Uncanny navigation across the Scotia seaJul 26, 2007
Of all the impossible small-boat journeys detailed in true-life stories, the passage that Sir Ernest Shackleton made with five companions aboard the 22-foot James Caird is still the one that boggles the mind.
Most by now are familiar with the Endurance expedition to Antarctica that Sir Ernest led in 1916. His ship, Endurance, was stove in by the ice, and Sir Ernest got his men safely to Elephant Island - about 800 miles from the nearest possible rescue. So with five members of his crew, including F.A. Worsley, navigator, they set off in the lifeboat James Caird bound for South Georgia Island where there was known to be a whaling station, an 860-mile passage through the most unforgiving seas in the world. The remainder of his crew waited on Elephant island, fully believing that the indomitable Sir Ernest would return to get them. And return he did! The passage to South Georgia Island took 14 days. Sir Ernest and two others in his crew then had to climb the mid-island mountains before they could get to the whaling station. Eventually all members of the crew were rescued.
Worsley wrote of his trip in a book titled Shackleton's Boat Journey. With only a Heath sextant and the one remaining chronometer - they had started the trip to Antarctica with 24 - Worsley was able to find South Georgia in what must have been a navigator's nightmare. Relying on his uncanny skill at dead reckoning, he was barely able to shoot any sun sights. But those sights he got he used. Worsley's Nautical Almanac was reportedly so water-saturated that he could hardly turn the pages in fear that they would fall apart. Here is how Worsley described a typical sight. "Dead reckoning ... had become a merry jest of guesswork. Once, perhaps twice, a week the sun smiled a sudden wintry flicker through storm torn clouds. If ready for it, and smart, I caught it ... I peered out from our burrow - precious sextant cuddled under my chest to prevent seas falling on it. Sir Ernest stood by under the canvas with chronometer, pencil, and book. I shouted 'Stand by' and knelt on the thwart - two men holding me up on either side. I brought the sun down to where the horizon ought to be and as the boat leaped frantically upward on the crest of a wave, snapped a good guess at the altitude and yelled, 'Stop'. Sir Ernest took the time and I worked out the result."
For this nav problem we will, hopefully, have the comfort of a warm home to work from. Also included is a problem that requires the use of Table 3 of HO 249. Worsley didn't have the luxury of this table and his sights were only meridian passages of the sun. We will also use the 2001 edition of the Nautical Almanac. The height of eye is six feet. The sextant has an index error of 2.6' off the arc. The chronometer couldn't be checked, but we will assume that it is was spot on. All sun sights are of lower limb. Much of this problem is about plotting so please pay attention to all the details so that should the time ever come you can navigate your boat home safely with just your sextant and your courage.
A. On April 26 Worsley has a DR position of 58° 30' S by 52° W. Worsley gets a shot of the sun at noon. Hs is 17° 32.8'. Calculate time to the nearest minute in GMT for meridian passage. Plot latitude line. Mark an estimated position (EP).
B. After 22 hours steering a true course of 036° at an estimated speed of 3.5 knots, Worsley gets another glimpse of the sun at 13hrs 26 min GMT. Hs is 14° 35.6'. How far have they traveled in 22 hours? What is the DR? Draw the LOP. What is the EP? Use Table 3.
C. From this EP James Caird gets hit with a storm, and they heave-to for 10 hours. The wind is from the north, and Worsley estimates they have been set 25 miles in a direction of 170° true. What is the new DR position?
D. On the following day, April 28, Worsley gets another noon sight. From his last DR he changed course to 057° true, and he estimates that his new DR is 57° 09' S by 48° 54' W. Hs of noon sight is 18° 13.6'. Calculate the GMT time of noon to the nearest minute; from his new DR position, plot latitude line, and find the EP.