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Distance Off and Quick EP’s

Chris Riley by Chris Riley Updated on August 16, 2019. In navigation

There are many times when knowing the distance off to a fixed object will be of considerable value to the navigator making his way through crowded channels or just cruising off the coast on a sunny afternoon. You may want to print this section out and put it in your navigation notebook for future reference.

What if you were cruising around the Sakonnet River just south east of Sachuest and wanted to make sure you were going to avoid Cormorant Rock (approximately 41º 27.6’N 71º 14.9’W) because you have a fondness for the shape of your keel and don’t really want it altered by running aground…

There are, of course, several different ways to make sure you give the hungry rock a wide berth. You can take a relative bow and beam bearing off the silo on Sachuest peninsula at 45º and 90º as was described in the Bow Beam Bearings Navigation Question, however, since the rock in question is almost directly south of Sachuest — using a 90º relative beam bearing to the silo would most certainly put on the same LOP as the rocks (or on top of them!) — exactly where you don’t want to be. You can still use two relative bearings, as long as the second bearing doubles that of the first to accomplish the same thing (best between 30º and 90º when the object is to starboard – i.e.: 30º/60º, 40º/80º, 45º/90º and keep an equal relationship to the starboard bearings when the object is to port – i.e.: 300º/330º, 280º/320º, 270º/315º…). Or, you could estimate your distance off to create a COP from the silo and be sure to stay outside of that COP. If you’ve ever heard the silly anatomy trivia like your foot is just as long as the space between the crook of your elbow and the bend of your wrist, you understand the natural ratios that exist in the human body. A quick, simple way to determine your distance off works based on the odd tidbit of trivia and physiological assumption that almost everyone’s arm is ten times longer than the distance between their eyes. Extend your right arm straight out in front of your face holding up a pencil or your finger and close your left eye. Place the pencil or your finger on the object that you are sighting, then switch eyes — open the left and close the right keeping your arm very still. Next, estimate how far the pencil or finger appeared to move away from the object you are sighting. That distance in feet, yards or miles is then multiplied by ten to give you your distance off to that object.

Try it right now — extend your arm from your position in your chair, hold up your finger and sight off something across the room: your door jamb, a corner, a bookshelf – anything. Now switch eyes and estimate how far your finger moved. Now multiply that by ten and that is your rough distance to that object. Find a measuring tape and check to see how close you were.

Working of this same basic idea, you can also determine distance off by knowing the distance between two charted objects and using the your whole hand rather than just a finger. In our example above on your 1210TR chart you have the silo at Sachuest and the spire off Easton Point. They are approximately 1.6nm apart. To determine your distance off, extend your arm again, this time holding your entire hand up, palm flat and thumb tucked in. Close one eye and bring your hand closer to your face until it covers both sighting objects simultaneously (you may have to tuck in some fingers or spread them apart to cover both objects). Next, measure the distance your hand is from your eye and the width of your hand. Take the known distance in nautical miles and multiply that by the distance from your hand to your eye, then take that product and divide it by the width of your hand. This seems a bit complicated, but after you’ve used the method several times, you’ll be able to quickly recall the width of your hand and be able to judge the distance between your hand and eye to achieve a fast distance off.

As in the example above, you know the distance between the silo and spire is 1.6nm. Your hand is 4 inches across your palm and you are holding it 10 inches from your eye to cover both objects. What is your distance off?

Take 1.6nm (the distance between the charted objects) times 10 (the distance between your hand and eye) and then divide that by 4 (the width of your hand) and you should get 4nm.

Once you have determined your distance off to a charted object, you can put a COP on the chart. Then, take a compass bearing to that same object and place an LOP on the chart. Where the LOP and COP meet is your EP – easy peasy…

Recap: to determine a distance off using the “blink” method:

  1. Extend your arm, holding up a pencil or your finger and close the opposite eye.
  2. Place the pencil or your finger over the sighting object.
  3. Switch eyes, keeping your arm still.
  4. Estimate the distance in feet, yards, or miles that the pencil or your finger appeared to move.
  5. Multiply this estimate by ten and that is your distance to the object.
  6. To estimate your position, draw a COP on your chart based on the distance off you determined above, then take a compass bearing to the sighted object. Where the LOP from your compass bearing meets the COP is your estimated position.

Recap: to determine a distance off using the “hand” method:

  1. Using your chart, determine the distance in nautical miles (or statute miles depending on your location) between two visible objects in near proximity.
  2. Extend your arm, holding up your hand and close one eye.
  3. Move your hand closer to your eye until your hand completely covers both objects (you may have to tuck in some fingers or spread them apart).
  4. Measure the distance between your hand and your eye.
  5. Measure the width of your hand that it took to cover both objects.
  6. Multiply the charted distance by the distance from your hand to your eye, then divide that product by the width of your hand. The answer is your distance off in nautical (statute) miles.
  7. To estimate your position, draw a COP on your chart based on the distance off you determined above, then take a compass bearing to the sighted object. Where the LOP from your compass bearing meets the COP is your estimated position.

Test your new-found knowledge of quick Distance Off methods.

Using your 1210 TR training chart solve the following problem:

You are cruising south east off Sachuest and your binnacle compass reads a course of 282º. Your boat speed is 5 kts. On this heading you have a deviation of 3ºE. You note on the chart the rocks about .8 nm south of Sachuest and want to make sure you don’t hit them. You quickly spot the silo on Sachuest and hold up your right arm and finger, closing your right eye. You line up your finger on the silo and switch eyes. Your finger winds up right on the southern most tip of the Sachuest peninsula (almost forming a range between your finger and the radio towers on the western shore of Newport). Next, you take a bearing over the ship’s compass of 303º to the silo.
What is the approximate distance your finger moved?
Based on that distance, what is your approximate distance off to the silo.
What is your estimated position?
Using the same scenario, you decide to test the “hand” method using the silo and the buoy G “1” BELL. Your hand is 4″ wide and about 6.66 inches from your face to cover both the buoy and the silo. Your compass bearing to the silo is again 303º, your deviation is the same and of course your variation is listed on the chart.
What is your distance off to the silo?
Do these two estimations match (Both finger and hand)?
If you continue on this heading, assuming there is no current or leeway, will you miss the rocks?

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