What Is a Knot Speed? Nautical Miles, Conversions and Use at Sea

One Knot Is One Nautical Mile Per Hour

What is a knot speed in plain terms? A knot is a unit of speed equal to one nautical mile per hour. The international nautical mile is defined as exactly 1,852 meters. That makes one knot exactly 1.852 km/h, about 1.15 statute miles per hour and about 0.514 meters per second. The knot is not an SI unit. It is formally accepted for use alongside SI, and it remains the standard speed unit for marine and air navigation worldwide.

The 1,852 meter value was agreed in 1929 at the First International Extraordinary Hydrographic Conference in Monaco. The United States kept its own slightly longer nautical mile of 1,853.248 meters until 1954, which is why some older American publications give conversions that do not quite match modern tables.

Why the Nautical Mile Is Tied to the Shape of the Earth

One nautical mile approximates one minute of arc of latitude, so one degree of latitude equals sixty nautical miles. That relationship is the reason the unit survived: it converts position directly into distance without any arithmetic.

It also drives a piece of chartwork every deck officer learns early. On a Mercator chart, the latitude scale up the side of the sheet is the distance scale. Measure a distance with dividers against the latitude scale at roughly the same latitude as the leg you are measuring, and never against the longitude scale along the bottom, where a degree shrinks as you move away from the equator.

Because the Earth is an oblate spheroid, a minute of latitude is not constant. It runs from roughly 1,843 meters near the equator to roughly 1,862 meters near the poles. Fixing the international nautical mile at 1,852 meters removed that variation from calculations.

Where the Word Knot Came From

The unit is named after a piece of equipment. The chip log was a weighted wooden quadrant, the log ship, streamed astern on a line. Knots were tied into the line at fixed intervals, and a sand glass was turned as the line ran out. Counting how many knots passed through the hand before the glass emptied gave the speed directly.

The common arrangement paired a 28 second glass with knots at intervals of 47 feet 3 inches. That is not an arbitrary number: 47.25 feet in 28 seconds works out at 1.6875 feet per second, and one nautical mile per hour is 1.6878 feet per second. The instrument was calibrated to give the answer with no conversion at all.

The practice of heaving the log and recording the result is where the ship's log book got its name, and the entries a bridge team makes today are the direct descendant of a sailor counting knots.

Speed Through the Water Versus Speed Over the Ground

Two different speeds appear on a modern bridge and confusing them causes commercial arguments as well as navigational errors.

• Speed through the water comes from the ship's log, electromagnetic or Doppler, and measures motion relative to the water the ship is in.
• Speed over the ground comes from the satellite positioning system and measures motion relative to the seabed.

The difference is the set and drift of the current plus any wind driven surface movement. Charter party speed and consumption claims are assessed on speed through the water, because the propeller only ever works against the water, and a favourable current does not mean the ship burned less fuel than warranted. Passage planning and estimated arrival times use speed over the ground, because that is what closes the distance to the pilot station.

Radar presentation depends on the same distinction. A sea stabilised display is what you want for collision avoidance, because a target's apparent motion then reflects how the two ships are actually moving through the same water. A ground stabilised display in a strong tideway shows true vectors that are correct for navigation but misleading for judging a risk of collision.

Conversions and Mental Arithmetic Worth Memorising

• Knots to km/h: multiply by 1.852, or add roughly 85 percent for a quick estimate.
• Knots to mph: multiply by 1.15.
• Knots to meters per second: halve it. The exact figure is 0.514, so halving is high by about three percent, which is close enough for a manoeuvring estimate.
• The six minute rule: distance run in six minutes, in nautical miles, is the speed in knots divided by ten. At 14.4 knots you cover 1.44 miles in six minutes.
• The three minute rule: in three minutes a ship covers roughly one hundred yards for each knot of speed. At 12 knots that is about 1,200 yards, which is the version used for close quarters work.
• Time for a given distance: minutes equals distance in miles multiplied by sixty and divided by speed in knots.

What Ships Actually Do, and Why an Extra Knot Is Expensive

Service speeds vary by type. Large container ships are designed for the low twenties in knots but frequently run in the middle teens. Laden crude tankers typically work in the low to mid teens, bulk carriers a little lower, and ro-pax and fast ferries considerably higher.

The reason operators slow down is in the physics. Propulsion power rises roughly with the cube of speed, so a ten percent reduction in speed cuts propulsion power by close to thirty percent. Run that in reverse and the cost of picking up one extra knot to hold a berth window becomes obvious.

A great deal of modern voyage optimisation, including just in time arrival coordination with ports, exists purely to avoid paying that cube law penalty to arrive early and wait at anchor.

Reading the Number Correctly on the Bridge

Know which sensor feeds which display. The log usually feeds radar and the voyage data recorder, while the positioning system feeds the AIS speed over ground and course over ground fields that other ships see. An AIS speed that disagrees with reality is far more often a configuration or sensor problem than a broken transmitter.

Log errors have recognisable causes: marine growth over an electromagnetic sensor, aerated water alongside the hull in heavy weather, and a Doppler log switching between bottom track and water track as depth changes. Compare log speed against the distance actually run between noon positions, and a drifting log calibration shows up long before anyone else notices it. The Navigation and Seamanship section covers passage planning arithmetic in more detail.

Verification of the number sits outside the ship. Class surveyors from DNV, Lloyd's Register or ClassNK examine the speed and distance measuring equipment at periodical survey. Port state control officers under the Paris MoU and Tokyo MoU inspect the same gear alongside the radar and the voyage data recorder. An unserviceable log is recorded as a deficiency, and navigational equipment defects remain a recurring detention item.

The faults themselves are mundane. Paint applied over a sensor head in dry dock shifts the calibration, and the resulting error gets misdiagnosed as current for months. A Doppler log loses bottom track over soft mud and reports water track without announcing the change. Damaged sensor wiring fails the log outright, and a casualty investigation working from voyage data recorder files then finds that channel empty.

Sources and further reading

• What is a knot? - NOAA National Ocean Service
• Knot | Speed, Unit, & Definition - Britannica
• Knot (unit)

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