105.3 Dead Reckoning Flashcards
“Dead Reckoning”
Uses only two factors – direction the boat is steered, and the distance it has travelled
DR Direction is…
defined by magnetic rose on chart
DR Distance…
is simply the number of miles the boat has run through the water
Direction sailed + distance sailed will give you a
DR position
What does DR not factor into its calcuation?
currents and leeway
To lay out a DR plot (three steps)
- Draw a course line(s) between desired locations
- Measure direction of each line using magn rose
- Measure the distance along each line
Notations on a DR plot line would include
Course C095M - written above the line
Speed S6.0 – written below the line
Distance (D=10NM) – written to the right of speed, enclosed in parenthesis
Can estimate time from S and D
Tidal currents increase in strength when passing through…
narrow passages (Golden Gate 6 knots tidal current)
Current that are open to the sea in non-narrow passages will typcially flow at
about one knot
A taffrail log was much like a car’s
odometer – spinning torpedo in the water directly increasing a guage on-board
What’s the difference between a knot meter vs. a knot log
Car’s speedometer vs. odometer
Most important habit in DR is…
recording the time of any DR position along the way (waypoint, passing something, changing course)(
In noting time on a chart customary to use
24 hour clock
ETA
Estimated time of arrival (ETD + ETE)
ETE
Estimated time en route
ETD
Estimated time of departure
ETE is always measured in…
Hours and tenths of an hour, not hours and minutes (e.g., .5H equals 30 minutes; .2H equals 12 minutes
“Yawing” is the….
… normal and unavoidable back and forth from a desired course, but ideally evens out in non-rough seas or wind
Factors affecting accuracy of DR (besides current and leeway)
yawing; inaccurate log readings; speed variations
What is the rule of thumb of accuracy of a DR plot?
Accuracy within 10% of the Distance RUN
Why is it called “Dead Reckoning”
It was originally “Deduced Reckoning” (deduced from direction and distance), then abbreviated Ded., hence Dead…
Instead of plotting a straight course to a destination, for a safety valve, select a direction ____, then look or turn ____
5 to 10% UPWIND/WINDWARD of the target; downwind to find intended location
A DR safety valve route is useful (three situations)
navigating through fog
navigating at night
approaching an unknown coast
What’s different about using DR in heavy weather or heading upwind
Have to create the DR plot lines AS YOU TRAVEL, recording the tacks made to either sail up wind or steer the boat into heavy water.
When using DR, at the very start do two things:
- Set the log to zero or note current reading (e.g., the start trip odometer reading)
- Note the exact time of departure from starting point
When DR upwind, sail as ____ ______ as possible, and note the average ______ at that point, then ___ that heading as the first DR _____. When needed, tack the boat, noting the _____ run on prior course and noting ____ ____ on the chart. Rinse and repeat.
close-hauled; heading; plot; course; distance; “tacking point”
Three cautions on using DR: 2 wind, 1 water
- No allowance for leeway (4 to 8 degrees on sailboat) (wind)
- Works well when no currents (water)
- Winds can shift (wind)
A useful thing to do is __-___ DR lines on most frequently used charts
pre-plot
Be certain a DR plot line does not cross…
dangerous or restricted areas
Helpful to plot DR lines from ____ to ____, when possible
buoy to buoy
Particularly in fog, you should double check your boat compass with
a second and even third compass
If boat has a knot meter, record the ____ ____ on the chart near the plot line
average speed
Plot position as a pre-set time _____
interval; set timer to do next recording
For nighttime navigation, plot the circle showing the likely distance from a ____ ____ ____ as the radius of the circle
known light source
If off course, when should you plot a new DR course?
If your course to return to your track is more than 10 degees different, then plot a new DR. If it is not so radical or your destination is a long way off, “walk” back to original DR line with 5 to 10 degree correction.
