I just happened to notice this eclectic group of boats all with Butterfly hatches. I think the design, past and present, to achieve the 'driest' butterfly hatch, is fascinating. So much work! I think they were primarily used for extra light below in the dark cabins of many low-mahogany dark boats. Ventilation is pretty meager in comparison to cowl vents or forward facing hatches. But they supply ventilation in a rainstorm. Here's a retired butterfly hatch showing the deep broad gutters that are the basis of their function. The hatch survived due to the craftsmenship, this cabin soon to be compost.

Maintenance, but the owners love these boats(and apparently these hatches).