Nainoa Thompson.

Finding Rapa Nui: The Latest Strategy
Wednesday, September 29, 1999
by Sam Low, interviewing Nainoa Thompson

Rapa Nui (64 sq. miles, 1,674 ft.high) is only a little larger and a little higher than the Hawaiian island of Kaho'olawe (45 sq. miles, 1,477 ft. high). Because Rapa Nui is so small, it would be unreasonable to expect that we could, even with favorable winds, navigate without instruments 1150 miles from Pitcairn straight to Rapa Nui; the traditional system of navigation we use is simply not that accurate and mechanical. It requires constant thought, memorization, and estimates of distances and directions traveled; it invovles adjusting to constantly changing wind, sky, and ocean conditions. The navigator rarely sleeps, except for catnaps; the longer a voyage lasts, the more prone to error he may become. To find Rapa Nui using our navigation system, we've created a search strategy which we hope will work, although we have never tried it before. It is a unique navigational challenge to find an island so small and isolated, without a large population of land-based sea birds. Instead of targeting a number of closely spaced islands in a chain or following seabirds flying to or from the target island, we have to sail within sight distance of Rapa Nui--within 30 miles of it--in to order to find it.

Original Plan (Before Leaving Hawai'i, based on the winds we expected to encounter during the voyage): We established a block of ocean for the search--a block which begins 300 miles to the west of Rapa Nui, between 25 degrees S to 29 degrees S. Rapa Nui is at 27 degrees 09' S, centered between the north and south limits of the eastern end of the block. Targeting this block of ocean--300 miles long and 240 miles wide, 72,000 square miles in all--rather than a small island 13 miles long and 10 miles wide, 64 square miles, allows us to compensate for any mistakes in our dead reckoning (the estimates of miles we sailed east from Pitcairn) or in determining our latitude. When we reach the western end of the block, 300 miles west of the Rapa Nui, we assume that Rapa Nui is somewhere in this block to the east. So we sail Haka Ko'olau (N by E) up to 25 degrees S, then Haka Malanai (S by E) down to 29 degrees S, then up north again and so on, in a zigzaging pattern, slowly moving eastward through the block. At some point in this zigzag pattern, we would pass within 30 miles of the Rapa Nui and be able to see it.

Modification of the Original Plan (Sept. 29, 1999): The incredibly good winds we've gotten now allows us to modify the strategy. Because we've been able to sail directly toward our destination in less time than we had originally planned (both of these factors reduce the possibilities of errors in our dead reckoning), we have now decided to begin our search 200 miles west of Rapa Nui instead of 300 miles to the west. This decreases the area we have to search by 1/3, from 72,000 square miles to 48,000 square miles, and will save some time in tacking in search of the island.

Contingency Plan: Our ability to be successful in tacking in search of Rapa Nui depends on the wind direction. In order for us to tack north and south in a zigzag pattern, we need easterly or westerly winds. Northerly winds will not allow us to tack north, and southerly winds will not allow us to tack south. So far the winds have been mainly northerly. If these winds continue to prevail we may not be able to tack north. So instead we will try to get a good reading of our latitude stars at night, and then sail directly east for Rapa Nui along the latitude of the island: 27 degrees 09' S. This strategy, called "Latitude Sailing,"requires some precision in determining latitude...because the island is so small, we need to be within half a degree of accuracy or less than 30 miles off on either side of Rapa Nui for it to work. In this scenario, after a period of time, if we have not found the island by sailing directly east at the latitude we think is 27° 09'S, plus or minus a half a degree, we may decide that we've sailed past the island and will turn around and search back to the west. We have placed a time limit of 30 days on our search for Rapa Nui, which began Sept. 21; so we are giving ourselves till Oct. 21 to find the island....but we have 42 days of food and water on board, so conceivably we could stay at sea till the end of October.

The Moon: The favorable winds and our quick passage to the vicinity of Rapa Nui has thrown this timetable off. Now it looks like we may be searching for the island under a waning moon. It is difficult to see an island at night. Originally we timed the voyage so that we would be searching for Rapa Nui around the full moon (October 24). The moon will disappear on Oct. 8-9 and the night sky will be completely dark, except for the light of the stars and planets. So we plan to stop sailing at night during the search, closing our sails two hours after sunset and opening them again two hours before sunrise. We cannot sail more than fifteen miles in two hours of darkness, so we won't sail past the island without seeing it in the two hour of darkness before sunrise and the two hours after sunset.

The dark sky will also make determining latitude more difficult. The readings depend on measuring the height of stars above the horizon as they cross the meridian. (See above.) As the moonlight diminishes and nights get darker, the horizon gets less distinct, the ocean surface less easy to distinguish from the night sky.