile is a unit of length. It corresponds approximately to one minute of latitude along any meridian.

Definition
The international standard definition is: 1 nautical mile = 1,852 metres exactly.

History

International agreement was achieved in 1929 when the International Extraordinary Hydrographic Conference held in Monaco adopted a definition of one international nautical mile as being equal to 1,852 metres exactly, in excellent agreement (for an integer) with both the above-mentioned values of 1,851.851 historical metres and 1,852.216 standard metres.

Since the 1929 agreement, all nations have now adopted the international definition. In the United States, which formerly used a value of 1,853.248 m (6,080.2 ft), the new definition has been in use officially since July 1, 1954.

Webster's dictionary defines a nautical mile to be 6080.2 feet. They need to up date their diffinition and define it to be the present standard of 6076.1 feet

Peter Wrote:

When I first started boating on the Great Lakes, I used knots, but I think I was the only one at our marina, or certainly our side of Lake Erie, so I give up.

I also flip flopped between the two different miles. While checking out exactly what a knot was, I found this.

According to Chapman, "The statute mile is used on inland bodies of water such as the Mississippi River and its tributaries, the Great Lakes, etc. (also the Atlantic and Gulf Intercostal Waterways since 1969)."

That's good enough for me. All my settings will be changed to statute miles. It will certainly be easier for me to relate to.

yikes...I stand corrected...