True,or Apparent?

Sail trim and reefing is done according to the wind acting on your sails, so according to apparent wind as it is the wind your sails are acted upon by. Reefing should be done before your sails and vessel is overpowered by the wind. Any table of wind speeds vs sail set (reefing) are simply guidelines. Sail trim, boat heel, crew abilities, sea state, sail condition and apparent wind will all influence when you should be reefing.

Short answer: true wind. "Wind speed" usually refers to true wind, unless explicitly stated otherwise. Long answer: You want to be able to sail safely and comfortably in any direction, both upwind (maximum apparent wind) and downwind (minimum apparent wind). So when making a decision about reefing, you should base it on true wind because it will not change every time you turn. There is, of course, a subtlety (which only matters if you are racing): you actually care about the apparent wind as well. In borderline (true) wind, if you decided to go with a full sail you would be fast sailing downwind but overpowered and slow upwind; if you went with a reef, you would be fast upwind, but slower downwind.

I agree with the first response - apparent wind and don't base the decision solely on a preconcieved table. You need to consider your crew and the sea state and different boats react differently. Also, think ahead. You may be on a run but have to come around and to turn up a river to get to the desired destination. The apparent wind will increase with the turn, so reefing early in anticipation of that need to change course would be the safe move. I was always taught that if you are wondering if you should put a reef in, the answer is yes. Adding a reef early is a safe move, it can always be shaken out later. I would not want be in an overpowerd boat regretting not having put a reef in earlier.

True Wind is your guide. At sea, wind direction, wind strength, and tides change, which can lead to conditions getting windier and rougher. At 20 knots TWS, it is time to consider reefing before you go out depending on the boat. As said earlier, it is much harder reefing at sea than in the harbor. When in doubt, reef. You can always pull it out later.

Hi Vladimir, I believe the true wind would be your true indicator for reefing. True wind being the speed of wind moving across the land or water and apparent wind is generated from true wind and boat movement. I have found that if you think you should reef due to the strong wind, you probably should have already done it. Keeping a good watch on the weather has taught me to be a little over cautious.

It's quite simple really ! True wind is what you would feel if your boat were standing still on the water (boat speed = 0 kts). As soon as your boat starts to move in a given direction it creates its own induced wind. To understand this, imagine you are riding a motorcycle on a windless day. When you stop your motorcycle, you feel no wind, but when you travel on your motorcycle at 50 km/hr, you obviously feel a 50 km/hr wind in your face. This wind is induced by your movement. Now you can understand that Apparent wind is the combination of True wind and boat speed. So, if your boat is sailing in the same direction as the wind, what you or your sails would feel is an apparent wind equal to True wind minus the speed of the boat. Apparent wind becomes weaker than True wind. Vice versa, if your boat is sailing against the wind, what you would feel is an apparent wind equal to True wind plus the speed of the boat. Apparent wind in this case becomes stronger than True wind.

Continuation from yesterday's contribution. So, Apparent wind is the result of True wind combined with boat speed. But it's not a matter of simply adding or substracting one from the other. Mathematically, velocities (speeds) are vectors and vectorial additions or substractions are a bit more tricky than simple algebraic operations. Without going in details, what you should know is that when you add vectors , you are not just computing a resultant value; you are also computing a resultant angle. This means that Apparent wind will not only have a different speed than True wind, but it will also have a different angle. The only time both will have the same speed and angle is when boat speed is zero. Now, if TW and AW have different speeds and angles which should you consider when deciding how to set or trim your sails. Consider this: what moves your boat is the force created by the pressure differential caused by the wind on your sails, and the wind that your sails feel ( or the wind that flows over your sails) is Apparent wind not True wind. So the wind that moves your boat and makes it heel is Apparent wind. The setting and trimming of your sails is therefore performed in relation to apparent wind not true wind. To convince yourself of this concept, stand on beach one day and watch a fun-boarder or hobbie-cat sail at high speed with a True wind blowing at 90 degrees across his track (the wind you feel standing on the beach). You will also notice how close in his sails will be sheeted (as if he was sailing close hauled). That is because he's got his sails trimmed accordind to his own Apparent wind and certainly not according to True wind. Clear now that Apparent wind should always be your primary factor to consider when trimming your sails or when setting them for any given point of sail. This does not mean you should forget about True wind which is your secondary factor. Knowing your TW is what allows you to estimate the expected AW speed should you have to change your heading. But keep in mind that on a boat, TW is a calculated parameter and not a measured parameter like AW and boat speed. So if your onboard wind vane, anemometer and loch sensor are not reading accurately, then your calculated TW will be out of whack and not to be trusted. In a nutshell, always rely and trust your AW because it is measured and not calculated like TW, and also beacuse it is the wind that you feel in your face and the one driving your sails. If you sail downwind, AW will weaken and if you sail upwind AW will strengthen. That's it, and I hope you're less confused than before.

I use true wind as an appropriate guide to make the decisions to change sail or reef. My rational is that on a run you can be fooled by the relatively low apparent wind, due to boat speed (somewhat) reducing the wind you feel on a run/broad reach. When you make a turn upwind the wind you feel will seem stronger with apparent wind based on a close-hauled heading in which boat speed is (somewhat) additive to the true wind. The resultant higher apparent wind may wish you made a sail change or reef to cope with this stronger wind on deck.