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Lesson 15: Voyage Planning and Time

Lesson 15: Voyage Planning and Time. 1/2/2020. Lesson 15: Voyage Planning and Time. AGENDA: ETA/ETD Determining Zone Time Date-time Group (DTG) Format Planning an Extended Ocean Voyage Plotting a Great Circle Route OTSR Applicable reading: Hobbs pg. 252-274. Time Terminology.

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Lesson 15: Voyage Planning and Time

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  1. Lesson 15: Voyage Planning and Time 1/2/2020

  2. Lesson 15: Voyage Planning and Time • AGENDA: • ETA/ETD • Determining Zone Time • Date-time Group (DTG) Format • Planning an Extended Ocean Voyage • Plotting a Great Circle Route • OTSR • Applicable reading: Hobbs pg. 252-274.

  3. Time Terminology • Estimated Time of Arrival (ETA): the time and date of arrival that must be achieved. Usually specified by higher authority. • Estimated Time of Departure (ETD): a computed estimate of the time and date of departure that will allow the ship to arrive on time.

  4. Zone Time • The rate at which the sun moves across the sky varies day to day due to the earth’s elliptic orbit about the sun. • This makes use of actual time rather difficult. • Instead, the theoretical mean sun, which passes completely around the earth once each 24 hours, is used to measure time.

  5. The Mean Sun & Zone Time • Thus, the mean sun completes one circuit around the earth every 24 hours, or 15 degrees of longitude each hour. • Each 15 degrees of arc is a time zone. • Within each zone, time is reckoned according to the position of the mean sun in relation to the central meridian of the zone.

  6. Greenwich Mean Time (GMT) • Centered on prime meridian. • Extends 7.5o east and west of the prime meridian. • Referred to as “zulu” time, because the time zone is designated by “Z.” • Each additional time zone (as we move east or west) has a boundary every 15 degrees past 7.5o.

  7. Figure 15-1 in Hobbs (page 254)

  8. The International Date Line • The 24th time zone is split in half, with each zone on either side of the 180th meridian being only 7.5o wide. • To remember what day it is on each side of the line, the phrase “San Francisco to Manila, Sunday to Monday” is used

  9. Determining the Time Zone • To determine the time zone, simply divide the longitude of a location by 15. • After this is done, round to the nearest whole number. • For eastern longitudes, place a negative sign (-) in front of the number; for western longitudes, place a positive sign (+) in front of the number.

  10. Time Zone Example • The longitude of Norfolk, Virginia is 076o 18.0’ W. What time zone is this? • Answer: • 076o 18.0’ =076.3o • 076.3/15 = 5.09, or rounded to the nearest whole number, 5. • Since longitude is west, time zone is +5. • The time zone indicator letter for this zone is R, so the zone is completely described as +5R

  11. Daylight Savings Time • As an example, when Norfolk observes daylight savings time, it keeps +4Q instead of +5R time. • The time kept at any particular location and time of the year can be determined from either Coast Pilots or Sailing Directions.

  12. Time Conversions • During voyage planning, all times are normally expressed in terms of GMT to avoid confusion. • As necessary, times are converted to the local time zone for ease of use. • The formula for all conversions is: • zone time + zone description = GMT

  13. Time Conversion Example • It’s 0800 (local zone time) in Naples, Italy. What is this time, expressed as GMT? • Answer: • Longitude of Naples is 14o 16’ E, so the time zone is -1A. • GMT= ZT + ZD = 0800 - 1 hour = 0700 Z

  14. Time Zone Conversions • Note that the date may change as a result of the time zone conversion. • For example if it is 0030A 30 June in Naples, this translates to 2330Z 29 June.

  15. Date Time Group (DTG) • Official Navy format for expressing time and date. Also in all naval message traffic. • An example: 171300SOCT00 translates to: 17 1300 S OCT 00 DAY 4-DIGIT TIME 3 LETTER LAST TWO OF TIME ZONE MONTH DIGITS OF YEAR MONTH INDICATOR

  16. Shipboard Time • During a transit, the ship’s clocks are set to the time zone in which the ship is located. • As the ship transits eastward, clocks are periodically advanced 1 hour to conform to the proper time zone, and vice versa for westward travel.

  17. The Voyage Planning Process • Obtaining and updating charts and publications. • Determination of arrival and departure dates. • Plotting the intended track and use of Optimum Track Ship Routing (OTSR).

  18. Homework Chapter 15: Omit the following: Section 1: #1, 2 Section 2: #1-4, 7, 8, 9, 12

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