There are situations where it is useful to quickly and precisely derive a reverse course. The scariest scenario is a man overboard when shorthanded.
On a small boat there might only be two persons on board. While underway, the crewmember might go over the side without the helmsperson even realizing it. Imagine the dread when you turn around from the helm and your crewmember is not aboard!
In addition to marking your current location on the chartplotter, it is important to immediately note your heading. Then you must establish a reciprocal course and estimate how much time has elapsed since you last saw or talked to your teammate.
OK, so what is the quickest way to figure out the precise reciprocal course without adding or subtracting 180? It’s very simple:
“Add Two, Subtract Two”
Example – your heading is 160, the reciprocal course is 340. You add 2 to the first digit, and subtract 2 from the second digit.
In cases of larger initial headings, the total would be over 360 which would be nonsensical. In those cases you subtract first:
“Subtract Two, Add Two”
Example – 245 on the compass – the reciprocal is 065. Subtract 2 from the first digit, and add 2 to the second digit.
In this way you can figure out your exact reciprocal course in a couple of seconds!
There are a few sectors on the compass where you need to adjust the method a bit. For headings between 100 and 119 degrees, there is some additional mental math. Think about it and you’ll see. Same for 000-019, 180-199, and 280-299.
NOTE – Remember that there is a difference between course and heading. The course is what you intend to steer, and the heading is which way the boat is pointed at any particular instant.