Why Ships and Planes Measure Speed in Knots

One of the pleasures of living by San Francisco Bay is that the water is always full of sailboats, and I have been lucky enough to crew on a friend’s boat now and then. The first time I looked at the instruments, everything was in knots, and I asked the obvious landlubber question: why not miles per hour like the car?

I got a slightly smug answer that turned out to be correct. The knot is not stubborn tradition — or not only that. Its underlying unit, the nautical mile, is built into the geometry of the planet.

A knot is a nautical mile per hour

One knot is one nautical mile per hour, and one nautical mile is defined as exactly 1852 meters. That makes one knot equal to 1.852 kilometers per hour, or about 1.151 statute miles per hour.

For everyday translation, a few anchors help:

• 10 knots is about 18.5 km/h, or 11.5 mph
• 30 knots — a fast ferry — is roughly 56 km/h, or 35 mph
• To convert km/h to knots, divide by 1.852

The knotted rope was real

The name is delightfully literal. Sailors once measured speed with a chip log: a wooden board tied to a rope knotted at regular intervals.

Toss the board overboard, let the rope pay out, and count how many knots slip through your hands during an interval timed with a sandglass. The knot spacing and glass were matched so the count gave speed directly. Count eight knots, and you were making eight knots. Every time I ride the ferry across the Bay, I think about some poor sailor doing arithmetic with wet rope.

Why charts make the nautical mile the right tool

Here is the genuinely good reason the unit survives. The nautical mile was historically defined as one minute of latitude — one sixtieth of a degree along a meridian.

Charts are drawn in degrees and minutes, so a navigator can take a distance straight off the latitude scale with dividers: one minute on the chart is one nautical mile on the water. Speed in knots then plugs directly into time-and-distance calculations with no conversion at all. For anyone navigating by angles on a round Earth — which is every sailor and pilot — the nautical mile is the natural unit, and the knot follows automatically.

From the bridge to the cockpit

Aviation inherited nautical units from marine navigation, and international aviation still uses knots for airspeed and nautical miles for distance almost everywhere. Marine and aviation weather reports give wind in knots too, keeping the whole operational picture in one unit — meteorologists tracking hurricanes work in knots internally even when the public bulletin says km/h or mph.

So when a ferry advertises 30 knots, read it as roughly 56 km/h — and appreciate that the unit connects a sailor’s knotted rope to the latitude lines on every chart ever printed.