True wind is the wind as it actually blows over the surface, measured from a stationary point rather than from a moving boat. It is described by true wind speed (TWS) and true wind direction (TWD) in compass degrees, or true wind angle (TWA) relative to the boat’s heading. The wind you feel on deck under way is the apparent wind, which is the true wind combined with the wind created by the boat’s own motion. A beginner needs to know that the two are different and that the sails respond to apparent wind. An experienced sailor adds a subtlety: strictly, “true wind” as most instruments calculate it is referenced to the water, using boat speed through the water, while ground wind is referenced to the seabed and includes the effect of tide. In a strong current the two can differ by several knots and many degrees.
True wind and apparent wind
Stand on a jetty in ten knots of breeze and you feel true wind. Step onto a boat doing six knots to windward and the wind on your face is stronger and comes from further ahead: the boat’s motion adds a headwind of six knots to the true wind, and the vector sum is the apparent wind. Turn downwind at the same speed and the apparent wind drops to four knots and swings aft. The sails, the masthead wind vane and the telltales all respond to apparent wind, which is why a fast boat sails with its sails trimmed far tighter than the true wind angle would suggest and why a slow boat running dead downwind feels almost becalmed in a decent breeze.
Instruments compute true wind from three inputs: apparent wind speed and angle from the masthead, boat speed from the paddlewheel or GPS, and heading from the compass. Small errors in any of them produce large errors in the result, especially when true wind is calculated downwind, where apparent wind is light and the arithmetic is unforgiving. Calibrating the masthead unit, the speed log and the compass is the unglamorous foundation of every performance sailing instrument system.
Why true wind matters on the racecourse
Tactics depend on true wind. A shift is a change in true wind direction, and you cannot see it in apparent wind alone because your apparent wind swings every time boat speed changes. Racing navigators watch TWD over time to identify oscillating shifts, persistent shifts and the geographic effects near shorelines. Polar diagrams, which tell you the fastest boat speed at each wind angle, are drawn against true wind speed and angle, so the target speeds the trimmers steer to are meaningless without a trustworthy true wind number. Laylines to the windward mark are calculated from TWD. And sail selection is made on true wind speed: a genoa that is perfect in twelve knots true upwind is the wrong sail for twelve knots true downwind, where the apparent wind may be only six.
The sailmaker’s angle
Sails are designed to a true wind range, because true wind is what the sailmaker can predict from the boat’s polar. A light-air genoa built for four to twelve knots true will see far higher apparent wind at the top of that range going upwind, and the cloth weight and construction are chosen with that in mind. Downwind sails are the clearest case. An asymmetric spinnaker or a code sail for a fast boat is designed knowing that in eighteen knots true the boat may be sailing at twelve, so the apparent wind angle is well forward and the sail must be cut flatter and stronger than a running kite for a heavier displacement cruiser at the same true wind speed. When we discuss a sail inventory with an owner, we talk in true wind ranges and then translate to the apparent wind each sail will actually feel on that particular boat.
Frequently Asked Questions about True Wind
Why do my instruments show a different true wind after I tack?
Almost always because of calibration. A masthead unit that is a few degrees off centre, a speed log reading high or low, or heel error in the wind vane will show a false “shift” of several degrees every tack. Correct the offsets rather than chasing the phantom shift.
Does the tide change the true wind?
It changes the water-referenced true wind that your instruments calculate. A boat drifting with a three-knot tide feels three knots of extra wind from ahead of the tide even when the ground wind is calm. Racing in strong current, this is why the wind appears to shift when the tide turns.
Why does the wind seem to drop when I bear away?
The true wind has not changed, but bearing away means the boat’s motion subtracts from the wind instead of adding to it, so the apparent wind falls. Reef by true wind speed, not by how it feels on a run.
True wind is the fixed reference in a moving world. The sails answer to apparent wind, but the forecast, the shifts, the polars and every sensible decision about reefing and sail selection come back to what the wind is really doing over the water. Learn to think in both and switch between them without effort, and you will trim better, plan better and be surprised far less often.