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Posted by on Apr 10, 2013 in Boating Law, Do It Yourself, How To, Powerboating, Safety and Seamanship, Sailing, US |

Collision Course with a Crossing Boat? How to Know

Here are a couple of ways to manage the chronic headache of 'constant bearing, decreasing range.'

converging boats

The bearings of these two boats aren’t changing as their distance apart decreases. They’re converging on a single collision point. The give-way boat should take action to avoid the stand-on boat, and make the action obvious so there’s no doubt in the mind of the stand-on helmsman.

The other day I wrote about a seemingly goofy way of using thumbs and fists to measure angles on the horizon and show people where distant objects are. To follow up, I thought it would be good to review how to tell if you’re on a collision course in a crossing situation.

For people new to boats, or who don’t have much experience at the helm, it can be a bit alarming that boats on the open water are allowed to scamper around with no traffic lanes, no stop signs, and no traffic lights. The Rules of the Road are there for a reason, and anyone at the helm of a boat should know them. However, knowing them and knowing how to gauge situations and act on them are different things.

One of the most vexing problems for new boaters is the crossing situation, and knowing how to judge who’s going to cross whom. If it’s not obvious, then it usually comes down to the concept of “constant bearing, decreasing range,” which is the perfect definition of a collision course. Here’s how to tell:

  1. Hold your course.
  2. Take a bearing on the other boat, either by sighting across an open compass or with a hand-bearing compass.
  3. A little while later, take the bearing again.
  4. If the second bearing is the same as the first, your courses are converging on a single point. It will be a collision or a close call.
  5. If you’re the give-way vessel, start altering your course sooner than later, so the other boat knows what you’re up to. If he’s the give-way vessel, watch him like a hawk and be prepared to avoid him. Most people on the water don’t appear to know the right-of-way rules.
  6. If the bearing is changing, you won’t collide. The faster the bearing changes, the farther apart you’ll be when you cross.

But wait, there’s more. If you can see land behind the boat you’re converging with, you can gauge the danger of a collision without compass bearings. Some people call this concept “gaining land” or “making land.”

  1. Again, hold your own course steady.
  2. Watch a feature of the distant land near the bow of the crossing boat, and keep watching.
  3. If that land feature disappears behind his bow, he’s gaining land on you, and will cross you. The faster the land disappears behind him, the more distance he’ll cross you by.
  4. If the land feature draws ahead of his bow, as if he’s falling behind on the land from your perspective, then you’re gaining land on him, and will cross him.
  5. If the land feature stays right where it was on his bow when you first started looking at it, then you’re on a potential collision course. See point five above.

Radar, AIS, and other electronics can help you gauge range and bearing to other vessels, but they all require you to take your eyes off the water and look at them. In normal boating situations in decent visibility, and especially if there are a lot of boats milling around, your eyes, your compass, and fixed references will serve you better.

If you have another way of judging who’s ahead in a crossing situation, tell us about it in the comments section.