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How the Wayfinder Calculates His Distance Made Good
The wayfinder uses a dead reckoning system in to determine his course. He knows his starting point and the distance and
direction to his destination. He develops a course strategy and draws an imaginary reference course (based on average
conditions) to his destination. As he estimates his direction and distance traveled each day, he mentally plots his position
against this reference course to his destination and estimates how many more days it will take him to reach his destination.
He sets the direction in which the canoe is heading by celestial bodies
and ocean swells, and subtracts leeway; he estimates his distance
traveled by multiplying his estimated speed by the time lapsed since
his last position determination.
An experienced wayfinder can estimate speed by watching the motion of the water as it passes the canoe. An inexperienced
wayfinder can determine the speed of the canoe by timing marks (bubbles or objects in the water) moving past two points on
the canoe. On Hokule'a, the wayfinder times bubbles moving between the front and back
'iako (crossbeams joining the two
hulls together), a distance of 42.2 feet.
The approximate speed in knots for various time intervals of objects passing Hokule'a can be memorized in the following
table:
3 seconds - 8.5 knots
4 seconds- 6.5 knots
5 seconds- 5 knots
6 seconds- 4 knots
7 seconds- 3.5 knots
8 seconds- 3 knots
10 seconds- 2.5 knots
12 seconds- 2 knots
15 seconds- 1.5 knots
25 seconds- 1 knot
An estimate of speed can be gotten by dividing 25 by the number of
seconds it takes the object to travel 42.2 feet: e.g. 25 divided by 3
seconds = 8.33 knots, rounded up to 8.5 knots. The exact formula is:
nautical miles per hour equals distance, converted to nautical miles,
divided by time, converted to hours: 42.2 feet = .007 nautical miles
(42.2 divided by 6077 feet in a nautical mile); 3 seconds = .0008 hours
(3 divided by 3600 seconds per hour); nautical miles per hour equals
.007 divided by .0008, or 8.75 knots.
Time during the day can be estimated by the position of celestial
bodies; twenty-four hours from sunrise to sunrise; around twelve hours
from sunrise to sunset; around six hours from sunrise till noon; and so
on. The sun, moon, or stars travel at about one degrees every four
minutes, taking about three hours to get 45 degrees on the celestial
sphere. Knowing direction, speed, and time elapsed, the wayfinder can
estimate the course and distance made good per day. The calculations
can become quite complex if the canoe changes direction during the day,
or when wind speed varies; the calculation for the day is then the sum
of various separate calculations. To simplify calculations and to
monitor his food and water supplies, the wayfinder may use sailing days
rather than miles to keep track of how far is from his departure point
and how far his destination is. One sailing day for Hokule'a is about
120 miles in the trade wind zones where winds average 10-20 knots; two
sailing days from Hawai'i equal 240 miles.
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