The other day out on the water, my crew-mate Alex and I were standing bow-watch together. We were looking out at the waves, and Alex asked what wind force I guessed it was. “I think seventeen knots. Twenty at most,” I said. He ran back to the helm where we have an anemometer, a wind gauge. “The anemometer says twenty-three but our speed downwind is 5. That makes it eighteen knots!”
When I was First Mate on the Bill of Rights, I used to quiz my crew (and myself) on wind force. There’s a guide called the Beaufort Scale which allows you to estimate wind speed based on sea-state. Zero knots is “surface like a mirror”. Force one, or 1-3 knots, shows ripples. Force two, or 4-6 knots, ripples begin to form crests. Force three, or 7-10 knots, crests begin to break…you get the picture.
It’s important to be able to estimate wind speed by observing the state of the ocean. We decide how much sail area is safe to carry based on wind speed, and being able to judge that without running over to the anemometer could help you out someday. GPS isn’t the only bit of technology one which the modern sailor has become too reliant. Here’s a clip of the full Beaufort Scale. Note that sea state is also affected by any nearby landmasses, and the length of the storm.