NOTE: In all situations the first vector triangle must be completed before attempting the second vector plot.
Before the second vector triangle can be developed, four decisions must be made on how you will avoid a close quarters situation with the contact.
1. How much new clearance do you want to give the contact?
2. When are you going to take the avoiding action?
3. Which side of own ship do you want the contact to pass on?
4. What type of avoiding action will you take - a course change, a speed change, or both a course and speed change?
DETERMINATION OF OWN SHIP'S NEW COURSE OR SPEED TO REDUCE RISK OF COLLISION.
(1) Draw a circle around the center of the plotting sheet equal in radius to the new clearance (label this the Danger Zone). (If the 12-mile distance scale is used and a 2-mile Danger Zone is required, this will be the first ring.)
(2) Locate the Execution Point: This is the contact's projected position provided it continues to advance along the predicted RML. The most practical method is to decide at what distance from own ship the avoiding action will take place, measuring from the center of the plotting sheet to a point on the RML which is the desired distance from own ship. Label this the execution point (MX). Time may also be used to locate (MX) by using the relative speed of the contact to determine how far it will advance along the RML from its last plotted position to the time of execution.
(3) Draw a line from (MX), tangent to the Danger Zone and passing on the side of own ship that you want the contact to clear. This line is the New Relative Motion Line that you want the contact to follow. Label it NRML. Make sure you put an arrowhead at the end of this line, indicating the new direction desired for the contact's relative motion.
NOTE: Make sure the NRML is drawn in the correct direction, if the contact is clear ahead, the NRML must cross own ship's course line. If the contact is to pass on own ship's port or starboard side, the NRML must be drawn to the left or right side of the plotting sheet, as viewed along own ship's course line.
(4) Parallel the NRML to the last position of the contact, and dmy it in, terminating it at the contact's last position (m). Put an arrowhead at this position to indicate the direction of the line. Remember, when you finish drawing the NRML into the first vector triangle, check to be sure it is going in the same direction as the NRML drawn between (MX) and the desired clearance circle. Don't extend this line beyond position (m). This is the New Relative Motion and Reference Line, and is the "Reference Line
for own ship's course and/or speed changes. Label this line NRM.
(5) To determine own ship's new course, at the same speed, put one leg of the dividers on point(e) of the First Vector Triangle and draw an arc, equal in radius to that of the (e-r) line, until it intersects the NRM. Label this intersection point (r'). Connect point (e) with the arc intersection on the NRM and Reference line, point (r'). Terminate this vector with an arrowhead pointing toward (r'). Parallel the (e-r') line to the center of the plotting sheet and read own ship's new course (NC) on the compass rose.
NOTE: In many situations, thc arc's radius may intersect the NRM at two points, either course change will yield the same Danger Zone. You stop and think about the two possible solutions to the Second Vector Triangle. One of the solutions may violate the maneuvering requirements of the Rules of the Road. also, hazards to navigation and other vessels in the vicinity should also be considered in choosing one or more possible solutions which provide the same Danger Zone.
(6) To determine own ship's new speed on the original course, measure the distance from point (e) to where the NRM crosses our (e-r') line. Convert this distance to speed, using the time interval between the contact's first and last positions. In case your (e-r') line is too short to intersect the NRM the (e-r') line should be extended to where it intersects the NRM. This means you need more speed.
(7) Where both new course and speed is desired: first calculate own ship's distance run at the new speed and then arc this new distance around point (e) until it intersects the NRM.
NOTE: For calculating our new speed or new distance run the time used is always the time interval between the contact's first and last positions.
