Rules of Thumb

Thumbs

This may be apocryphal, an urban legend that goes back to Amerigo Vespucci, or maybe to Henry the Navigator, but the story is that the original Thumb in the original Rule of Thumb, was the Navigator's. And he used his illustrious thumb to keep his vessel off rocks and shoals by laying it adjacent to a hazard on a chart and setting his course to the outside of said thumb.

This worked and works pretty well. As the scale of the chart gets bigger, the thumb covers a smaller geographic area. On small scale charts where continents are but inches, the thumb provide miles and miles of clearance. On a 1:80,000, my thumb covers about a mile. And that seems about right for planning purposes.

Rules of Fingers

Sunset is about 20 minutes off when the sun appears tangent to three fingers held at arms length - parallel to the horizon

The width of one finger held vertically at arms length subtends about 4 degrees of arc

Two fingers held tightly together at arms length covers 10 degrees of arc

Four fingers together with thumb extended: 25 degrees

The rules aren't accurate enough for navigation, but useful when someone says, "Look right 15."

Estimating the Wind

If you see small wavelets, crests glassy with no breaking, you're in 4-6kts.

Large wavelets, crests begin to break, scattered whitecaps, that's 7-10

Small waves 1-4 ft. becoming longer, numerous whitecaps: 11-16

Larger waves 8-13 ft, whitecaps common, more spray: 22-27

For the rest, the terrifying part, click Beaufort Scale

Distance with the Right Rev. Isosceles

How far am I from the beach?

Distance off can be estimated by taking two bearings on the same object several minutes or more apart. The navigator takes the first bearing when the object lies at an angle of 45 degrees off the bow.

He starts his clock, DR tracer, GPS or some other device for measuring distance and waits until the object is 90 degrees off the bow.

The distance traveled between the 45 degree mark and the 90 degree mark is equal to the distance from the object.

Here are a couple of more distance estimators:

You're in to less than a mile when you can discern individual trees.

You can see individual windows in a house just inside two miles

When the junction of beach and water becomes indistinct, you're about 3 miles off

If you're 6' tall standing on a deck 4' above the water the horizon is almost 3.7 miles off.

If you're 5' tall, the horizon is 2.6 miles away.

Near the top of a 60' foot mast, the horizon is 9 nm and change.

Swim Call

MOB in the water when sea temp is 50-60 degree F can probably keep swimming 1-2 hours, when they're likely to lose consciousness.

They are likely to survive 1-6 hours if wearing a Type I pfd or other device that will keep the head out of the water. See Cold Water Survival for the rest of the chilling table.

Anchor Holding

With a rope & chain rode, a scope of 4:1 yields about 55% of the maximum holding power of the anchor. At 6:1 it's about 70%, 8/1: 80% and with 10/1 about 85%. That's the practical maximum.

One hundred percent holding would be achieved with a horizontal rode. That would require infinite rode or could occur when the bitter end is not secured and the whole mess is lying on the bottom.

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