Visit Us!

Visit our Sail School and Sail Charters on the Chesapeake Bay!

Sunday, May 13, 2007

True Wind vs Apparent Wind

True wind is very simple concept. On any windy day, stand outside and turn your face directly into the wind. This is the true direction of the wind and the true wind speed. A stationary anemometer and wind vane can easily determine this true direction and true speed.

Most of us have experience apparent wind when we rode our bikes as a child. When we tried to ride into the wind, we discovered that we had to work hard and use more energy. When we rode away from the wind, (i.e. in the same direction that the wind was blowing), we could ride faster and easier without working as hard. This is the basic concept of apparent wind.

When we are sailing, we are using the apparent wind. Anytime that the true wind is forward of the beam of our sailboat, the apparent wind speed will increase in speed and also move further forward of the beam. Anyone, who has studied physics in high school, will remember all those wonderful vector problems. Fortunately for us sailors, we do not need to apply vector theory in order to use the apparent wind to sail, although for those of you, who love to work math problems, you are more than well to review your high school physics and use vectors to calculate the apparent wind speed and apparent wind direction.

What we do need to remember, is that the increased wind speed and the change of direction of the apparent wind will effort how we trim our sails and what sail configuration we must use. For example, let’s assume that the true wind is blowing at 15 knots and we are sailing on a close reach. This true wind of 15 knots will actually feel more like 18 to 20 knots and will cause more heel and more weather helm. For a cruising sailor, this will cause the helmsman to work harder and to become fatigued much quicker. Also the ride will be much less comfort due to the heel. Therefore, the cruising sailor may find that it is necessary to reef the mainsail. By reefing and reducing the sail area, we can reduce the heel and the excessive weather helm.

So what happens to the wind aft of the beam of the vessel? The apparent wind will move further aft of the boat and will decrease in speed. Using our 15 knot wind from the previous paragraph, this will now be more like 10 – 12 knots and we will not need to reef and reduce our sail area. When my daughters were young children, we would often sail from Middle River to St. Michaels. Our favorite route was to sail south under the Bay Bridge and around Bloody Point and up the Eastern Bay. During the summer the winds often tends to come out of the southeast so we had a great close reach or beam reach down the Bay. As soon as we turned Bloody Point and headed up the Eastern Bay, my children always wanted to know what happened to the wind. It always seemed to die down. This was the apparent wind. Every sailor needs to understand this and be prepared to change the sail configuration as the vessel either heads up into the wind or falls off of the wind.

Here is a simple set of rules for apparent wind:

  • Wind abeam or forward of the beam will increase in speed and move further forward of the beam. Be prepared to trim sails and/or reef the sails.
  • Wind aft of the beam will decrease in speed and move further aft of the beam. Be prepared to ease the sails and shake out a reef in the sails.

More information is available in The Annapolis Book of Seamanship by John Rousmaniere.