The way boats historically measured speed was by dragging a rope behind them. The rope has knots tied with exact spacing. You drop one end of the rope in the water, and count how many knots pass you in a given time. That's then your speed in knots.

Using this method repeatedly to guess how far you've moved over the course of days is, historically, a fantastic way to crash into the side of France in the middle of the night.

Well the other problem is knowing where you are. The sun/stars can give you latitude. Longitude was nearly impossible until the advent of the marine chronometer in the latter part of the 18th century, and not "standard" on ships until the mid-1800s. There were earlier versions, which had poor accuracy and were not much better than dead reckoning

> You drop one end of the rope in the water, and count how many knots pass you in a given time.

Given that you're dragging the rope behind you, won't this number be zero?

The rope has a mechanism for creating drag (a wooden board) at the end, and regularly spaced knots. You throw the board in the water, let the rope play out through your hands, and count the knots as they pass through your hands while watching a timer.