Apparently it is windy. Apparent wind explained.

James CloseClass 40 racing, Dream or Two Sailing, James, Tips & Tricks, Uncategorised, wind

I talk about apparent wind a lot, it is a term that applies to all sailing boats, but is more noticeable on a fast boat like Fortissimo, as high boat speed means we generate a bigger headwind, which distorts the wind we feel onboard.  To explain how we manage the wind to increase performance I will start with the basics, and then elaborate…

We have the true wind, which is the wind we feel if we were standing still. Once we start moving, we generate a head wind which will be combined with the true wind to create apparent wind. The wind we feel when we are sailing.

Imagine we have a true wind of 10 knots blowing. If we motored at 10 knots towards it we would feel 20knots of apparent wind (10 knots head wind + 10 knots true wind). If we turn around and motor at 10 knots away from it, the apparent wind we feel would be zero knots as we are traveling at the same speed as the wind. So on a day with a steady 10 knots of wind and a boat travelling at 10 knots, by just changing direction, the wind we feel varies between 0 and 20 knots!

On fast yachts we use our high speed to increase the apparent wind. The latest ultra fast hydrofoil boats that travel at 25 knots or more are generating their very own 25 knot headwind! This is often more that the true wind speed and results in the strange effect where by even though you are sailing down wind, because you are going so fast, the wind you feel is still coming from ahead of you. This is why the America’s cup boats do not have big spinnakers, the sails are always trimmed in tight as apparent wind is always forward of the beam no matter what direction they sail.

Upwind

In this diagram, the true wind is blowing from top to bottom. The boat is travelling at 10 knots, so is experiencing a 10 knot headwind, shown as the red arrow. When the true wind and head wind are combined graphically, we can work out the apparent wind speed and direction, shown as the green arrow. This is the same geometry that instruments use to calculate the true wind when the yacht is moving.

SAILING UPWIND

We all learn sailing upwind requires us to fly the tell tales on the jib. The sails are close hauled, they can’t go any tighter, so it is up to the helmsman to steer the sails perfectly into the wind. The wind splitting both sides of the sail, illustrated by the tell tells streaming on each side of the sail.

On fast yachts you need to be careful you don’t fall into the trap of sailing too far off the wind. As you go faster the apparent wind will come more ahead of you, so the luff will start to flutter and the windward tell tell will flicker and lift. This is usually the cue to bear away. If you do this, the tell tells will correct, speed will increase, apparent wind will increase and come ahead so you bear away again. It is feedback loop, that means you could be sailing at 70 degrees to the wind, with the sails still trimmed in tight and tell tells flying perfectly. We need to recognise this trap and sail with our tell tells lifting as long as we are up to target speed. Our target upwind speed is around 8 knots. If we drop below, we bear away slightly, once up to speed, feather the sail into the wind with the windward tell tell lifting to ensure we point close to the wind (45 degrees to the true wind).

Downwind

Here the yacht is still doing 10 knots, but away from the wind on a broad reach angle. The apparent wind is on the beam and at 11 knots for decent propulsion. In the ghost image you can see what would happen if the boat bears away too much. With all other variables staying the same, the apparent wind halves, power drops dramatically and the boat will slow down.

SAILING DOWNWIND

Sailing away from the wind our head wind takes away from the apparent wind, sailing into the wind, our headwind adds to the apparent wind speed.

Performance boats can quite happily sail in traditional slow boat mode, if we let them and do not unlock the speed potential. If we have allowed the boat to slow to say 5 knots sailing downwind in 10, we feel about 6 knots which is not enough to get the boat moving any quicker, we’re stuck.

If we head up on to a beam reach our headwind will add to the wind speed and dramatically increase the apparent wind and move the angle ahead of us. The yacht will take off, our speed builds, apparent wind increases and comes more ahead and we can start to bear away. The sweet spot is to keep the apparent wind on the beam (90 degrees to the boat), this represents the fundamental boundary between our headwind adding to the apparent wind or taking away. We sit in this zone and adjust our course depending on what we want, if we need more wind (more power) we can head up 5 degrees and go more against the wind, this will increase apparent and enable us to build boat speed. If we get a big gust and have too much wind, we can bear away and run with it (our speed will reduce our apparent wind). This swerving to windward to power up is called “heating it up”.

Of course it is not always as simple as that, as the hot angle may not take us to where we want to go, so we have to strike a compromise between outright speed and heading in the right direction. Velocity Made Good (VMG)

HeatingUp

In this example the boat has increased speed to 15 knots, but is sailing at the same angle to the true wind. Interestingly the apparent wind has not increased in strength, but the angle has moved forward of the beam. Hence fast boats like AC45s not using spinnakers. Boat b has headed up slightly, but dramatically increased the apparent wind vector. This is a very sensitive point of sail where small course changes dramatically affect the apparent wind and the power the boat can tap in to. The key is to alter course to keep the power on and the speed up. If you get a gust or a wave you can afford to bear away, to reduce power, then head up to keep power on.

I hope this has all made sense, it is just an introduction but covers the fundamentals. If you have any questions, please feel free to ask in the comments box below and I will do my best to answer them. The complexity and the number of variables is what makes sailing so intriguing, it is a constantly changing picture that requires some technical understanding but also good feeling. It is a Science and an Art.