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Let's assume that the relevant tracts of land are under the control of a single prosperous state with a good reason to dig a canal roughly equal to OTL's Kiel Canal. Let's also handwave away any wars that might get in the way for now. How early could such a project conceivably be completed?
I know of the Danish Eider Canal of 1784, but I am interested in knowing if it would be possible for one to be completed significantly earlier than that.