RELATIVE MOTION AND THE MANEUVERING BOARD87284
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Format: 16mm
Description: This United States Navy training film describes the principles of relative motion to be applied in naval maneuvers for ships at sea. It opens with a cartoon depicting a caveman hunting with a rock (:39). Without understanding relative motion, when he throws the rock at the deer, he misses every time as the deer is in motion (1:00). When begins to guess the two motions of the moving deer and the rock he begins to understand relative motion and becomes a skillful hunter (1:58). From the deer’s perspective it appears the rock is coming straight for him although it was actually thrown ahead of him (1:17). A forward pass is shown in a football game which also uses relative motion (1:52). On the road, relative motion is showed by two cars driving at 50 mph and when one speeds up or slows down it appears the cars are moving away from one another or closer to one another, respectively (2:25). In naval operations, relative motion problems could be plotted to receive accurate solutions (2:56). The definition is provided which is: relative motion is the apparent motion of one moving object to another moving object (3:15). A demonstration follows of two ships crossing one another (3:23). Ship A in the demonstration is to watch the apparent motion of ship B and vice versa (4:01). Ship A’s relative motion is parallel to the relative motion of ship B (4:33). A vector line is shown which represents the direction of ship A and another vector line which shows the direction of ship B (4:49) and when the two are joined they form the two sides of a vector triangle (5:24). The third side of the vector triangle represents the true course and speed of ship B. This vector diagram is the key to solving all relative motion issues (5:44). The direction and speed of it’s third side can be discovered if any of the other two sides are known (6:02). An example is provided of two ships moving on opposing courses (6:23) and when their vectors are added together, the relative speed between the two ships is determined (6:39). A destroyer is pointed to on the water (7:52) and the film sets up an example where this ship has just received word to proceed with maximum speed to join a convoy course. The convoy was to be north and moving east, therefore the destroyer was to head north east (8:13) which would mean it was leading the convoy until a vector diagram could be formed to discover the actual course. The first step would be to draw a relative plot showing the destroyer’s position as well as the convoy’s position (8:25). The line which represents the destroyer’s direction of relative motion is pointed to with the convoy as a reference (8:39), but if this line was used as a course, the destroyer would miss the convoy (8:47). The convoy course and its true speed combined form one side of the vector diagram (8:59). Next, the captain would plot the direction of relative motion (9:07). Without knowledge of the relative speed, the line is shown extended indefinitely (9:07). The third side is the destroyer’s true course and speed (9:20). The diagram is completed when the vector line is swung from the starting point until touching the direction of relative motion (9:33). From here, as the definite length for the direction of relative motion has been acquired, the length can be measured and the speed of relative motion can be deduced (10:02). Another example of a problem with relative motion follows, which is usually solved with only the seamen’s eye (10:22). A cruiser ship is to move alongside of an oiler (10:26). The oiler is moving ahead of the cruiser which meant the cruiser would have to move towards it and slightly to the right so that the direction of relative motion will go towards the oiler (10:42). His experience will be used in order to determine how much of an increase of speed is necessary, how far to the right he should move and when he was to return to his base course (10:53). This film begins to wrap up with a run through of what was learned (11:13) and it was of the Sea Power for Security films (12:09).
Transcription
you this man lived in a cave many thousands of years ago he wanted to learn to be a good hunter but until he learned to apply the principles of relative motion he often went without his dinner this forced him to figure out why he always missed the deer when he threw towards the target the trouble seemed to be that the deer moved at the same time he threw the rock well he would allow for the two movements deer and rock guessing at the amount of the two motions he soon became a skillful hunter because he always remembered to apply the principles of relative motion to the deer the rock seemed to come straight toward him actually of course the rock was thrown well ahead of the target but from the deers viewpoint the rock appeared to approach like this he would see its motion relative to his own motion here's another skillful use of the same principle of relative motion safe driving often depends upon accurate judgments of the speed of relative motion our actual speed is 50 miles an hour the car ahead also is doing 50 so there is no relative motion between us but if we slackin our speed to 40 miles an hour the car ahead appears to be moving away from us at the rate of ten miles an hour as we increase our speed to 55 we are in the sense of relative motion approaching the car ahead at a relative speed of five miles an hour a car coming toward us and an actual speed of 50 miles an hour will have a relative speed of 105 in a passing situation our driver needs a good sense of relative motion oh or else at sea while you need both skill and common sense your relative motion problems can be plotted to obtain accurate solutions this film will introduce you to the principles of relative motion as applied in naval maneuvers here is a definition of relative motion it is the apparent motion of one moving object to another moving object these ships are in a normal crossing maneuver we are seeing their actual or Geographic motion now let's go back and this time we will travel with ship a and watch the apparent or relative motion of ship be ship B is on course 0 90 degrees true but her relative motion to ship a is in this line of positions the direction of which is 130 degrees true let's go back again and this time we will move with ship be ready to observe the relative motion of ship a the course of ship a is 030 degrees true but because we are moving with ship be the direction of ship A's relative motion appears to be in this direction which is 310 degrees true now remembering that relative motion is the apparent motion of one moving object to another moving object note that the direction of relative motion of ship be as it appears to ship a is parallel to the direction of relative motion of ship a as it appears to shift be the directions of relative motion are reciprocal let us apply the principles of relative motion to a naval maneuver this vector line represents the direction of own ship A's true course its length of 10 spaces represents our speed of 10 knots this vector line represents the direction of ship B's direction of relative motion observed from ship a it is measured to a scale of 13 spaces for ship B's relative speed by joining the vector line representing ship B's direction of relative motion and relative speed to the vector line representing ship A's true course and speed we form two sides of a vector triangle the third side represents the true course of ship be the length of the third side of our vector measured to the same scale as that used for the other two sides represents the true speed of ship be the vector diagram is the key to the solution of all problems in relative motion it is not a geographic plot each line represents a direction and a speed if the course or direction and speed of any two sides of the vector diagram are known the direction and speed of the third side can be determined here we know the course and speed of own ship and the course and speed of the other ship measuring to the same scale used for the other vector lines in this diagram we can determine the other ships direction of relative motion and its speed of relative motion here are two ships on opposite courses ship a on course 270 degrees true speed 15 knots ship be on course 0 90 degrees true speed 20 knots by adding the vectors the relative speed between the two ships can be measured as 35 knots let's bring ship a around to course 180 degrees true and see what happens to the vector diagram the vector for ship a is in Direction 180 degrees 15 units long for her speed of 15 knots the vector for ship be drawn from the same point of origin is in the direction 090 degrees and is 20 units long for B's speed of 20 knots the direction of relative motion is always away from the reference ship from ship a the direction of relative motion of ship B is toward 054 if ship B is the reference ship the direction of relative motion of ship a is toward to 34 degrees the length of the line connecting the two measured vectors is the speed of relative motion in this case 25 knots this speed a relative motion would be apparent from either ship as commanding officer of a destroyer you're about to receive this message proceed maximum speed on two boilers join convoy alpha convoy course zero-nine-zero 9 knots position 29 30 north 6145 west you know that the convoy is north of you moving east so common sense prompts your first move you head northeast leading the convoy until you can make a vector diagram to find your actual course you first draw a relative plot it shows your own position and the position of the convoy bearing 010 degrees true distance 32 miles this line represents your direction of relative motion using the convoy as the reference however if you use this relative direction as your course you will miss the convoy now to make the vector diagram to find your true course first the vector for the convoy course zero-nine-zero true speed 9 knots that's one side of the vector diagram next you plot the direction a relative motion 010 degrees true as determined on the relative plot since you do not know the relative speed the line is extended to an indefinite length the third side of the diagram is your own true course and speed your orders call for maximum speed so you lay off vector length for 27 knots with the same scale you use for the convoys vector swing the vector line from the point of origin until it touches the direction of relative motion vector line this completes your vector diagram the direction of your vector away from the point of origin fixes your course 0 to 9 degrees true you have already scaled it to your maximum speed 27 knots now that you have obtained a definite length for the direction of relative motion vector you can measure its length and find the speed of relative motion 23 and a half knots this will enable you to predict the time you will meet the convoy 32 miles away now let us look at a problem in relative motion usually solved by the use of Siemens I let's see what the captain of this Cruiser is doing as he moves from his position on the quarter to a position alongside the oiler it is obvious that the captain will not set a course directly toward the oiler since the oiler is moving ahead the cruiser must increase speed and come right a little so that its direction of relative motion will go toward the oiler how much does the captain of the cruiser increase speed how much does he come right and when will he return to base course and speed he depends on his experience with relative motion and maneuvers using Siemens I this requires a great deal of experience especially when the movement is between two ships that are close to each other we have observed relative motion from a number of aspects moving cars on the highway our friend getting his dinner and the completion of a forward pass we have seen how it is possible to solve a problem in relative motion which involves a ship taking station in a distant convoy the solution was found by plotting a vector diagram we have seen an example of solving a problem in relative motion by the use of Siemens I remember that relative motion is the apparent motion of one moving object to another moving object all problems in relative motion involve this concept the ability to make use of the principles of relative motion is essential to your performance as an officer of the deck as a navigator as a watch officer in CIC and throughout your naval career
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