True and Apparent Winds
Wind in sailing falls into two main categories - true and apparent wind. They differ by their nature, their use and the way we have to approach them.
When you stand on the harbour, or in the cockpit of your boat, and there's a wind on your cheeks (you can sense its direction and strength) - this is the true wind. The wind indicators on land give you information about its speed and direction - yet it often changes, it is fickle and influenced by many factors.
When you ride a bike, a motorcycle, a surf, a yacht or another open vehicle, you feel a wind created by yourself (the whole entity you are with the moving object you're on/in). This, combined with the true wind makes up the so called apparent wind. Fortunately the wind indicators on the yacht (flags or threads, or whatever you use) show the apparent wind, as it is the driving force that makes you move - it is very useful to see the direction of the apparent wind, because if you get the true wind you will can get confused.
Once again, make sure you remember this equation:
True wind (A) + The wind you create by moving (B) = Apparent Wind (the actual driving force = C)
Slight problem arises when A and B have different directions. Well, you need to revise your knowledge about vectors from high school, because it is the only way you can calculate the apparent wind without computer.
Boas that cannot exceed 6-8 mph have little effect on a strong true wind, so for them the difference between a true and apparent wind is slight. You just need to look at the indicators and adjust the sails accordingly.
Fast and massif boats make their own wind. If you are sailing a race boat or high performance catamaran like Tornado, you need to really have in mind the apparent wind. Speed boats (like the mentioned catamarans) can increase their speed very fast upon meeting a strong gust, even though the true wind may not have changed, you have to trim the sails to close-hauled position and be ready to react very fast. After the gust or lull passes, the boat naturally slows down and you can gradually ease the sails, until you are at beam reach. Remember - you have to seize every opportunity to move faster, and not take unreasonable risks. Even more extreme examples are the windsurfers and ice boats. Notice what hey do in strong breeze - they stay trimmed to close-hauled regardless of their course in relative to true wind (even when the boat is at broad reach).
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