A “double-bottomed” ship, a kind of catamaran, designed by Sir William Petty. Here’s a quote from this review of Ted McCormick’s William Petty and the Ambitions of Political Arithmetic by Steven Shapin:
The ship was an experiment in science, technology and entrepreneurship. No one had ever seen such a vessel; it violated the conventions of the shipwright’s art; and among the knowledgeable it was generally an object of ridicule and opposition. There was, Petty acknowledged, ‘scarce a good word for it’ from anyone in the business. Most experts thought it wouldn’t work, and some worried that, in the unlikely event it did, it would be a technology too far. Samuel Pepys, the Navy Board’s clerk of the acts, was a supporter, while the master shipwright Anthony Deane said Petty’s design ‘must needs prove a folly’. The navy commissioner Peter Pett told Pepys that the double-hulled ship was ‘the most dangerous thing in the world’: if it was successful, the secret would get out, and it would be the ruin of English trade and sea power. The Dutch, with whom England was about to fight the second instalment of a series of naval wars, might use the shallow-draught ship to sail right up the Thames and lay London waste.
Here’s a photo of a model of a similar multi-hulled ship of the period.