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Posted by on Mar 27, 2013 in How To, Powerboating, Sailing, UK, US |

Rule of Thumb: Distances and Angles on the Horizon

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. (Picture a lookout on an aircraft carrier whistling the admiral over: “Yessir, just put your chin on my shoulder and follow my finger. See that leeeeetle dark speck over there under the cloud that looks sort of like a dog under some broccoli?”)

Even with established verbal references it can be frustrating, especially if the second person isn’t familiar with the deep-water custom of using angles off the bow or beam. Old-timers used points off; more modern mariners use degrees, but even experienced people have trouble following a verbal cue down to a few degrees.

Rule of thumb channel markers

Going out the channel, the first green can is about a thumb to the right of the big island, and about a fist to the right of the nun. The second green can is about half a thumb to the right of the nun, a couple hundred yards farther away.

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 closed fist held at arm’s length covers about 8-10 degrees of the horizon. Three fingers near the tips cover about 4-5 degrees. A thumb’s width covers about 2-3 degrees. Go with the lower numbers for smaller hands. Most people’s fists, fingers, and extended arms are close enough in size and length to get these estimates well within the ballpark.

You can even dispense with degrees if there’s a reference point close to the object you’re sighting. Then you can just call back to the cockpit, “It’s two fists to the right of the light. It’s just a thumb to the left of that anchored blue boat, maybe a mile beyond.”

Two things to remember: Sight with one eye closed to avoid parallax confusion, and keep your hand at arm’s length when sighting.

If you have some rule-of-thumb tips to share, put them in the Comments below.

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  1. Boats.com Blog » Blog Archive How to avoid a collision course with another boat - [...] other day I wrote about a seemingly goofy way of using thumbs and fists to measure angles on the …