Thought recycle/share some stuff I posted on in our Seamanship forum.
Most of you probably know this, but from some of the reactions of other skippers on the race
course, apparently it's not common knowledge. So many times you know you're going to clear that stand-on vessel, but he freaks out anyway, alters course and protests. Those are the ones that don't know about bearing drift. And some of you may be self-taught and new to sailing. I've also talked with experienced sailors that think they just "know" if they're clear but can't express why. Their brain is looking at bearing drift without them knowing.
So, in short, you can use bearing drift to figure out if you're on a collision
course with another vessel.
- Look past the boat in question and find a reference. The farther the better. Land is good, celestial bodies are best, a static cloud formation is okay. Pick something that stands out like a radio
tower. A hand held compass
is of course great, but isn't always handy and distracts you if you're at the helm
- As you sail on a steady course, if the boat is drifting to the left, he will pass to the left. If the boat is steady on the reference, you will collide. In the Navy
it's called "CBDR". Constant bearing Decreasing Range.
Notes: This is for "eyeballing" it. You have to inject some logic and take appropriate action. If the bearing drift is slow, it will be a close meeting. If the vessel you're looking at is a 100ft long, you could have left bearing drift on the bow and right drift on the stern... meaning you're on a collision
course with the middle. So take the drift of the part of the boat you need to clear. Also consider where your eye is. If it's a close one and you're spotting form the bow, you probably want to be sure that the stern of you boat will clear as well. In racing
you're usually checking if your stern will cross his bow without a problem. Lastly, if either boat changes course or speed, it's a whole new ball game
That was an early seamanship lesson at the Naval Academy and I shared that to a few wide-eyed sailors. You could see the light bulb over their head
when it registered. Of course the Skipper
of High Flight is a pilot so it was old news. The pilot equivalent is putting yout finger on the canopy covering the contact. If he's still under your finger after a bit, you're on a collision course.
Frankly I wish everyone on land knew this because it works for driving and walking too! I hate having to dodge morons in a crowd because they can't figure that out. 8)