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Dead reckoning, piloting. John Huth. Topics. Measuring speed of boat Currents Leeway Errors from dead reckoning Piloting Curvature of the earth Landfall. Forces on boat. Wind Resistance on hull Current Lee-way (wind on sailing vessel) . Forces on a sailboat, and resultant motion.

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Presentation Transcript
  • Measuring speed of boat
  • Currents
  • Leeway
  • Errors from dead reckoning
  • Piloting
    • Curvature of the earth
    • Landfall
forces on boat
Forces on boat
  • Wind
  • Resistance on hull
  • Current
  • Lee-way (wind on sailing vessel)

Forces on a sailboat, and resultant motion

Direction of motion


Steering direction


Lateral force from keel



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


A good navigator can estimate

hull speed from the shapes of

the bow wave and stern wave.


Estimating speed relative to water

Boat length L

Start counting now

Piece of flotsam in water

Stop counting now

Speed is L/time


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.


How Polynesians estimated currents

Current direction

Initial bearing

Initial position

Final bearing

Position after drift


Estimating leeway

“Slick” of calmer water

Wake is tilted

Water piles up higher on bow


Compensating for leeway and current


Desired heading


Actual motion

Direction of travel


Example: compensating for current in a blind crossing

Current draining bay = 1 knot

Heading we chose

Desired heading

Speed = 4 knots


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


Expanding the target of landfall

Color of the sea




color of the sea
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

Range formula for objects

Curvature of the earth causes objects to be hidden

by the horizon

D = distance of object in nautical miles

H=height of object in feet

h= height of observer in feet



Sequence: approaching an island

Far distance – deep blue color