For just-learning sailors perhaps the most difficult concept to grasp is that of true and apparent wind, yet nothing is more critical to the success of the learner.

True wind is the actual wind. It is the wind as it would be if there were no boat to deflect its direction and speed. The wind as caused naturally. This is not a constant. The wind may back or veer and it may increase or decrease in strength. The whole art of sailing is for the helmsman to react to any such changes by changing the boat's heading, however slightly, and/or by trimming, or even changing/reefing the sails.

The complication is that, as the boat moves through the water, it 'bends' the true wind so that, when beating, the wind seems to move ahead of its true direction. This modified wind is called the apparent wind and it is what the sailor has to consider all the time.

Not only does the angle change, because of the boat's forward speed the wind appears to strengthen. The best way to illustrate this, strangely, is by explaining the converse, which is easier to grasp.

When a smallish boat is running at a speed of eight knots with the wind blowing at 18 knots, the apparent wind speed is 10 knots. When this boat turns into the wind its speed will drop - probably to five knots - and the apparent wind strength will be 23 knots.

This may well be too much wind for this boat to handle with a full main, so a flattening reef, or even a full slab reef may be called for. The only person who can really make this judgment is probably the skipper, as normally one would expect him or her to be the person on board who knows the boat best.

Really good racing crews will have collected data on what sails suit their boat best in particular winds and are meticulous in sticking to that plan unless they have a very good reason to alter it.

The corollary is that, when approaching the next run, the reef will be shaken out, unless the conditions have changed.

When sailing, it's not just the strength of the wind that the skipper needs to take into consideration, but also its direction.

Knowing when (or whether) to shake out a reef when going on a run and reef again when turning onto a windward leg is one of the hardest skills to achieve, but one of the most important.