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:
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.