Knot (kn) - Unit Information & Conversion

Quick Answer

What is a Knot? A knot is a unit of speed equal to one nautical mile per hour.

Quick Conversions:

• 1 knot = 1.15 mph (miles per hour)
• 1 knot = 1.852 km/h (kilometers per hour)
• 1 knot = 0.514 m/s (meters per second)

Who Uses It?

• Sailors (all ships worldwide)
• Pilots (all aircraft worldwide)
• Meteorologists (wind speed, hurricanes)

Quick Comparison Table

Knots	mph	km/h	Common Example
1 kn	1.15 mph	1.85 km/h	Slow river current
5 kn	5.75 mph	9.26 km/h	Rowing speed
10 kn	11.5 mph	18.5 km/h	Sailboat cruising
20 kn	23 mph	37 km/h	Cargo ship
30 kn	34.5 mph	55.6 km/h	Fast ferry
64 kn	74 mph	119 km/h	Hurricane threshold
120 kn	138 mph	222 km/h	Small plane cruising
500 kn	575 mph	926 km/h	Jet airliner cruising
1,000 kn	1,150 mph	1,852 km/h	Supersonic jet (Mach 1.5)

Definition

The Mathematical Definition

1 Knot = 1 Nautical Mile per Hour

In SI Units: $$ 1 \text{ knot} = 1.852 \frac{\text{km}}{\text{h}} = 0.514444 \frac{\text{m}}{\text{s}} $$

In Imperial Units: $$ 1 \text{ knot} = 1.15078 \frac{\text{miles}}{\text{hour}} = 1.68781 \frac{\text{feet}}{\text{second}} $$

Why the Nautical Mile?

The nautical mile is not arbitrary—it's based on the Earth's geometry.

Definition: One nautical mile = one minute of latitude along a meridian.

The Math:

• Earth's circumference ≈ 40,075 km (at equator).
• 360 degrees × 60 minutes/degree = 21,600 minutes around the Earth.
• 40,075 km ÷ 21,600 = 1.855 km per minute of latitude.
• Standardized to exactly 1.852 km (1,852 meters).

Why This Matters: If you're at 40°N latitude and sail due north at 60 knots for 1 hour, you'll be at 41°N latitude. The math is perfect for navigation.

Knot vs. Statute Mile

Unit	Length	Use
Nautical Mile	6,076 feet (1,852 m)	Maritime, aviation navigation
Statute Mile	5,280 feet (1,609 m)	Land travel (cars, roads)
Difference	796 feet longer	Nautical mile is 15% longer

History: From Rope Knots to GPS

Ancient Navigation (Before 1500s)

Before the knot, sailors had no reliable way to measure speed. They used:

• Dead Reckoning: Estimating speed by watching foam, debris, or seaweed pass the ship.
• Guesswork: Experienced sailors "felt" the speed.

This led to massive navigation errors. Ships would miss islands, run aground, or get hopelessly lost.

The Chip Log Invention (1600s)

The chip log (or common log) revolutionized navigation.

Components:

1. The Chip: A triangular wooden board weighted to float upright.
2. The Log Line: A rope with knots tied at intervals of 47 feet 3 inches (14.4 meters).
3. The Sandglass: A 28-second or 30-second timer.

The Process:

1. Sailor throws the chip overboard from the stern.
2. The chip stays relatively stationary in the water (drag keeps it in place).
3. As the ship sails away, the log line unspools.
4. Another sailor flips the sandglass.
5. A third sailor counts the knots passing through his hands.
6. When the sand runs out, they note the count: "7 knots!"

The Math: The knot spacing (47 ft 3 in) and timing (28-30 sec) were calibrated so that:

• 1 knot on the line = 1 nautical mile per hour of ship speed.

Example:

• If 7 knots passed in 30 seconds, the ship was traveling at 7 knots (7 nautical miles per hour).

Why "47 Feet 3 Inches"?

This seems random, but it's brilliant math:

• 1 nautical mile = 6,076 feet.
• 1 hour = 3,600 seconds.
• 30 seconds = 1/120 of an hour.
• 6,076 ÷ 120 = 50.63 feet.

Early sailors used 47 feet 3 inches (close enough) because it was easier to measure with the tools available.

Modern Standardization (1929)

The International Hydrographic Bureau standardized the nautical mile to exactly 1,852 meters in 1929. This fixed the knot at exactly 1.852 km/h.

Today:

• Ships use GPS and electronic speed logs.
• The chip log is obsolete, but the term "knot" remains universal.

Real-World Examples

Maritime Vessels (Ships & Boats)

Vessel Type	Typical Speed	mph Equivalent
Rowboat	2-4 knots	2.3-4.6 mph
Sailboat (Cruising)	5-8 knots	5.8-9.2 mph
Racing Yacht	10-15 knots	11.5-17 mph
America's Cup (Foiling)	40-50 knots	46-57 mph
Cargo Ship	15-25 knots	17-29 mph
Container Ship (Large)	20-25 knots	23-29 mph
Cruise Ship	20-24 knots	23-28 mph
Ferry (Fast)	30-40 knots	35-46 mph
Aircraft Carrier	30+ knots	35+ mph
Destroyer (Naval)	30-35 knots	35-40 mph
Submarine (Submerged)	20-35 knots	23-40 mph
Hydrofoil	40-50 knots	46-57 mph
Speedboat (Racing)	80-100 knots	92-115 mph
World Record (Water)	276 knots	317 mph

Aviation (Aircraft)

Aircraft Type	Typical Speed	mph Equivalent
Ultralight	30-50 knots	35-57 mph
Cessna 172 (Small Plane)	110-120 knots	126-138 mph
Piper Cub	75-85 knots	86-98 mph
Cirrus SR22	180-200 knots	207-230 mph
King Air (Turboprop)	250-300 knots	288-345 mph
Boeing 737 (Cruise)	450-480 knots	518-552 mph
Boeing 777 (Cruise)	490-510 knots	564-587 mph
Airbus A380 (Cruise)	490-510 knots	564-587 mph
Business Jet (Gulfstream)	500-550 knots	575-633 mph
F-16 Fighter (Cruise)	400-500 knots	460-575 mph
F-16 (Max Speed)	1,200+ knots	1,380+ mph (Mach 2)
SR-71 Blackbird	1,900+ knots	2,185+ mph (Mach 3.3)
Concorde (Cruise)	1,150 knots	1,323 mph (Mach 2)

Weather & Wind

Condition	Wind Speed (Knots)	mph Equivalent
Calm	<1 knot	<1 mph
Light Air	1-3 knots	1-3 mph
Light Breeze	4-6 knots	5-7 mph
Gentle Breeze	7-10 knots	8-12 mph
Moderate Breeze	11-16 knots	13-18 mph
Fresh Breeze	17-21 knots	20-24 mph
Strong Breeze	22-27 knots	25-31 mph
Near Gale	28-33 knots	32-38 mph
Gale	34-40 knots	39-46 mph
Strong Gale	41-47 knots	47-54 mph
Storm	48-55 knots	55-63 mph
Violent Storm	56-63 knots	64-72 mph
Hurricane	64+ knots	74+ mph
Category 3 Hurricane	96-112 knots	111-129 mph
Category 5 Hurricane	137+ knots	157+ mph

Common Uses

1. Maritime Navigation

Why Knots?

• Chart Compatibility: Nautical charts use latitude/longitude. 1 knot = 1 minute of latitude per hour.
• Mental Math: Easy to calculate distance and time.
• Universal Standard: All ships worldwide use knots.

Example:

• "We're at 40°N, heading north at 30 knots."
• "In 2 hours, we'll be at 41°N." (30 knots × 2 hours = 60 nautical miles = 1 degree).

2. Aviation Navigation

Why Pilots Use Knots:

• International Standard: All air traffic control uses knots.
• Wind Reports: "Winds 270 at 15 knots" (from west at 15 knots).
• True Airspeed vs. Ground Speed: Pilots calculate wind correction using knots.

Example:

• True Airspeed: 450 knots (speed through air).
• Headwind: 50 knots.
• Ground Speed: 400 knots (speed over ground).

3. Meteorology

Wind Speed Reporting:

• Surface Winds: Reported in knots for marine forecasts.
• Upper-Level Winds: Jet stream speeds in knots (can reach 200+ knots).
• Hurricane Intensity: Measured in knots (64+ knots = hurricane).

4. Oceanography

Ocean Currents:

• Gulf Stream: Flows at 3-5 knots (fastest ocean current).
• Tidal Currents: Can reach 5-10 knots in narrow straits.

Conversion Guide

Knots to mph (Miles per Hour)

Formula: $$ \text{mph} = \text{knots} \times 1.15078 $$

Quick Mental Math: Add 15%

• 20 knots: 20 + 3 = 23 mph.
• 100 knots: 100 + 15 = 115 mph.

Knots	mph
10	11.5
20	23.0
50	57.5
100	115.1
500	575.4

Knots to km/h (Kilometers per Hour)

Formula: $$ \text{km/h} = \text{knots} \times 1.852 $$

Quick Mental Math: Double it (close enough)

• 20 knots: ~40 km/h (actual: 37 km/h).
• 100 knots: ~200 km/h (actual: 185 km/h).

Knots	km/h
10	18.5
20	37.0
50	92.6
100	185.2
500	926.0

Knots to m/s (Meters per Second)

Formula: $$ \text{m/s} = \text{knots} \times 0.514444 $$

Knots	m/s
10	5.14
20	10.29
50	25.72
100	51.44

Fun Facts About Knots

• The Fastest Sailboat: The Vestas Sailrocket 2 reached 65.45 knots (75.3 mph) in 2012—faster than most speedboats!
• Aircraft Carrier Launch: When launching jets, carriers steam into the wind at 30+ knots to give planes extra lift.
• The Gulf Stream: This ocean current flows at 3-5 knots. Ships sailing with it get a free speed boost!
• Jet Stream Winds: Can exceed 200 knots (230 mph) at high altitude. Pilots use them to save fuel on eastbound flights.
• The Beaufort Scale: Created in 1805 by Admiral Francis Beaufort to standardize wind descriptions. Still used today!
• Why "Log Book"? Ships' daily records are called "log books" because they originally recorded the chip log readings.

Comprehensive Conversion Table

Knots	mph	km/h	m/s	ft/s
1	1.15	1.85	0.51	1.69
5	5.75	9.26	2.57	8.44
10	11.51	18.52	5.14	16.88
20	23.02	37.04	10.29	33.76
30	34.52	55.56	15.43	50.63
50	57.54	92.60	25.72	84.39
100	115.08	185.20	51.44	168.78
200	230.16	370.40	102.89	337.56
500	575.39	926.00	257.22	843.91
1,000	1,150.78	1,852.00	514.44	1,687.81

Navigation Techniques Using Knots

Dead Reckoning

Definition: Calculating your current position based on speed, time, and direction from a known starting point.

Formula: $$ \text{Distance (nautical miles)} = \text{Speed (knots)} \times \text{Time (hours)} $$

Example:

• Starting Position: 40°N, 70°W
• Course: Due East
• Speed: 15 knots
• Time: 4 hours
• Distance Traveled: 15 × 4 = 60 nautical miles
• New Position: 40°N, 69°W (1 degree east)

Time-Speed-Distance Calculations

The "60D Street" Rule: A navigation shortcut using the "60-to-1" rule.

Formula:

• Distance = Speed × Time
• Speed = Distance ÷ Time
• Time = Distance ÷ Speed

Example:

• Question: How long to travel 120 nautical miles at 30 knots?
• Answer: 120 ÷ 30 = 4 hours.

Wind Correction

Pilots must account for wind when calculating ground speed.

Example:

• True Airspeed: 450 knots (speed through air)
• Headwind: 50 knots
• Ground Speed: 450 - 50 = 400 knots (speed over ground)

Crosswind:

• Requires trigonometry to calculate drift angle.
• Pilots use "wind correction angle" to stay on course.

Speed Records in Knots

Water Speed Records

Category	Record	Holder	Year
Unlimited	276 knots (317 mph)	Spirit of Australia	1978
Propeller-Driven	200 knots (230 mph)	Problem Child	1989
Sailing (Outright)	65.45 knots (75.3 mph)	Vestas Sailrocket 2	2012
Sailing (Foiling)	50.17 knots (57.7 mph)	America's Cup AC75	2021
Human-Powered	18.5 knots (21.3 mph)	Decavitator (hydrofoil)	1991

Air Speed Records

Category	Record	Holder	Year
Manned Aircraft	1,905 knots (2,193 mph)	SR-71 Blackbird	1976
Jet Aircraft	1,905 knots (2,193 mph)	SR-71 Blackbird	1976
Propeller Aircraft	528 knots (607 mph)	Rare Bear (modified)	1989
Helicopter	249 knots (287 mph)	Westland Lynx	1986
Glider	190 knots (219 mph)	Klaus Ohlmann	2003

Wind Speed Records

Type	Record	Location	Year
Surface Wind	253 knots (291 mph)	Barrow Island, Australia	1996
Tornado	261-318 knots (301-366 mph)	Bridge Creek, Oklahoma	1999
Jet Stream	230+ knots (265+ mph)	Various high-altitude	Ongoing

Additional Unit FAQs

Do submarines use knots?

Yes! Submarines use knots for both surface and submerged speeds.

Typical Speeds:

• Surface: 10-15 knots (slow, inefficient).
• Submerged: 20-35 knots (fast, efficient).
• Nuclear Submarines: Can exceed 35 knots submerged.

Why Faster Underwater?

• Hydrodynamic Design: Optimized for underwater travel.
• No Wave Resistance: Submerged vessels don't create surface waves (major drag source).

How accurate was the chip log?

Surprisingly accurate! Within 5-10% of actual speed.

Sources of Error:

• Current: Ocean currents affected the chip's position.
• Wind: Strong winds could drag the chip.
• Rope Stretch: Wet rope stretched, affecting knot spacing.
• Human Error: Counting mistakes, timing errors.

Modern Comparison:

• Chip Log: ±5-10% accuracy.
• GPS: ±0.1% accuracy.

What is "indicated airspeed" vs. "true airspeed"?

Indicated Airspeed (IAS):

• What the airspeed indicator shows.
• Affected by air density (altitude, temperature).
• Used for aircraft performance (stall speed, landing speed).

True Airspeed (TAS):

• Actual speed through the air mass.
• Increases with altitude (thinner air).
• Used for navigation (calculating ground speed).

Example:

• At Sea Level: 150 knots IAS = 150 knots TAS.
• At 35,000 feet: 150 knots IAS = 280 knots TAS.

Why do weather forecasts use knots?

International Standard:

• ICAO (Aviation): Requires knots for all weather reports.
• WMO (Meteorology): Recommends knots for marine forecasts.

Practical Reasons:

• Marine Safety: Ships need wind speed in knots.
• Aviation Safety: Pilots need wind speed in knots.
• Consistency: Same unit for surface winds and upper-level winds.

How fast is a "fresh breeze" in knots?

Beaufort Scale (Detailed):

Force	Name	Knots	Conditions at Sea	Conditions on Land
0	Calm	<1	Mirror-like sea	Smoke rises vertically
1	Light Air	1-3	Ripples	Smoke drifts
2	Light Breeze	4-6	Small wavelets	Leaves rustle
3	Gentle Breeze	7-10	Large wavelets	Leaves/twigs move
4	Moderate Breeze	11-16	Small waves	Small branches move
5	Fresh Breeze	17-21	Moderate waves	Small trees sway
6	Strong Breeze	22-27	Large waves	Large branches move
7	Near Gale	28-33	Sea heaps up	Whole trees move
8	Gale	34-40	High waves	Twigs break off
9	Strong Gale	41-47	Very high waves	Roof damage
10	Storm	48-55	Exceptionally high waves	Trees uprooted
11	Violent Storm	56-63	Air filled with foam	Widespread damage
12	Hurricane	64+	Sea white with spray	Devastation

Fresh Breeze: 17-21 knots (20-24 mph). Small trees sway, moderate waves at sea.

Can you go faster than the wind in a sailboat?

Yes! Modern sailboats can sail faster than the wind.

How?

• Foiling: Hydrofoils lift the hull out of the water, reducing drag.
• Apparent Wind: As the boat accelerates, it creates its own wind.
• Aerodynamics: Modern sails act like airplane wings, generating lift.

Records:

• Vestas Sailrocket 2: Reached 65.45 knots in 25-knot winds (2.6x wind speed!).
• America's Cup AC75: Regularly sails at 40-50 knots in 15-20 knot winds.

What is "ground speed" vs. "airspeed"?

Airspeed: Speed through the air (what the plane "feels"). Ground Speed: Speed over the ground (what GPS shows).

The Difference: Wind!

Example (Headwind):

• Airspeed: 500 knots (flying into 100-knot headwind).
• Ground Speed: 400 knots (actual progress over ground).

Example (Tailwind):

• Airspeed: 500 knots (flying with 100-knot tailwind).
• Ground Speed: 600 knots (actual progress over ground).

Why It Matters:

• Fuel Planning: Based on ground speed (how long to destination).
• Flight Time: Based on ground speed.
• Aircraft Performance: Based on airspeed (stall speed, climb rate).

How do you measure knots on a modern ship?

Methods:

1. GPS: Most accurate. Measures ground speed directly.
2. Doppler Log: Sends sound waves to ocean floor, measures reflection (speed over ground).
3. Electromagnetic Log: Measures water flow past hull (speed through water).
4. Pitot Tube: Measures water pressure (speed through water).

Difference:

• Speed Through Water: What the ship "feels" (affected by currents).
• Speed Over Ground: Actual progress (what matters for navigation).

Example:

• Ship Speed: 20 knots through water.
• Current: 3-knot favorable current.
• Ground Speed: 23 knots over ground.

Glossary of Nautical Terms

• Knot (kn): Unit of speed = 1 nautical mile per hour.
• Nautical Mile (NM): 1,852 meters = 1 minute of latitude.
• Statute Mile: 5,280 feet = 1,609 meters (land mile).
• Chip Log: Historical device for measuring ship speed.
• Dead Reckoning: Navigation by calculating position from speed/convert/time/direction.
• True Airspeed (TAS): Actual speed through air.
• Indicated Airspeed (IAS): Speed shown on airspeed indicator.
• Ground Speed (GS): Speed over ground (GPS).
• Beaufort Scale: Wind force scale (0-12).
• Gale: Wind force 8 (34-40 knots).
• Hurricane: Wind force 12 (64+ knots).
• Headwind: Wind blowing against direction of travel.
• Tailwind: Wind blowing in direction of travel.
• Crosswind: Wind blowing perpendicular to direction of travel.
• Apparent Wind: Wind experienced by moving vessel (combination of true wind and vessel motion).