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Weather Instruments

Wind Vane

Measures apparent wind direction.

Measures apparent wind speed.

Anemometer

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Wind

Definitions

Apparent wind. The speed and direction (true

or relative) from which the wind appears to blowwith reference to a moving point.

True wind. The speed and direction from which

the wind blows with reference to a fixed point.

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Wind

Examples

Apparent wind: From 090 T (000 R), 15 knots.

True wind: East wind, 10 knots.

Ship: course 090 T, speed 5 knots.

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Wind

Examples

Apparent wind: 270 T (180 R), 5 knots.

True wind: West wind, 10 knots.

Ship: course 090 T, speed 5 knots.

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Wind

Anemometers and wind vanes give

apparent wind

(unless the vessel is stopped).

We must solve for True wind for our

weather observations.

Best done graphically on maneuvering

board, plotting sheet or compass rose.

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Wind

Step 1: Plot ships course& speed.

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WindStep 3: Draw the apparent wind from the tip of the

ships course & speed vector.

(Remember, draw the direction the wind is blowingtowards!)

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WindStep 4: Complete the triangle. Result = True wind.

Direction = 315 T

Speed = 20 knots

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Wind

Example 2: A ship is proceeding on

course 140 at a speed of 15 knots. Theapparent wind is from 110Tat 20

knots. What is the true wind?

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Wind

Step 1: Plot ships course& speed.

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Wind

Step 2: Convert relative direction ofapparent wind to true direction, if

necessary.

True direction of apparent wind = 110 T

Example 2: A ship is proceeding on course 140 at a

speed of 15 knots. The apparent wind is from 110Tat

20 knots. What is the true wind?

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WindStep 3: Draw the apparent wind from the tip of the

ships course & speed vector.

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WindStep 4: Complete the triangle. Result = True wind.

Direction = 071 T

Speed = 11 knots

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Wind Measurement

Beaufort Scale

If no instruments are available,

speed may be estimated by observing

the effect on the water.

Scale was devised in 1806 by English

Admiral Sir Francis Beaufort (1774-1857).

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Force 0

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Force 1

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Force 2

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Force 3

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Force 4

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Force 5

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Force 6

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Force 7

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Force 8

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Force 9

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Force 10

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Force 11

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Force 12

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Introduction to Navigation

Questions?