AEROGRAPHER'S MATE 3 & 2 



An example of how the computer is used 

 for the determination of true wind direction 

 and speed is set out below: 



Assume that the apparent wind direction and 

 speed are 300° and 18 knots, and the ship's 

 course and speed are 080° and 16 knots. 



1. Slide the rotor disc along the ship's speed 

 reference line until the center index of the 

 rotor disc is opposite the ship's speed, 16 knots, 

 and then rotate the disc until the ship's heading, 

 080° true on the compass rose of the rotor 

 disc, is directly over the 000°/360° bearing 

 radius of the base plate. 



2. Plot with a grease pencil a dot on the 

 rotor disc at the point determined by the apparent 

 wind direction, 300° true, and speed 18 knots, 

 utilizing the base grid. 



3. Slide the rotor disc to the zero of the 

 ship's speed reference index (the center of the 

 concentric circles of the base plate) and rotate 

 the disc until the grease pencil dot, previously 

 plotted, lies along the 000°/360° bearing radius 

 of the base plate. The true wind direction, 

 328°, can now be read directly off the rotor 

 disc over the 000°/360° bearing radius of the 

 base plate. The true wind speed, 17.5 knots, 

 can now be read directly opposite the grease 

 pencil dot by utilizing the ship' s speed reference 

 index. 



For the determination of a ship's course and 

 speed required to produce the desired apparent 

 wind direction to produce the desired wind 

 direction and speed, assume that the apparent 

 wind direction desired is 5° off the port bow and 

 the apparent wind speed is 40 knots, actual 

 apparent wind direction and speed are 300° 

 and 18 knots, present ship's course is 080° 

 true, and present ship's speed is 16 knots. 



1. Repeat steps 1, 2, and 3 of the above 

 example to find the true wind direction and speed 

 of 328° and 17.5 knots. 



2. Manipulate the rotor disc (by rotation and 

 sliding along the 000°/360° bearing radius of the 

 base plate) until the grease pencil dot, the head 

 of the true wind vector, is directly over the 

 reference point on the base plate as determined 

 by the given requirements of the desired apparent 

 wind (direction at 5° off the port bow and speed 

 of 40 knots across the deck) , utilizing the grid 

 of the base plate. 



3. Read directly off the rotor disc the ship's 

 course, 340° true, which is directly over the 

 0007360° bearing radius of the base plate. 

 Read directly the ship's speed, 23.5 knots, 

 opposite the center of the rotor disc, from the 

 ship's speed reference index. This ship's course, 

 340° true, and a speed of 23.5 knots, with the 

 given true wind direction and speed, will produce 

 the desired apparent wind direction, 5° off 

 the port bow, and a speed of 40 knots across 

 the deck. 



Checking Computed Winds 



The following may be used to check computed 

 wind values: 



1. The true direction of the wind is always 

 on the same side of the ship as the apparent 

 direction, but farther from the bow (port or 

 starboard) than the apparent direction. 



2. The true speed of the wind is greater than 

 the apparent speed whenever the apparent 

 direction is aft of the beam. 



3. The true speed of the wind is less than 

 the apparent speed whenever the true direction 

 is forward of the beam. 



TRUE WIND OBSERVING METHOD 



In the absence of a computer, the true wind 

 direction may be observed by noting the direction 

 from which small wavelets, ripples, and sea 

 spray are coming. To accomplish this, sight 

 along the wave crests, then turn 90 degrees to 

 face the advancing waves; this is the true 

 direction, which is evaluated to the nearest 

 10 degrees, (See table 3-1.) 



The true wind speed may be estimated by 

 observing the sea condition and referring to 

 table 3-3 or Appendix VI. However, the observer 

 should be aware of the assumptions upon which 

 these estimations are based, such as that the 

 wind is well removed from land and that the 

 wind has been blowing from a constant direction 

 and speed long enough to cause the present 

 sea condition. 



Some of the factors which will result in an 

 underestimation of true wind speed are as 

 follows: 



1. Off-shore winds within sight of land. 



2. Moderate to heavy precipitation causing 

 a smoother than normal sea condition. 



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