Sailing has it’s own terminology, including knots, keels, fore and aft. Sooner or later, new sailors ask want to know how fast a knot is. The answer is not very fast. I know because I looked it up. Here is what I found:
A moderate sailboat speed is 3-4 knots. A knot is about 1.15 mph (1.85km/h). Three knots is equivalent to 3.4523383441 mpg (5.556 mk/h), or about the same speed as an adult walking quickly, so not very fast. Faster speeds are possible.
In this article I will cover knots and comparisons to other measurements. Then I will cover some other questions. How this measurement came about is rather interesting.
What is a Nautical Knot?
A “knot” is a measure of speed and is defined as one nautical mile per hour. A nautical mile is defined by multiplying the number of degrees in a circle, 360 degrees, times 60 minutes, to arrive at 21,600 units, which (at the equator) are 6080 feet in length.
What is a Nautical Mile?
A Nautical Mile is defined as exactly 1852 metres (6076 ft; 1.151 mi). It used to be defined as one minute (1/60 of a degree) of latitude. A Nautical Mile is also used in air, marine, and space navigation, and for the definition of territorial waters.
The earth year is made up of approximately 365 days. The similarity to a circle with 360 degrees is not likely a coincidence. However, the exact details are apparently lost to recorded history, at least for now.
360 Degree Circle
The concept of a circle having 360 degrees dates at least from the Sumerians who created a calendar with 360 days in the year. The Sumerians used a base-60 numbering system. They divided each degree into 60 units. This is that we still use today as minutes of arc.
The Gregorian calendar is a circle of 360 divided by 12 months. The zodiac is also a circle of 360 degrees, divided by 12 signs. Most other (Jewish, Muslim, Bahá’í) calendars are based on a 360 degree circle, with adjustments for the extra days in the year.
Some time after that, the Greek scholar Eratosthenes (276 BC – 195 BC) accurately determined the circumference of the earth.
60 Degrees
If you multiply 360 by 60 you will arrive at 21,600 units. This number is a convenient measure of the earth’s circumference. This makes for easy math with no fractions or rounding errors. This is also very useful for a number of reasons, not just convenience. Early navigation was difficult enough without fractions.
One Nautical Mile
One 60th of a degree, or one minute of arc, equals one nautical mile. This is equivalent (at the equator) to being 6080 feet. This is where the expression “a minute’s a mile” comes from.
Some have said the definition of a nautical mile is 6080 feet. While this is true, it’s not accurate. This a result of the calculation, not the cause of it. All of this was 2200 years before GPS and “Where in the world is Marco Polo.”
How Fast Can You Sail a Sailboat?
Most sailboats cruise at a speed of 4-6 knots or 4.5-7 mph (6.4-11.26 km/h), with a top speed of 7 knots or 8 mph (13 km/h). Larger racing yachts can easily reach speeds up to 15 knots or 17 mph (28 km/h), with an average cruising speed between 6-8 knots or 7-9 mph (11.26-14.48). Cruising speeds of over 8 knots are not common.
These speeds are not consistent. Wind speeds change intensity and direction frequently. Predicting how long it will take to get to a destination is highly dependent upon multiple factors beyond your control.
How Many Nautical Miles Can You Sail in a Day?
In ideal conditions, a typical cruising yacht can manage between 150–200 nautical miles in 24 hours. Powered boats can achieve 500 Nautical Miles in a day. A multi-hull racing yacht set the 24-hour world record in 2009 at 908 nautical miles.
"The top speed is usually a function of the "hull speed"
Sailboats (4-6 knots)
Assuming you have consistent wind, a cruising sailboat’s speed is limited by the “hull speed”. The hull speed is a calculation equal to 1.34 times the square root of the waterline length.
To better understand this, let’s take an example. My 40′ sailboat has a waterline length of 32.5′ yielding a hull speed of 7.6 knots or about 180 nautical miles per day. Obviously winds and/or currents will contribute to the actual results. Further, not everyone sails at night.
Catamarans (9-10 knots)
The narrow-hulled catamarans and racing catamarans can have a slightly larger factor. Maximum hull speed is approximately 1.3 times the square root of the waterline length. Some yachts can spend some of the time semi-planing. As a result, their hull speed is faster.
Conclusions
In conclusion, the knot makes perfect sense. Being a land based person for much of my life, I had often wondered about knots and Nautical Miles, and how they came to be. Now I know.
Nautical charts, even the digital ones, have a grid of lines that mark the longitude and latitude lines. A Nautical Mile has a direct relationship to these lines. A knot is the measure of speed based on the Nautical Mile.
Sources:
https://www.britannica.com/science/mile
https://en.wikipedia.org/wiki/Mile
https://en.wikipedia.org/wiki/Nautical_mile
https://www.unitconverters.net/speed/knot-to-mph.htm
https://www.checkyourmath.com/convert/speed/per_hour/mph_km_per_hour.php
https://oceanservice.noaa.gov/education/tutorial_currents/06measure2.html