Cruise News July 2022

Bella Vita

Richard Loseby

True vs ApparentWind

If you're hanging out at any yacht club bar, reading your instruments or talking about sail trim, then chances are you've heard people talking about things like true wind speed and apparent wind angle. Well what does all that mean anyway?

What is True Wind? Imagine you are standing still in a paddock. There is a light breeze of 5 knots coming from the north. This is the true wind speed and the true wind angle or direction. It is the actual speed of the wind as it passes over the land or surface of the sea. It is also the apparent wind as this is the same wind that you are feeling when you are standing still.

What is Apparent Wind? Imagine you are still in the paddock with the 5 knot northerly, and you start running north at 5 knots. You are now 'feeling' 10 knots of wind. The 5 knots of true wind, plus the fact that you are running at 5 knots - gives you 10 knots of 'apparent wind'. It isn't actually blowing 10 knots but because you are running that fast in to the wind that is what you feel.

If however you run south - with the wind behind you, and you run at 5 knots - i.e. the same speed as the wind, then you will feel no apparent wind at all.

Apparent wind is a combination of wind direction and the direction and speed of the moving object. The two winds, the actual wind and the wind created by the motion, are added together and this is your apparent wind.

Picture by Richard Loseby of Bella Vita setting off on the #pacificrally2022

Why do we need to know this? It is important to understand the difference between true and apparent wind for a number of reasons. Weather forecasts will give you the true wind speed and direction. This will also determine the sea state.

When you are sailing your vessel, you will be wanting to set a sail plan based on the apparent wind. The wind we feel on our face is the same wind that the sails feel. So we sail to the apparent wind, and not to the true wind and then we can also take advantage of apparent wind to sail faster.

For example if you are going to be sailing down wind in 25kts, and your boat speed with your spinnaker up is 10kts speed over the ground, the actual wind hitting your sails is the equivalent of just 15 kts (25kts - 10kts = 15kts).

However if you turn around and head upwind and your upwind boat speed is 6kts, then the apparent wind feels much stronger like 31 knots! (25kts+6kts = 31kts).

Apparent wind speed is entirely dependent on how fast you are travelling and at what angle to the true wind speed.

True wind speed helps you to forecast the sea state, but your direction of travel will help determine how you will experience this wind.

Reading Your Instruments Modern instruments are quickly able to measure apparent wind speed and, by a simple calculation based upon the vessel’s speed and direction they will instantly inform us of the true and apparent wind speed. However, it’s important to make sure your instruments are calibrated and regularly checked, otherwise the information you will receive will be of little use.

Pic - by Crystal Cowan, shows the Archer crew experiencing lots of apparent wind as they cruise along through the Lau group in their dinghy

Sailing with Apparent Wind You might have noticed on the Americas Cup & SailGP foiling boats that they sail everywhere with their sails sheeted in the entire time. The faster they go the apparent wind angle decreases and moves further forward as the apparent wind speed increases.

Imagine you are back in the paddock with the 5 knot northerly blowing. If you start running east at 5 knots, the apparent wind speed will feel like it is coming from the north east. If you run faster - like 10 knots, the wind angle will move forward to be coming at you directly from the east - or right on the nose.

Apparent wind can also be dramatically affected by changes in the tidal stream. Sailing with the tide will increase your boat speed over the ground and with it, apparent wind. So when it comes to really using the boat effectively you must always trim the sails according to the apparent wind speed and angle, not the true wind.

So in summary, the difference between true wind and apparent wind will always depend on our course, and its strength will be different with a tailwind or crosswind. Plus, it changes depending on the speed at which we sail. During the voyage, we don’t change power depending on the direction of the real wind (according to the weather and other influences) but also the apparent wind, which is related to our course and speed, which is constantly changing.

Picture by Martin Bennett of Champagne sailing down the East Coast of the South Island on the #southislandrally2021