Set and Drift or Direction and Rate Problem

Just for practice – solve the following problem on the Long Island Sound Chart. This is the chart that we use in the NauticEd Coastal Navigation Course.

 

At 2245 your GPS fixes your position at LAT 41 deg 01.75′ N and LONG 72 deg 48.40′ W. You are steering course 086 deg psc at a speed of 6.0 knots. At 2400 you fix your position at LAT 41 deg 04.2′ N and LONG 72 deg 38.85′ W. What were your set and drift?

Use the following
(1) Here is a pdf of the chart for you to work on
(2) On the chart, the variation is 14 deg W
(3) Since the problem says psc (per ships compass) we need to account of the ships compass deviation. In the NauticEd Coastal Navigation course exercises we used the following table.

 

Ships Compass Deviation Table

HDG MAG DEV HDG MAG DEV HDG MAG DEV
000° 2.0°E 120° 1.0°E 240° 3.0°W
030° 3.0°E 150° 1.0°W 270° 1.5°W
060° 4.0°E 180° 2.0°W 300° 0.0°
090° 2.0°E 210° 3.5°W 330° 1.5°E

Ans:

Set up the TVMDC table

T 074
V 14W
M 088
D  2E
C 086

Thus, your True heading on the chart is 074 deg T. Your water speed along this line is as given is 6 knots.

The time difference is 1 hour and 15 minutes = 1.25 hours. Thus in 1 hour and 15 minutes, you would travel 7.5 nautical miles.

Scribe a line 7.5 nm from the origin along 074 deg T line. This is your water position. The ground position is described by the GPS coords. Draw a line from your water position to the ground position. This is your 1.25 hour long current vector. It is headed due north and is 0.5 nm long. Since this happened in 1.25 hours the current speed is .5/1.25 = 0.4 nm/hr (knots).

Thus:

  • Set (or Direction) is 0 deg T
  • Drift (or Rate) is 0.4 knots

Note: Current is always expressed in deg True and always expressed in the direction it is heading whereas wind is expressed as where it comes from. Note and remember the difference – important.

No Cheating – do the problem first – here is the answer plot

Here is the real answer plot.

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