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Distance Off and Quick EP’s – Answers

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

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?

0.2 nm

Based on that distance, what is your approximate distance off to the silo.

2 nm

What is your estimated position?

41º 27.8’N 71º 12.3’W

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?

2 nm

Do these two estimations match (Both finger and hand)?

Yes

If you continue on this heading, assuming there is no current or leeway, will you miss the rocks?

Yes

Step by Step Explanation

  1. Measure the distance from the silo to the southern most tip of the penunsila. As a hint, if your finger is on the southern most tip and your finger forms a range with the radio towers on the Newport coast, then the tip is on that range as well. You can plot that LOP between the towers and the tip of the penuninsila. Measure the distance with your dividers between the silo and the tip of the penunsila (in range with the towers) and you get .2 nm.
  2. Multiply that distance by ten. (.2 nm x 10 = 2 nm)
  3.  Determining your position you should come up with 41º 27.8’N 71º 12.3’W or within .2 nm.
  4.  Calculate this by multiplying the distance on the chart from the silo to the G “1” BELL buoy (1.2nm) and the distance between you hand and eye (6.66 inches) and then dividing the product by the width of your hand (4 inches). ( [1.2 nm x 6.66 in = 7.992] / 4 in = 1.998 nm rounded to 2 nm) NOTE: The inches are irrelevent in this equasion, you’re looking for ratios in nautical (or statute depending on where you are) miles.
  5.  Yes – they are both 2 nm.
  6.  Yes, if you continue at 282º PSC (270ºT) you’ll pass between the rocks and the tip of the penninsulia at about .2nm (.1 nm if you’re working off the cross rather than the green squigley) north (Whew! That was close!).

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