Apparent or not?
|12 March 2012||Posted by Alex from Lagoon under Technical world|
After dealing with the VMG and speed polar diagram notions, this new article is about another one, very much linked to the previous: the apparent wind. To know more about this wind that we all know for sure but that not everyone understands, we have to start with two things: the true wind and the boat speed wind.
- The true wind is the atmospheric wind, the wind that we talk about every day and that your wind vane indicates when your boat is at a stop.
- The boat speed wind is the wind created by the motion of the boat. Its direction is always facing the boat.
The combination of these two notions is the apparent wind. This wind is the one indicated by your wind vane when you’re sailing. As a matter of fact, the choice between apparent wind and true wind is available on your wind speed display.
The apparent wind is the one that you need to take in count for your sails trimming because it is the one that really acts on them. As you can see on the drawing, the apparent wind is always more heading than the true wind; it means that the wind direction is closer to the route of the boat. So on an abeam reach course you need to trim your sails as if you were on a close reaching course.
Let’s look at 3 realistic possibilities:
1. What happens when the true wind increases?
If we look at the drawing and if we increase the true wind (original wind in light color and increasing in dark), we can see that the wind is lifting. It means that the wind recedes from the boat’s route.
This is why we have to differentiate the wind situation between the top and the lower part of the mainsail. For example, if you take a Lagoon 620 with a 30 metre high mast, the wind up there is totally different from that at the boom level: it is stronger because there is less friction from the surface of the sea at a 30 meter altitude. So the wind is stronger and creates a lift at the upper part of the mainsail. It is why the mainsail twist adjustment is really important!
2. How does the apparent wind evolve under different points of sail?
If we follow the geometrical construction of the apparent wind, we can see that:
- On courses at less than 90°off the wind, the apparent wind is stronger than the true wind,
- On courses of more than 90° off the wind, the apparent wind is weaker than the true wind.
This explains why on a downwind course we feel less wind than on an upwind course, even with strong gusts. Nevertheless, some performance multihulls are so fast that they can « create their own wind » and so they are in a different situation on a downwind course. Indeed their apparent wind is always stronger than their true wind. This also explains the flat shape of their downwind sails compared to a conventional spinnaker.
3. Why should we consider the apparent wind first to trim the sails?
In general the apparent wind is really important with multihulls as this kind of boat is more able to change wind into speed than a monohull. A good understanding of this concept and of its use when sail trimming is fundamental for the speed of the boat.
If we look at the drawing of a Lagoon 560 on a close hauled course we can see:
- Stage 1: the true wind is 20 knots, the boat speed wind is 8 knots at 55° off the wind and so the apparent wind is 25 knots (you can calculate the apparent wind here).
- Stage 2: the apparent wind is stronger than the true wind so the boat speed is increasing.
- Stage 3: the boat speed wind has increased, the apparent wind is increasing also but it is heading too, which means it gets closer to the boat’s course.
- Stage 4: We can see clearly the direction change of the apparent wind due to the increase of the boat speed wind.
Unfortunately you cannot keep increasing your speed for ever with this phenomenon. The increasing of the apparent wind and consequently the boat speed are limited by the angle of the apparent wind which is heading a little more as speed increases and so prevents from keeping to accelerate.
We find the same situation when the boat speed increases due to the increasing of the true wind:
- Stage A: The true wind is increasing,
- Stage B: The apparent wind is increasing and lifting,
- Stage C: the boat speed increases as well as the boat speed wind which leads us to stage 2 on the previous drawing,
This effect is particularly visible on landyachts and even more on « ice boats » which have a very important potential for transforming wind energy into speed thanks to the low friction force that they face. So it is not rare to see them sheet in their mainsail at the maximum.
Even if you probably will not face all these situations with your Lagoon catamaran, the understanding of these elements will give you the opportunity to sail better!