A Sailor’s Dilemma

In 1968, a British sailor named Robin Knox Johnston set off on an around-the-world race. He sailed from England down the Atlantic, around the coast of South Africa, past Australia, across the Pacific, round the bottom of South America, then back up the Atlantic to England.  30,000 miles in 313 days, becoming the first person to sail single-handedly, non-stop around the world.

Back in ’68, sailors didn’t have the help of satellites for navigation and communication. Navigation was performed the old-fashioned way, with sextants, looking at the stars, charts, rulers, dividers and, oh yeah, luck. Now, the only form of communication that Robin had was a radio, which was powered by lead acid batteries, which were charged by a little three-horsepower gasoline engine connected by a fan belt to a small generator.

At one point he tried to start the charger engine for a routine charge of the batteries, but it wouldn’t start. Foolishly he dismantled the engine, cleaned the various parts, reassembled it and guess what, it still wouldn’t start. Then he decided to read the manual, which told him that it was essential that he should set the ignition points and spark plug gaps correctly. Each needed to be set at 15/1000ths of an inch. Failure to set the gaps correctly would mean that the engine would run poorly and inefficiently, if at all. Now he had brought along 35 gallons of gasoline, just enough to run the engine for the 300 or so days that he thought the voyage would last. If the engine ran inefficiently, he’d run out of fuel and would lose his only means of communication, and if something went wrong, he’d probably die a horrible and lonely death sprawled out on the deck, with seagulls feasting on his decaying carcass.

Well, Robin clearly had a problem. He had a carefully selected tool kit on board–you know– wrenches, saws, screwdrivers, but nothing to measure 15/1000ths of an inch. What he needed was something called a feeler gauge. Now for those who don’t know about such things, feeler gauges are strips of metal of a given thickness, and printed on them is what the thickness is, so you hopefully have one that says .015 inches. They’re used, of course, to measure the gaps between two surfaces, like ignition points.

Without a feeler gauge, Robin had just no chance of measuring this gap of 15/1000ths of an inch. He couldn’t even begin to guess how big a gap 15/1000ths is. After all, he was a man of the sea; he knew about jibs, and sheet lines, and mainsails, and fathoms, but 15/1000ths of an inch? Not a chance. So he cursed in frustration and resigned himself to losing his radio and probably dying a horrible and lonely death on the high seas. Well, that night as he sat down with his diary to write about the day’s events and his certain demise at sea, he had an inspiration and he figured out how to measure that gap of 15/1000ths of an inch using the various things that he had on board.

How did he do it?