True & Relative Wind

What the anemometer reads, and what the wind is actually doing — both ways round, with the wind over the ground and the wind over the water kept apart.

Scratch working — nothing is saved

The ship

What the anemometer reads — relative wind

The answer

Enter the ship’s heading and speed, then the wind.

Three conventions that each look plausible when they are wrong
A wind direction is where it comes FROM. A vector points where things GO. A wind “from 270” is air moving towards 090. Every direction here is turned through 180° on the way into the arithmetic and 180° back on the way out. Invert that once and the answer still looks entirely reasonable — which is why it is the commonest bug in a wind calculator, and why the working above shows both the bearing and the vector.
Relative wind is referenced to the heading, not the course. The anemometer is bolted to the mast and the mast points along the ship’s head. Using COG in its place is only right when set, drift and leeway are all zero — which is to say, almost never. That is why heading is asked for separately and is never optional.
“True wind” is two different quantities. Take the ship’s motion over the ground out of it and you get the wind over the ground — what a weather observation, a noon report and a routing service all mean. Take the motion through the water out instead and you get the wind over the water. With a current running they are different, and neither is wrong. Both are shown, labelled, along with the drift angle between heading and COG — because that angle is precisely why they differ.
Calm has no direction. Below 0.05 in whatever speed unit is in use, a direction carries no information — atan2(0,0) is 0, and 0 is not north. A ship steaming into a dead calm reports calm, not a heading.
Beaufort is a scale of bands, not a unit. A speed converts to a force exactly; a force converts back only to a range. The bands are the WMO ones, defined for a standard height of 10 m over the open sea. A masthead anemometer sits well above that and reads high — no correction is applied here.
What this is. Vector arithmetic on the numbers you type. It reads no instrument and corrects nothing — not anemometer height, not calibration, not the airflow distortion around the accommodation block. The readings come from your instruments and your judgement, and the Master remains responsible for the weather decision.

A web version of a tool from DeckWatch, the work planner for ship’s deck officers. Nothing here is approved, certified or class-accepted.