Modern cruisers typically have instrumentation that tells you the boat speed in a nice easy digital readout. However, it is also possible to measure the speed in a more primitive way. By tossing something – preferably biodegradable – into the water at the bow and measuring the amount of time it takes for the object to pass the stern, you can calculate the speed of the boat.
Speed is simply the distance traveled (the length of the boat) divided by time. Of course it’s important to convert the quantities into the desired units.
In fact, the origin of the term knots being used as a unit for nautical speed is rooted in this crude method of measurement. Sailors used to throw a rope into the water and based on how many knots in the rope passed through their hands in a given time period, they could determine the boat speed.
History aside, a knot is simply a nautical mile per hour. Formally, a nautical mile is equal to one minute of latitude, which converts to about 1.15 miles. The usefulness of the knot unit becomes obvious if you ever try to navigate using a nautical chart, which is laid out on a grid of latitude and longitude lines. For a given speed in knots and time in hours, no conversions are necessary to map a distance traveled onto a nautical chart.
