In the initial stages of most studies of Maneuvering Board methods the significance of relative movement has not been properly accentuated. Various approaches to the meaning of this term, which embodies motion with respect to some other motion, are somewhat abstract to the new student. A modern reference manual presents the reader with: “ . . . the apparent movement of celestial bodies across the sky”* as a familiar example of this phenomenon. Also too general is the comparison which concerns the hunter shooting a bird in flight. He must shoot ahead, and lead his target in order to score a hit. An overworked simile involves two trains proceeding in the same direction. Passengers aboard the faster train suppose that the slower one is traveling backwards. These are interesting concepts but can hardly be said to demonstrate a type of relative movement that is applicable to the Maneuvering Board. If plotted upon such a diagram, these examples would be ineffectual as illustrations.
Possibly the principal objection to the aforementioned cases is that they fail to represent visually the problem at hand. They do not serve the purpose of introducing the subject because they are not geared to graphic representation of the variety of apparent movement common to maneuvering problems. An example of this type should follow through with a graphic plot to accomplish its aim and correlate the concept of relative motion with the Maneuvering Board diagram itself.
The speed and direction triangle is the basis for the solution of Maneuvering Board problems. In the phenomenon of true and apparent wind we have a situation which, by way of introducing the subject, is characterized by the following merits:
(1) The presentation is in terms of an everyday occurrence to the mariner.
(2) The speeds and directions are analogous to the actual movements of two ships involved in a maneuver.
(3) It serves to exemplify the direction and speed of relative (apparent) movement—the relative speed (gin) line.
(4) Simple introductory sketches convey the action involved.
(5) A vector plot representative of a speed and direction triangle equivalent to the one employed in maneuvering problems and similarly labeled summarizes the presentation.
Consider a vessel at anchor; wind blowing from east at 10 knots:
The vessel sets out on course 000° (T); speed 20 knots:
What is the apparent direction and force of the wind? See the following speed and direction triangle.
Notice that the gm line in the apparent wind plot represents the direction and speed of the wind relative to the ship, which is itself in motion. The gm line in maneuvering problems represents the direction and speed of the maneuvering vessel relative to the guide, which is itself in motion. In other words, the wind in the one case is comparable to the maneuvering vessel in the other. The gm line, therefore, in both types of problems represents a relative speed and a relative direction between two objects in motion.
“In addition to conformance to the statutory laws of the land, namely, the rules of the road quoted in the last chapter of the Navy Regulations, and at least with a consideration of courtesy, naval vessels operate in accordance with certain maneuvering instructions and in prescribed formations, both of which within the Navy have the effect of law.
“The second cruiser, being in a prescribed formation, is required to maintain a certain distance and bearing from the leading cruiser, and she is not free to maneuver except by orders or Permission of her senior, or to avoid immediate danger. In other words in the absence of orders or permission she will hold on in her position relative to the leader until the circumstances in the situation justify a departure.
"Now ordinarily an individual vessel has the right of way over any steam vessel or vessels on her port bow on crossing courses, but in the past, where private vessels have collided with men-of- war in formation, some such cases have been judged under the proviso of special circumstances, sometimes the individual ship being held at fault for adhering to her course and sometimes the men-of-war for failing to give way."—Marsden’s Collisions at Sea.
* Maneuvering Board Manual (H. O. No. 217)