The Shark's Paintbrush – More paint drops

Another nibble from the book …

It was November 1985, and a pitch- black, steamy night in the Java Sea. En route to Singapore from Australia, our fifty- foot sailboat should have been lit up by at least the tricolor navigation light at the top of her mast. Instead, the night before we had been hit by lightning, which shorted out all the boat’s electrical systems. There was no wind, so the main engine was propelling us and I had rigged a small flashlight, covered with red cellophane, to the compass so we could see to steer by. About midnight, I looked behind us and was enraptured by a broad trail of exquisite sparkling lights stretching back a mile or two from the stern of the boat. Millions of fluorescing organisms had been scintillated by the turbulent rotations, or cavitation, of our propeller. It looked like a scene from a Peter Pan fantasy— one of the most beautiful and grand sights I have ever seen.

Bioluminescence means “living light,” and that’s exactly what it is. While specific mechanisms differ across the many species that use bioluminescence as part of their survival or mating strategies, it always emits from a living organism and involves the oxidation of an organic molecule. By dramatically agitating the water with our churning propeller, we were, in effect, turning on their chemical lighting systems.

A few minutes later I heard the familiar whoosh of dolphins expelling air as they surfaced. I rushed up the deck to stand on the cathead, a small platform protruding from the bow. When you’re standing on it, you feel like you’re suspended in the air over the ocean. It’s an enthralling experience, especially when dolphins are swimming and breaching through the surface just six feet below your feet.

We were often accompanied by dolphins. As many as forty or fifty in a pod swam with us, in and out of our bow wave, sometimes for fifteen minutes or more. This night, as they leaped out of the water, they trailed cascades of sparkles. As they reentered the water, they pulled air and sparkles with them, clearly defining their tracks underwater. I was surprised to observe that after a few seconds below the surface, the dolphins created no more fluorescence until they broke the surface again. As I watched them play, admiring the fact that the dolphins did not disturb the fluorescing organisms, no matter how fast they swam, I began to wonder: Could traditional boat designs and propellers be biomimetically re-created to achieve such efficient motion?

Turbulence, or cavitation, is a negative consequence of boat propellers. It results from the rotating blades disturbing the water so vigorously that it essentially boils into millions of tiny bubbles of vapor. When the bubbles collapse, it can be with a force of as much as twenty thousand pounds of pressure per square inch. (A car tire is typically inflated to just thirty- five pounds per square inch. A can of Coca- Cola has about forty-five pounds of pressure per square inch.) In fact, these explosions are so violent that they tear chunks of metal away from the propellers and can eventually destroy them. I’ve heard of a ship that, on its maiden voyage from the United States to England, lost blades from its $100,000 propeller—in just one trip.