Unformatted text preview: Voyage
Time Voyage Planning and Time AGENDA: – ETA/ETD
– Determining Zone Time – Datetime Group (DTG) Format
– Planning an Extended Ocean Voyage
– Plotting a Great Circle Route
– OTSR Time Terminology
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. Zone Time
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. The Mean Sun & Zone Time
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. 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.
– Exceptions are M and Y around the International Date Line (180th meridian) Figure 15-1 in Hobbs (page 254) The International Date Line
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 Determining the Time Zone
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.
– “West is Best, East is Least” Time Zone Example
Time Zone Example The longitude of Norfolk, Virginia is 076o 18.0’ W. What time zone is this?
– 076o 18.0’/15 = 5 with 1o 18’ left Be mindful of the remainder. If it’s over 7.5o then you are in the next time zone – 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 Figure 15-1 in Hobbs (page 254) Daylight Savings Time
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. Time Conversions
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 Time Conversion Example
Time Conversion Example It’s 0800 (local zone time) in Naples, Italy (longitude 14o 16’ E). 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 Time Zone Conversions
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. Date Time Group (DTG)
Date Time Group (DTG) Official Navy format for expressing time and date. Also in all naval message traffic. An example: translates to:
TIME 271115ZSEP02 Z SEP TIME
DIGITS OF YEAR Shipboard Time
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. The Voyage Planning Process
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). OTSR
OTSR Advisory service – Time and fuel savings – Minimize storms, high seas, ice risks 1500 miles or more WEAX/AVWX not automatic Message request 72 hrs in advance of underway Advice covers 36 hrs prior to and thru underway Include “OTSR” in MOVREP Questions?
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- Spring '14
- Greenwich Mean Time, Time zone, DTG, Hobbs