An observer aboard a vessel proceeding through still air experiences an apparent wind which is from dead ahead and has an apparent speed equal to the speed of the vessel. Thus, if the actual or true wind is zero and the speed of the vessel is 10 knots, the apparent wind is from dead ahead at 10 knots. If the true wind is from dead ahead at 15 knots, and the speed of the vessel is 10 knots, the apparent wind is 15 + 10 = 25 knots from dead ahead. If the vessel reverses course, the apparent wind is 15 - 10 = 5 knots, from dead astern.

The apparent wind is the vector sum of the true wind and the reciprocal of the vessel's course and speed vector. Since wind vanes and anemometers measure apparent wind, the usual problem aboard a vessel equipped with an anemometer is to convert apparent wind to true wind. There are several ways of doing this. Perhaps the simplest is by the graphical solution illustrated in the nofollowing example:

• Example 1: A ship is proceeding on course 240° at a speed of 18 knots. The apparent wind is from 040° relative at 30 knots.
• Required: The direction and speed of the true wind.
• Solution: (Figure below) First starting from the center of a maneuvering board, plot the ship's vector "er," at 240°, length 18 knots (using the 3-1 scale). Next plot the relative wind's vector from r, in a direction of 100° (the reciprocal of 280°) length 30 knots. The true wind is from the center to the end of this vector or line "ew."Alternatively, you can plot the ship's vector from the center, then plot the relative wind's vector toward the center, and see the true wind's vector from the end of this line to the end of the ship's vector. Use parallel rulers to transfer the wind vector to the center for an accurate reading.
• Answer: True wind is from 315° at 20 knots.

On a moving ship, the direction of the true wind is always on the same side and aft of the direction of the apparent wind. The faster the ship moves, the more the apparent wind draws ahead of the true wind.

A solution can also be made in the nofollowing manner without plotting: On amaneuvering board, label the circles 5, 10, 15, 20, etc., from the center, and draw vertical lines tangent to these circles. Cut out the 5:1 scale and discard that part having graduations greater than the maximum speed of the vessel. Keep this sheet for all solutions. (For durability, the two parts can be mounted on cardboard or other suitable material.) To find true wind, spot in point 1 by eye. Place the zero of the 5:1 scale on this point and align the scale (inverted) using the vertical lines. Locate point 2 at the speed of the vessel as indicated on the 5:1 scale. It is always vertically below point 1. Read the relative direction and the speed of the true wind, using eye interpolation if needed.

A tabular solution can be made using Table 30, Direction and Speed of True Wind in Units of Ship's Speed. The entering values for this table are the apparent wind speed in units of ship's speed, and the difference between the heading and the apparent wind direction. The values taken from the table are the relative direction (right or left) of the true wind, and the speed of the true wind in units of ship's speed. If a vessel is proceeding at 12 knots, 6 knots constitutes one-half (0.5) unit, 12 knots one unit, 18 knots 1.5 units, 24 knots two units, etc.

• Example 2: A ship is proceeding on course 270° at a speed of 10 knots. The apparent wind is from 10° off the port bow, speed 30 knots.
• Required: The relative direction, true direction, and speed of the true wind by table.
• Solution: The apparent wind speed is: 30/10 = 3.0 ships speed units. Enter Table 30 with 3.0 and 10° and find the relative direction of the true wind to be 15° off the port bow (345° relative), and the speed to be 2.02 times the ship's speed, or 20 knots, approximately. The true direction is 345° + 270° (-360) = 255°.
• Answers: True wind from 345° relative = 255° true, at 20 knots.

One can also find apparent wind from the true wind, course or speed required to produce an apparent wind from a given direction or speed, or course and speed to produce an apparent wind of a given speed from a given direction.Such problems often arise in aircraft carrier operations and in some rescue situations.

When wind speed and direction are determined by the appearance of the sea, the result is true speed and direction. Waves move in the same direction as the generating wind, and are not deflected by Earth's rotation. If a wind vane is used, the direction of the apparent wind thus determined can be used with the speed of the true wind to determine the direction of the true wind by vector diagram.

• Zone Time In Celestial Navigation - 2009-04-21 - 01:09:16
• Some common local winds are listed by local name - 2009-04-21 - 12:35:43
• Wind Patterns - Modifications of the General Circulation - 2009-04-21 - 05:46:03
• Maneuvering to Avoid the Storm Center - 2009-04-21 - 05:03:03
• Weather Routing - Benefits - 2009-04-19 - 07:35:50
• Weather Routing - Communications - 2009-04-19 - 07:25:17
• Weather Routing - Southern Hemisphere Routing - 2009-04-19 - 07:21:29
• Weather Routing - Types of Recommendations and Advisories - 2009-04-19 - 07:16:12
• Special Weather and Environmental Considerations - 2009-04-19 - 07:06:59
• Weather Routing - Synoptic Weather Considerations - 2009-04-19 - 06:15:22

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