When should you reef your sails? This is a common question with no definitive answer. Some common guidelines used for determining when to reef are:
A set windspeed
The first sighting of a whitecap
The approach of storm clouds on the horizon
First sight of rain
Preparing to Heave-to
Many sailors put in reefs at arbitrarily set wind speeds. A common guideline is to put the first reef in at 15 knots, and the second reef at 20 knots of wind speed. From this guideline, you can then determine when you want to reef. If you feel fine at 15 knots, keep the sails up a little longer. When you finally decide you wanted to reef, look at the wind speed; next time reef a few knots sooner than the current windspeed.
On Wisdom, we put the first reef in at 20 knots, and the second reef at 27 knots. We also drop the Jib at 20 knots, and fly the staysail all the way through to really high wind speeds.
On Windpuff, we have no instruments, so we reef at the first sign of whitecap or approaching storm cloud. Reefing based on whitecaps is a good indicator, as they tend to form in response to building wind conditions. I like to reef at the first sight of a storm cloud because I prefer to go forward to the mast in calm weather and await the winds with reduced sails instead of being caught off guard.
Rain on the horizon usually accompanies an approaching squall with high winds. The dark curtain approaching your boat is an excellent signal to put the reefs in. The only thing worse than putting in reefs during high winds is being pelted with stinging water droplets!
On a completely different note that does not involve weather or storms is crew comfort. If you have a frightful person on board, you may have to put in a reef to keep them calm. I have taken friends sailing and they panic when we heel over, leading me to put in 2 reefs in winds of 12 knots. We hardly moved, but the boat did not heel. With time, they became more confident that the boat will not flip over and allowed me to shake out the reefs one at a time. Eventually we were sailing along under full sail at hull speed and everyone had a good time! If I had told them to tough it out and hold onto the heeling hull, they would have been traumatized and probably never gone sailing again. While crew comfort is not related to weather conditions or sail trim, it is a valuable indicator for when to reef.
One last reason to reef the sails is when you are preparing to heave to for the night. The weather may be calm and the seas tranquil, but they might change during the night. By reefing the sails all the way down before heaving to, you can sleep comfortably and confidently through the night!
The least scientific method (which is also my go to method) is sail trim. When the wind builds, we ease the sheets to control the amount of heeling. When the leech begins to flutter because the sail has been eased too far and we are still heeled over, we typically put in a reef. This allows us to trim the sail perfectly once again and maintain the keel in a more efficient orientation. If the wind eases up and we loose speed, we wait 15-30 minutes before we shake the reef out (in case it was simply a lull in the wind, we wait to make sure the wind doesn't pick up again and catch us over-canvassed).
When do you typically put in your reefs?