For a canoe to traverse the open seas between islands in an archipelago, it needed something to stabilize it. The ancient Austronesians lashed two canoes together side by side, or, better, separated them with horizontal beams, or, better yet, replaced one of the canoes with a long float. Under sail, the separation provided more stability than any fin keel, and their craft’s shallow draft meant they could safely sail across reefs and shallows.