Dead reckoning, piloting John Huth. Topics Measuring speed of boat Currents Leeway Errors from dead...
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Transcript of Dead reckoning, piloting John Huth. Topics Measuring speed of boat Currents Leeway Errors from dead...
![Page 1: Dead reckoning, piloting John Huth. Topics Measuring speed of boat Currents Leeway Errors from dead reckoning Piloting –Curvature of the earth –Landfall.](https://reader036.fdocuments.us/reader036/viewer/2022070403/56649f305503460f94c4a794/html5/thumbnails/1.jpg)
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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Wind
Direction of motion
Drag
Lateral force from keel
Leeway
Forces on a sailboat, and resultant motion
Steering direction
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A big factor in hull resistanceis caused by the bow wave and stern wave, creating a wake.
The number of wavelengthsbetween the bow-wave and stern-wave partly determinesresistance.
A good navigator can estimatehull speed from the shapes ofthe bow wave and stern wave.
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Estimating speed relative to water
Boat length L
Piece of flotsam in water
Start counting now
Stop counting now
Speed is L/time
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Ships log (or chip log)
The log gets thrown overthe stern of the vessel – asthe line gets played out, sailors count the numberof knots that pass the stern for a fixed periodof time. (where theterm “knot” for “nautical mile” comes from).
This can be easily improvised.
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Initial position
Position after drift
Current direction
Initial bearing
Final bearing
How Polynesians estimated currents
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Estimating leeway
“Slick” of calmer water
Water piles up higher on bow
Wake is tilted
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Leeway
Current
Desired heading
Direction of travel
Compensating for leeway and current
Actual motion
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Example: compensating for current in a blind crossing
Desired heading
Current draining bay = 1 knot
Speed = 4 knots
Heading we chose
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error -12o
error +12o
Desired heading
Errors (uncertainties) in position in dead reckoning
Uncertainty in speed
Uncertainty in heading
Uncertainties in speed and heading are typically a fixed percentage – so, as a journey progresses, the numericaluncertainty in position gets larger as time goes on
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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 photoof Black sea and Mediterranean
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hHD
Range formula for objects
Curvature of the earth causes objects to be hiddenby the horizon
D = distance of object in nautical milesH=height of object in feeth= 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