Lesson 7: Dead Reckoning

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Lesson 7 Dead Reckoning

• Learning Objectives
• Know the six rules of Dead Reckoning.
• Apply proper procedures to become proficient in
  the techniques of maintaining the DR plot.
• Apply the 3- and 6-minute rules in DR plotting.
• Applicable reading Hobbs, pp. 127-128, 132-136,
  141-143.

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Dead Reckoning

• Since the eighteenth century, navigators have
  calculated a ships current and future position
  by a procedure known as deduced - often
  abbreviated as ded - reckoning. The slightly
  altered term dead reckoning is still applied to
  the process.
• Dead reckoning The process of determining a
  ships approximate position by applying to the
  last established charted position, a vector or
  series of vectors representing all true courses
  and speeds subsequently ordered.
• The dead reckoning technique is essential because
  it provides a ships approximate or future
  position. A fix of the ships position only
  indicates the location for a specific point in
  time in the recent past.

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Dead Reckoning

• The Dead Reckoning Plot The symbol used to
  represent a fix on the plot is a circle, about
  one-eighth inch in diameter, placed over the
  intersection of the LOPs used to determine the
  fix. The fix is also labeled with the time it
  was shot. If a fix is shot on the half minute, a
  prime () is used to denote this fact (i.e. 0936
  means 0936 and thirty seconds). A DR position is
  labeled on a chart with a semi-circle about
  one-eight inch in diameter with its time printed
  nearby. The DR position label is further
  differentiated from the fix label by the fact
  that the time of fix is written horizontally
  while the DR time is written at an oblique angle.

4
Dead Reckoning

• Course line Each time a fix or running fix is
  plotted, a vector representing the ordered course
  and speed is originated from it the vector is
  known as a course line.
• The course lines direction represents the ships
  true course.
• The length represents the distance the ship would
  travel in a given time interval at ordered speed.
• In confined waters the navigator will normally
  take a fix every three minutes, while in less
  restricted waters, a fix may be plotted once
  every 15 minutes. The DR is laid out for
  whatever interval the fixes are being taken.

What do we base our interval on?
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Dead Reckoning

• The Six Rules of DR
• A DR position will be plotted every hour on the
  hour.
• A DR position will be plotted at the time of
  every course change.
• A DR position will be plotted at the time of
  every speed change.
• A DR position will be plotted for the time at
  which a fix or running fix is obtained.
• A DR position will be plotted for the time at
  which a single line of position is obtained.
• A new course line will be plotted from each fix
  or running fix as soon as it is plotted on the
  chart.
• Note It is important to remember that the DR is
  only an approximation as it does not take into
  account the effects of wind and current.

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Dead Reckoning

• A sample DR plot is provided below

7
Dead Reckoning

• Deck log entries for the plot
• 0800 Departed anchorage on course 090T, speed 15
  kts.
• 0815 Obtained fix from visual LOPs, Light A bears
  000T,
• Light B bears,020T, Light C bears 340T.
  
• 0830 Changed speed to 10 kts. to avoid sailboat.
• 0845 Obtained fix from visual LOPs, Light C bears
  330T,
• Light D bears,000T, Light E bears 025T.
  Changed
  
• course to 055T, changed speed to 20 kts.
• 0900 Obtained radar fix on brg 010T, range 7 nm.
• Changed course to 065T, changed speed to
  10 kts.
  
• 0915 Changed course to 090T, changed speed to 18
  kts
• 0930 Arrived at operating area S-2.

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Dead Reckoning

• The Track Whenever a ship gets underway, the
  pre-planned DR plot called a track is laid down.
  The track is a form of DR plot, with its course
  and speed vectors representing the intended
  courses and planned speed, rather than the
  ordered course and speed. A small section of a
  track is shown below
• The direction of the vectors are labeled with
  TR to denote the direction of the intended
  movement and SOA, called speed of advance,
  which is the intended speed.
• A DR plot is always plotted in conjunction with a
  pre-planned track. If a fix were to place the
  ships position off to the right of its intended
  track, the navigator would extend the DR course
  line from the fix in the direction of the ordered
  true course suitable to bring the ship back on
  track.

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Dead Reckoning

• Three and Six Minute Rules
• Three Minute Rule
• Distance traveled in yards in three minutes
  Ships speed in knots x 100
• Six Minute Rule
• Distance traveled in miles in six minutes
  Ships speed in knots x 1/10