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# Dead reckoning, piloting - PowerPoint PPT Presentation

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

John Huth

Topics
• Measuring speed of boat
• Currents
• Leeway
• 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

Direction of motion

Leeway

Steering direction

Drag

Lateral force from keel

Wind

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.

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

Current

Leeway

Actual motion

Direction of travel

Example: compensating for current in a blind crossing

Current draining bay = 1 knot

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

error +12o

Uncertainty in speed

Expanding the target of landfall

Color of the sea

Birds

Clouds

Mountains

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

D

Sequence: approaching an island

Far distance – deep blue color