The meaning of that is not very clear. htm you can see various other translation attempts, including "Leave an 18-inch opening below the roof all the way around the boat", which is still quite hard to visualise. There are also some commentaries which attempt to explain this further, but none of the explanations or wordings are completely clear to me as to what is being described.
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GPYIf you left an 18-inch opening below the roof all the way around the boat then wouldn't the roof fall down?If you literally had it all the way round, yes. You would have to have supporting posts at intervals.