Five people set sail across the Atlantic heading for Barbados in an old factory boiler. The Belgian captain and owner, Fons Oerlemans, fitted out the 45-foot concrete-reinforced boiler with a wheelhouse, a mast, two sails, a radio , a diesel engine, four watertight living compartments, and an underwater observation tube. With him are his fiancee, Marguerite Arens, and three Dutchmen, among them a marine biologist. One compartment extends below the body of the boiler and has glass portholes through which the crew will film and observe marine life. Mr. Oerlemans said the craft had stoop up well to gales in the English Channel as it sailed from the Netherlands to England.