Dead reckoning, piloting

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Dead reckoning, piloting

• John Huth

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Topics

• Measuring speed of boat
• Currents
• Leeway
• Errors from dead reckoning
• Piloting
• Curvature of the earth
• Landfall

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Forces on boat

• Wind
• Resistance on hull
• Current
• Lee-way (wind on sailing vessel)

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Forces on a sailboat, and resultant motion
Direction of motion
Leeway
Steering direction
Drag
Lateral force from keel
Wind
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A big factor in hull resistance is caused by the
bow wave and stern wave, creating a wake. The
number of wavelengths between the bow-wave and
stern-wave partly determines resistance. A
good navigator can estimate hull speed from the
shapes of the bow wave and stern wave.
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Estimating speed relative to water
Boat length L
Start counting now
Piece of flotsam in water
Stop counting now
Speed is L/time
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Ships log (or chip log) The log gets thrown
over the stern of the vessel as the line gets
played out, sailors count the number of knots
that pass the stern for a fixed period of time.
(where the term knot for nautical mile
comes from). This can be easily improvised.
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How Polynesians estimated currents
Current direction
Initial bearing
Initial position
Final bearing
Position after drift
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Estimating leeway
Slick of calmer water
Wake is tilted
Water piles up higher on bow
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Compensating for leeway and current
Current
Desired heading
Leeway
Actual motion
Direction of travel
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Example compensating for current in a blind
crossing
Current draining bay 1 knot
Heading we chose
Desired heading
Speed 4 knots
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Errors (uncertainties) in position in dead
reckoning
Uncertainties in speed and heading are typically
a fixed percentage so, as a journey
progresses, the numerical uncertainty in position
gets larger as time goes on
error -12o
Uncertainty in heading
Desired heading
error 12o
Uncertainty in speed
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Expanding the target of landfall
Color of the sea Birds Clouds Mountains
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Color of the sea

• Deep sea is typically a dark blue
• Reflects the color of the sky, plus absorption
• Depends, in part on content of algae
• Color in shallower waters are a combination of
  factors
• Color of the water itself
• Color of the bottom (sand, rock, etc)
• Examples
• Tropical waters can be azure

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Importance of local knowledge satellite photo of
Black sea and Mediterranean
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Range formula for objects
Curvature of the earth causes objects to be
hidden by the horizon
D distance of object in nautical miles Hheight
of object in feet h height of observer in feet
D
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Ship hull down in the distance
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Sequence approaching an island Far distance
deep blue color
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Sixty miles out spot non-pelagic birds, steer
toward land
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Forty miles out color of sea changes
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Thirty miles out clouds appear over land
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Twenty miles out two islands?
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Ten miles out one island