How does a sailor measure boat speed?

Answer 1

Boat speed is traditionally defined as how fast your boat is moving with respect to the water. This is an important definition as I've seen boats running at top speed but unfortunately for them they were caught in a strong current so the boat was not moving at all, or moving backward, with respect to the ground. (That is called Speed Over Ground.)

Speed with respect to the water is measured by dipping something in the water, like a log or a paddle-wheel, and measuring how fast the water is moving by. For example if you have a boat that is 10 long (feet, meters, I don't care), and you throw something in the water at the front of the boat and notice it takes 1 second to get to the back of the boat, then your speed is 10/second. A paddle wheel you would see how fast it turns and compare it to other known speeds. The advantage of this technique is it gives you continuous readings. Boat speed instruments are based on this technique and involve a miniature paddle installed on the outside of the hull.

Or you can use devices that track location, like a sextant or GPS, and see how far you have traveled over the last hour and that is your speed per hour.