This information, in different form, was originally published on boats.com.
It's not always easy to tell someone how to spot something far away. Mariners, like carpenters and tailors, can estimate with their thumbs.
There are plenty of times on boats when a lookout has to relay a sighting on the horizon to someone else on board. This can be tricky if the second person can’t focus right where the lookout is pointing.
Hand-bearing compasses and open binnacle compasses are the most accurate tools for sighting angles, but for quick-and-dirty measurements the handiest thing is...your hand. A hand or fist held at arm’s length covers about 8-10 degrees of the horizon. A thumb’s width covers about 2-3 degrees. But you don't really need to think about numbers. Most people’s hands and thumbs are close enough in size and length to transfer effective ballpark estimates back and forth.
If there’s a reference point close to the object you’re sighting, you can just call back to the cockpit, “It’s two hands to the right of the light. " Or, "It’s just a thumb to the left of that anchored blue boat, maybe a mile beyond.” And the helmsman or navigator can put his or her own hand up and orient their search to the same criteria.
Two things to remember: Sight with one eye closed to avoid parallax confusion, and keep your hand at arm’s length when sighting.
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:
- Hold your course
- Take a bearing on the other boat, either by sighting across an open compass or with a hand-bearing compass
- A little while later, take the bearing again. 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.
- 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. A lot people on the water don’t know the right-of-way rules, or don't know how to put them into action.
- 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.”
- Again, hold your own course steady.
- Watch a feature of the distant land near the bow of the crossing boat, and keep watching.
- 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.
- 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.
- If the land feature stays right where it was on his bow when you first started looking at it, then you’re on a collision course, or one that's too close for comfort.
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.