What Is a Knot?

Knots persist at sea because the nautical mile is tied to the geometry of the earth rather than to an arbitrary land measure. One nautical mile corresponds to one minute of latitude, so a navigator reading a chart can measure distance directly off the latitude scale in the margin. Speed expressed in nautical miles per hour therefore drops straight into time-and-distance calculations without a conversion step.

The name comes from the method. Sailors trailed a weighted log line knotted at fixed intervals, let it run out for a timed interval, and counted the knots that passed to derive speed. The unit outlived the technique.

Practical conversion: multiply knots by 1.15 for a close approximation in mph. Twenty knots is about 23 mph. Thirty knots is about 35 mph. That gap between the two scales is why a yacht advertising a 25-knot cruise sounds slower than it is to anyone thinking in car speeds.

On a Miami charter, knots matter for two reasons. First, fuel: on a planing motor yacht consumption rises steeply with speed, so cruising at 22 knots instead of 30 can meaningfully change fuel cost on a chartered day where fuel is billed to you. Second, marine forecasts report wind in knots. A forecast of 15 to 20 knots on Biscayne Bay means a chop that will make an open sandbar day uncomfortable, and reading the forecast in the same unit the captain uses is the fastest way to know whether to move a booking. Wind speed also drives the sandbar decision more than most guests expect, since these anchorages are open water with no lee, and a 20-knot easterly turns a swim stop into a chop you cannot stand up in. Current is quoted in knots too, and in Government Cut and Haulover Inlet a strong outgoing tide against an onshore breeze builds a steep, short sea that a captain will want to time around.