Quick Tips to Ponder Part 4 – Navigation

Estimating Distance Off

In clear weather, one can distinguish the shapes of tall houses, trees, lighthouses, etc. from about 8 miles offshore. The distance to the horizon however can be quite small. if, in a small boat, your eye is 5 feet above the water level, the distance to the horizon is only 2.5 miles away. Although the distance can be more accurately estimated using the formula

1.17 times the square root of your height of eye = Distance to the horizon in nautical miles

Some rules of thumb you can use are as follows:

  • A light-colored beach can be seen at approximately 4 miles offshore from the deck of a typical small boat.
  • Individual windows in buildings can be distinguished by day or night at about 2 miles off.
  • A large buoy is visible at 2 miles.
  • A small buoy is visible at 1.5 miles but color and shape will not be clear.
  • The shape of a small buoy can be distinguished at about 1 mile.
  • The color of a large buoy can be distinguished at about 1 mile.
  • A person is seen as a moving black dot without limbs at 1 mile.
  • Movement of a person’s legs or arms can be distinguished at about 400 yards.
  • Faces can be seen, but not necessarily recognizable, at 250 to 300 yards.

Pencil Radar

It is possible to use something as simple as a pencil to determine approximate distance off. In order to use pencil radar:

  • Locate an object that can be seen onshore which is shown on your chart with a charted height.
  • Hold a pencil up at arm’s length and site the top of the object with the tip of the pencil with one eye.
  • Switch eyes.
  • You should estimate the horizontal distance (HD) that the tip appears to have jumped relative to the known height of the object. For example, if you are spotting the top of a spire on a mountain (as shown on your chart at 600 feet), when you shift eyes the pencil appears to shift twice the horizontal distance as the height, you would have an estimated horizontal distance of 1200 feet.
  • Now you simply use the formula D (Distance Off in feet) = HD (in feet) X 10. In our example, if the spire was 600 feet and the horizontal distance (HD) is 1200 feet, the distance off the spire = 1200′ X 10 = 12,000′ or 2 approximately miles.

Navigating By Eye

When navigating in areas with uncharted coral reefs it is a good idea to wait until the sun is high and behind you – from 10AM to 4PM. (This is why charter companies encourage you to anchor no later than 4PM.) Height above deck is an advantage to the spotter and vision can be improved with polarized sunglasses. Calm, grey days are the most difficult when trying to look deep into the water. As a rule of thumb in reading the water:

  • Dark blue tones mean deep water, 20 fathoms or more.
  • The blue becomes lighter with decreasing depth, and the turquoise (green-blue) is a warning of shoaling. it is the color of the coral sand covering a flat expanse of reef with 4-6 feet of water coverage.
  • Dark brown indicates coral heads.
  • Brown or yellow indicates reefs with a depth of 3 or 4 feet over them.
  • Green-brown means a grassy bottom.
  • White means very shallow water.

Piloting Using Echoes

As kids we all thought that echoes were fun and interesting but did you know you could use them in piloting? Note the time in seconds from a signal to the return echo from a cliff, iceberg, wharf, or moored freighter. Every second’s delay indicates a distance off of 1 cable, or 200 yards. Every 10 second’s delay indicates a distance off of 1 mile.

This rule could be useful in fog some day. A blank pistol shot produces a sharp echo, but the ship’s bell or horn will work as well. Even a loud hailer works in close quarters. The Rule of Thumb at work here is that sound travels about 1 mile in 5 seconds.

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Filed under Boat Operation, Boating News, Boating Safety

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