With barely enough breeze to fill the sails on Symbiosis, our Tayana 37, we ghosted south through the Caicos Passage, heading toward the Dominican Republic. A wall of cumulus clouds stretched to the east beside us, looking only slightly menacing. Suddenly, a dark funnel descended from the sky several miles ahead. My pulse quickened. I put my wife, Noi, on the helm and went forward to douse the main.