Watched Master and Commander the other night, again. There's a scene where a kid is measuring speed with a knotted rope. 5 knots slip through your hand in X time equals 5 knots. Now imagine using someone else's knotted rope, tied at different intervals. Holy crap, now we're doing 8 knots! Or the anchor at the end is different, or your hour glass is smaller... As long as you use the same knotted rope with the same Anchor and same hour glass you will eventually going to be able to roughly predict your time to destination