A Method Of Finding The Course From One Relative Position To Another, Knowing The Course And Speed Of The Guide, Using Martin's Mooring Board
PROBLEM
It is required to find the course from a position on line AB to another, as A to B; A to C; or, B to A; C to A, knowing speed and course of guide.
SOLUTION
The guide is represented at center of circle.
Place ruler parallel to AB ahead of the guide at a point equal to the speed of the guide, using an arbitrary speed scale.
A line to the point where the ruler crosses the circle representing your speed, on the same arbitrary speed scale, will give the new course, measured from the center of the circle.
If your change in position is to the right, the line to the point where the ruler crosses your speed circle to the right gives the change in course.
If your change in position is to the left, the line to the point where the ruler crosses your speed circle to the left gives the change in course.
EXAMPLE (FIG. 1)
From position D to position B.
Speed of guide 8 knots, your speed 16 knots.
Place ruler parallel to DB, at the point H equal to 8 (speed of guide), ahead of J.
The line to the point where the ruler crosses the 16-knot circle at G, gives the change in course to the right, 30 degrees.
As a quick reference to know approximately the course to steer to take up a position on the beam of the guide. For example: Guide's speed, 12 knots; you have 2 knots reserve speed. Where the 14-knot circle cuts the horizontal line at 12 knots, gives 31 degrees as the change of course that will still maintain 12 knots along the course of the guide.
If guide slows to 8 knots, then 55 degrees change of course at 14 knots will give 8 knots along the course of the guide.
In connection with this quadrant diagram, the scale of speeds with times and distances as the elements has been found to be a ready help. For example: You wish to gain a position 1000 yards to the right or left of present position, speed of guide 12 knots, you have 2 knots in reserve. Reference to quadrant diagram where 14-knot circle cuts horizontal line through "12," gives 31 degrees as the change of course that will still advance you at a speed of 12 knots along course of guide, and this gives 7 knots as the sped at right angles to the guide.
The horizontal line through "500" crosses the 7-knot speed line at the 2.1-minute vertical line, which shows it takes 2.1 minutes to gain 500 yards and therefore 4.2 minutes to gain 1000 yards.
The reverse of this is often required at target practice. You are 100 yards outside or inside your line. Speed 10 knots, and you desire to get back in your line in about 1 minute. The 100-yard horizontal line crosses the 1-minute vertical line on the 3-knot line, therefore 3 knots to the right or left is the change you should make. What change of course at 10 knots will give 3 knots to the right or left? The perpendicular through 3 knots crosses the 10-knot circle at 17 degrees in the quadrant diagram, which is the change of course required.
It is desirable to know that a 10-degree change in course gives 1.75 knots to the right or left, and at that speed 100yards will be covered in 1.8 minutes.
These diagrams are useful to find quickly the course to increase or decrease the scouting intervals, and the time to reach the new positions.