21 knots is about 24 mph or 39 km/h , which is similar to a fast city driving speed or a decent cruising speed for many boats.

Basic conversion

  • 21 knots ≈ 24.2 miles per hour (mph).
  • 21 knots ≈ 38.9 kilometers per hour (km/h).
  • One knot means one nautical mile per hour, so it is slightly faster than one land mile per hour.

How that feels in real life

  • It’s like driving a car at roughly 40 km/h through town, but on water instead of a road.
  • For many small to medium boats, 21 knots is a comfortable cruising speed rather than an extreme top speed.

Quick reference table (HTML)

[3][1] [4]
Speed Knots Miles per hour Kilometers per hour
Given speed 21 kn ≈ 24.2 mph≈ 38.9 km/h
**TL;DR:** 21 knots is roughly **24 mph / 39 km/h** , a solid moderate speed for a boat or ship.

Information gathered from public forums or data available on the internet and portrayed here.