Chapter 3 — BEARINGS AND MOTION 



RELATIVE MOTION 



Relative motion is the apparent movement 

 of an object in relation to another object. Do 

 not confuse the concept of relative motion with 

 relative bearing. Relative motion is measured 

 as a true direction of apparent movement. 



You may have been unaware at the time, 

 but on many occasions you have witnessed the 

 solution of relative movement problems. When 

 a baseball player races to catch a high fly ball, 

 or when a football quarterback throws down- 

 field to a receiver, the players are estimating 

 relative movement. As an example, assume 

 your ship is on a course of 000° at a speed of 

 20 Imots. You are overtaking and will pass 

 close aboard a ship that is also on course 

 000°, but whose speed is only 10 knots. As 

 you close the ship, the bearings to him draw 

 aft. Eventually you pass him and he falls further 

 and further behind. His relative movement from 

 you is approximately 180°, but his true move- 

 ment is 000°. 



Following are three rules pertaimng to relative 

 motion you must remember when making an 

 attack on a submarine. Figure 3-10 illustrates 

 these relative motion situations. 



1. If range is closing and the bearings are 

 drawing toward the bow, your ship will 

 pass astern of the submarine. 



2. If range is closing and the bearing re- 

 mains steady, the ship will pass directly 

 over the submarine. (If the submarine 

 were on the surface, a collision would 

 result.) 



3. If range is closing and the bearings are 

 drawing aft, the ship will pass ahead of 

 the submarine. 



ADVANCE AND TRANSFER 



When a ship changes course to head for a 

 new bearing, she does not move as a car does 

 on land. Water is a fluid substance, and it 

 does not allow the ship the advantage of good 

 traction. As rudder is applied to a sMp, a 

 short period ensues before the rudder takes 

 hold in the water. During the turn, the stern 

 of the ship actually slides through the water. 

 As it slides, the ship tends to advance in the 

 same direction of her original course. The 

 distance the ship moves in the original direction 

 until she is on the new course is called advance. 

 The amount of advance depends on ship's speed, 

 amount of turn, and amount of rudder angle 

 applied. 



During the change in course, the ship also 

 moves at right angles to the original course. 

 Tlie distance the ship moves at right angles to 

 the original course during the turn is called 

 transfer. The amount of transfer, as with advance, 

 depends on the amount of turn, ship's speed, 

 and amount of rudder angle. 



Advance and transfer vary with each type 

 of ship, and even with ships of the same type. 

 Each ship, therefore, makes her own advance 

 and transfer tables for various rudder angles 

 and at different speeds. 



As you can imagine, advance and transfer 

 will affect target bearings during a turn. All 

 possible situations cannot be explained here 

 because there are almost unlimited combina- 

 tions of target and attacker relative movements. 

 If the target is on your port beam, for example, 

 and has the same course and speed you have, 

 when you turn left to head for him, his bearing 

 will change to the right. Figure 3-11 illustrates 

 the effect of advance and transfer on bearing 

 when the target is stationary, or nearly so. 



BEARING DRIFT 



When the sonar operator detects an under- 

 water target, he reports the bearing and range. 

 The conning officer then turns the ship to head 

 for the reported target. As the ship turns, the 

 bearing cursor is trained back and forth across 

 the target to check for bearing width, and the 

 tai'get is classified. The operator then places 

 the cursor in the center of the target pip. The 

 cursor shows direction of sound reception. Its 

 length indicates range when the cursor line is 

 adjusted to touch the target echo. With the ship 

 headed for the tai'get on a steady course, and 

 at a constant speed, with the cursor bisecting 

 the echo, any change in target bearing results 

 from target movement. If the target moves to 

 the right, the operator must train his cursor 

 to the right in order to remain on tai'get, 

 reporting "Bearing drift right." If the target 

 moves to the left, he must train his cursor to 

 the left to remain on target, and reports "Bear- 

 ing drift left." 



In an attempt to evade the attacking ship, 

 the submarine will maneuver. The attacking 

 ship, in turn, changes course as necessary to 

 reach optimum weapon firing position while 

 maintaining sonar contact. Regardless of the 

 number of course changes of the ship, or maneu- 

 vers by the submarine, the job of the operator 

 is to keep the cursor on the target. Correct 

 bearings, ranges, audio response, and other 



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