dead reckoning, piloting
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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)
Wind
Direction of motion
Drag
Lateral force from keel
Leeway
Forces on a sailboat, and resultant motion
Steering direction
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.
Estimating speed relative to water
Boat length L
Piece of flotsam in water
Start counting now
Stop counting now
Speed is L/time
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.
Initial position
Position after drift
Current direction
Initial bearing
Final bearing
How Polynesians estimated currents
Estimating leeway
“Slick” of calmer water
Water piles up higher on bow
Wake is tilted
Leeway
Current
Desired heading
Direction of travel
Compensating for leeway and current
Actual motion
Example: compensating for current in a blind crossing
Desired heading
Current draining bay = 1 knot
Speed = 4 knots
Heading we chose
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
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
Importance of local knowledge: satellite photoof Black sea and Mediterranean
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
Ship hull down – in the distance
Sequence: approaching an island
Far distance – deep blue color
Sixty miles out – spot non-pelagic birds,steer toward land
Forty miles out – color of sea changes
Thirty miles out: clouds appear over land
Twenty miles out – two islands?
Ten miles out – one island