1. Dead reckoning, piloting
John Huth

2. Topics Measuring speed of boat Currents Leeway
Errors from dead reckoning
Piloting
Curvature of the earth
Landfall

3. Forces on boat Wind Resistance on hull Current
Lee-way (wind on sailing vessel)

4. Forces on a sailboat, and resultant motion
Direction of motion
Leeway
Steering direction
Drag
Lateral force from keel
Wind

5. 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.

6. Estimating speed relative to water
Boat length L
Start counting now
Piece of flotsam in water
Stop counting now
Speed is L/time

7. 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.

8. How Polynesians estimated currents
Current direction
Initial bearing
Initial position
Final bearing
Position after drift

9. Estimating leeway “Slick” of calmer water Wake is tilted
Water piles up higher on bow

10. Compensating for leeway and current
Desired heading
Leeway
Actual motion
Direction of travel

11. Example: compensating for current in a blind crossing
Current draining bay = 1 knot
Heading we chose
Desired heading
Speed = 4 knots

12. 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

13. Expanding the target of landfall
Color of the sea
Birds
Clouds
Mountains

14. 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

15. Importance of local knowledge: satellite photo
of Black sea and Mediterranean

16. 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

17. Ship hull down – in the distance

18. Sequence: approaching an island
Far distance – deep blue color

19. Sixty miles out – spot non-pelagic birds,
steer toward land

20. Forty miles out – color of sea changes

21. Thirty miles out: clouds appear over land

22. Twenty miles out – two islands?

23. Ten miles out – one island