Ok so I’ve made a little sketch to hopefully show the point I’m trying to make. (not to scale)
In this situation COLREGS require the watch officer on vessel “A” to alter course to stbd and pass astern of vessel “B”. Problem is that for whatever reason it’s realized too late that a fishing vessel marked “x” puts a kink in that plan.
The issue here is that to be safe the watch officer should be watching the bearing change on F/V “X” visually in case of a change in course or speed because an ARPA vector might not update quickly enough.
But at the same time they are going to want to bury their head in the RADAR / ARPA to compute a course required to pass astern of vessel “B” at the required minimum CPA.
This time pressure and conflict between priorities is going to cause elevated stress in the watch officer which in turn will make it even more difficult to perform both tasks simultaneously.
But both tasks don’t need to be done simultaneously. The watch officer could just wait until the F/V is abeam and then turn the ship till the heading is aft of ship “B”. Then switch to the ARPA and calculate the exact course needed.